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Code Standards
This is a blatant copy and alteration of the google guide which we have based our standards on here at Maidsafe.
C++ is the main language used in MaidSafe's projects. As every C++ programmer knows, the language has many powerful features; but this power brings with it complexity, which in turn can make code more bug-prone and harder to read and maintain.
The goal of this guide is to manage this complexity by describing in detail the dos and don'ts of writing C++ code. These rules exist to keep the code base manageable while still allowing coders to use C++ language features productively.
One way in which we keep the code base manageable is by enforcing consistency.
It is very important that any programmer be able to look at another's code and quickly understand it. Maintaining a uniform style and following conventions means that we can more easily use 'pattern-matching' to infer what various symbols are and what invariants are true about them. Creating common, required idioms and patterns makes code much easier to understand. In some cases there might be good arguments for changing certain style rules, but we generally keep things as they are in order to preserve consistency.
Another issue this guide addresses is that of C++ feature bloat. C++ is a huge language with many advanced features. In some cases we constrain, or even ban, use of certain features. We do this to keep code simple and to avoid the various common errors and problems that these features can cause. This guide lists these features and explains why their use is restricted.
In general, every project will have an associated DashBoard. This allows us to keep track of all progress and test results. All projects must aim for at least 90% code coverage.
Yum Yums are an excellent sugary bun found in Scottish bakers and they are enjoyed by all the team. We particularly enjoy them when other developers are forced to purchase them for us.
All code that is committed and breaks the build on every OS means, go and buy Yum Yums. Do not pass go do not argue just go and get them, it's your way of saying sorry (and it's fun!). Open Source developers may have to think of ingenious ways of paying this penalty.
Additionally anyone in debate can request a 'Yum Yums' bet and the proven loser then must yet again, buy Yum Yums, no debate and no arguments, it's one of our very few golden and indisputable rules.
In general, every .cc file should have an associated .h file. There are some common exceptions, such as unittests and small .cc files containing just a main() function. Correct use of header files can make a huge difference to the readability, size and performance of your code.
The following rules will guide you through the various pitfalls of using header files.
All header files should have #define guards to prevent multiple inclusion. To guarantee uniqueness, they should be based on the full path in a project's source tree. For example, the file maidsafe/common/crypto.h in project MaidSafe-Common has the following guard:
#ifndef MAIDSAFE_COMMON_CRYPTO_H_
#define MAIDSAFE_COMMON_CRYPTO_H_
...
#endif // MAIDSAFE_COMMON_CRYPTO_H_Don't use an #include when a forward declaration would suffice.
When you include a header file you introduce a dependency that will cause your code to be recompiled whenever the header file changes. If your header file includes other header files, any change to those files will cause any code that includes your header to be recompiled. Therefore, we prefer to minimize includes, particularly includes of header files in other header files.
You can significantly reduce the number of header files you need to include in your own header files by using forward declarations. For example, if your header file uses the File class in ways that do not require access to the declaration of the File class, your header file can just forward declare class File; instead of having to #include "file/base/file.h".
How can we use a class Foo in a header file without access to its definition?
- We can declare data members of type
Foo*orFoo&. - We can declare (but not define) functions with arguments, and/or return values, of type
Foo. (One exception is if an argumentFooorconst Foo&has a non-explicit, one-argument constructor, in which case we need the full definition to support automatic type conversion.) - We can declare static data members of type
Foo. This is because static data members are defined outside the class definition.
On the other hand, you must include the header file for Foo if your class subclasses Foo or has a data member of type Foo.
Sometimes it makes sense to have pointer (or better, std::unique_ptr) members instead of object members. However, this complicates code readability and imposes a performance penalty, so avoid doing this transformation if the only purpose is to minimize includes in header files.
Of course, .cc files typically do require the definitions of the classes they use, and usually have to include several header files.
Note: If you use a symbol Foo in your source file, you should bring in a definition for Foo yourself, either via an #include or via a forward declaration. Do not depend on the symbol being brought in transitively via headers not directly included. One exception is if Foo is used in myfile.cc, it's OK to #include (or forward-declare) Foo in myfile.h, instead of myfile.cc.
Define functions inline only when they are small, say, 10 lines or less.
Another useful rule of thumb: it's typically not cost effective to inline functions with loops or switch statements (unless, in the common case, the loop or switch statement is never executed).
It is important to know that functions are not always inlined even if they are declared as such; for example, virtual and recursive functions are not normally inlined. Usually recursive functions should not be inline. The main reason for making a virtual function inline is to place its definition in the class, either for convenience or to document its behaviour, e.g. for accessors and mutators.
You may use file names with a -inl.h suffix to define complex inline functions when needed.
The definition of an inline function needs to be in a header file, so that the compiler has the definition available for inlining at the call sites. However, implementation code properly belongs in .cc files, and we do not like to have much actual code in .h files unless there is a readability or performance advantage.
If an inline function definition is short, with very little, if any, logic in it, you should put the code in your .h file. For example, accessors and mutators should certainly be inside a class definition. More complex inline functions may also be put in a .h file for the convenience of the implementer and callers, though if this makes the .h file too unwieldy you can instead put that code in a separate -inl.h file. This separates the implementation from the class definition, while still allowing the implementation to be included where necessary.
Another use of -inl.h files is for definitions of function templates. This can be used to keep your template definitions easy to read.
Do not forget that a -inl.h file requires a #define guard just like any other header file.
Use standard order for readability and to avoid hidden dependencies: C library, C++ library, other libraries' .h, your project's .h.
All of a project's header files should be listed as descendants of the project's source directory without use of directory shortcuts . (the current directory) or .. (the parent directory). For example, common/include/maidsafe/common/log.h should be included as
#include "maidsafe/common/log.h"
In dir/foo.cc or dir/foo_test.cc, whose main purpose is to implement or test the stuff in dir2/foo2.h, order your includes as follows:
-
dir2/foo2.h(preferred location see details below). - C system files.
- C++ system files.
- Other libraries'
.hfiles. - Your project's
.hfiles.
With the preferred ordering, if dir/foo2.h omits any necessary includes, the build of dir/foo.cc or dir/foo_test.cc will break. Thus, this rule ensures that build breaks show up first for the people working on these files, not for innocent people in other packages.
dir/foo.cc and dir2/foo2.h are often in the same directory (e.g. base/basictypes_test.cc and base/basictypes.h), but can be in different directories too.
For example, the includes in awesome-project/src/foo/internal/fooserver.cc might look like this:
#include "foo/public/fooserver.h" // Preferred location.
#include <sys/types.h>;
#include <unistd.h>;
#include <hash_map>;
#include <vector>;
#include "base/basictypes.h"
#include "base/commandlineflags.h"
#include "foo/public/bar.h"Unnamed namespaces in .cc files are encouraged to avoid runtime naming conflicts. With named namespaces, choose the name based on the project and possibly its path. Do not use a using-directive.
Terminate namespaces with comments as shown in the given examples.
namespace { // This is in a .cc file.
// The content of a namespace is not indented
enum class { kUnused, kEOF, kError }; // Commonly used tokens.
bool AtEof() { return pos_ == kEOF; } // Uses our namespace's EOF.
} // namespaceHowever, file-scope declarations that are associated with a particular class may be declared in that class as types, static data members or static member functions rather than as members of an unnamed namespace.
Do not use unnamed namespaces in .h files.
Named namespaces should be used as follows:
-
Namespaces wrap the entire source file after includes, definitions/declarations and forward declarations of classes from other namespaces:
// In the .h file namespace mynamespace { // All declarations are within the namespace scope. // Notice the lack of indentation. class MyClass { public: ... void Foo(); }; } // namespace mynamespace // In the .cc file namespace mynamespace { // Definition of functions is within scope of the namespace. void MyClass::Foo() { ... } } // namespace mynamespace
-
The typical
.ccfile might have more complex detail, including the need to reference classes in other namespaces.#include "a.h" DEFINE_bool(someflag, false, "dummy flag"); class C; // Forward declaration of class C in the global namespace. namespace a { class A; } // Forward declaration of a::A. namespace b { ...code for b... // Code goes against the left margin. } // namespace b
-
Do not declare anything in namespace
std, not even forward declarations of standard library classes. Declaring entities in namespacestdis undefined behavior, i.e., not portable. To declare entities from the standard library, include the appropriate header file. -
You may not use a
using-directiveto make all names from a namespace available.// Forbidden -- This pollutes the namespace. using namespace foo;
-
You may use a
using-declarationrarely in a.ccor in.hfiles. An example may be when accessing otherwise hidden methods in a base class from a derived class. In this case you ** must** reference the actual method itself.using ::foo::bar; -
Namespace aliases are allowed anywhere in a
.ccfile, anywhere inside the named namespace that wraps an entire.hfile, and in functions and methods.// Shorten access to some commonly used names in .cc files. namespace fbz = ::foo::bar::baz; // Shorten access to some commonly used names (in a .h file). namespace librarian { // The following alias is available to all files including // this header (in namespace librarian): // alias names should therefore be chosen consistently // within a project. namespace pd_s = ::pipeline_diagnostics::sidetable; inline void my_inline_function() { // namespace alias local to a function (or method). namespace fbz = ::foo::bar::baz; ... } } // namespace librarian
Note that an alias in a .h file is visible to everyone #including that file, so public headers (those available outside a project) and headers transitively #included by them should avoid defining aliases as part of the general goal of keeping public APIs as small as possible.
Although you may use public nested classes when they are part of an interface, consider a namespace to keep declarations out of the global scope.
A class can define another class within it; this is also called a member class.
class Foo {
private:
// Bar is a member class, nested within Foo.
class Bar {
...
};
};This is useful when the nested (or member) class is only used
by the enclosing class; making it a member puts it in the
enclosing class scope rather than polluting the outer scope
with the class name. Nested classes can be forward declared
within the enclosing class and then defined in the
.cc file to avoid including the nested class
definition in the enclosing class declaration, since the
nested class definition is usually only relevant to the
implementation.
Nested classes can be forward-declared only within the
definition of the enclosing class. Thus, any header file
manipulating a Foo::Bar* pointer will have to
include the full class declaration for Foo.
Do not make nested classes public unless they are actually part of the interface, e.g., a class that holds a set of options for some method.
Prefer nonmember functions within a namespace or static member functions to global functions; use completely global functions rarely.
Nonmember and static member functions can be useful in some situations. Putting nonmember functions in a namespace avoids polluting the global namespace.
Nonmember and static member functions may make more sense as members of a new class, especially if they access external resources or have significant dependencies.
Sometimes it is useful, or even necessary, to define a function not bound to a class instance. Such a function can be either a static member or a nonmember function. Nonmember functions should not depend on external variables and should nearly always exist in a namespace. Rather than creating classes only to group static member functions which do not share static data, use Namespaces instead.
Functions defined in the same compilation unit as production classes may introduce unnecessary coupling and link-time dependencies when directly called from other compilation units. Static member functions are particularly susceptible to this. Consider extracting a new class, or placing the functions in a namespace, possibly in a separate library.
If you must define a nonmember function and it is only
needed in its .cc file, use an unnamed
Namespaces or static
linkage (eg static int Foo() {...}) to limit
its scope.
Place a function's variables in the narrowest scope possible and initialise variables in the declaration.
C++ allows you to declare variables anywhere in a function. We encourage you to declare them in as local a scope as possible and as close to the first use as possible. This makes it easier for the reader to find the declaration and see what type the variable is and what it was initialised to. In particular, initialisation should be used instead of declaration and assignment, e.g.
```c++ int i; i = f(); // Bad -- initialisation separate from declaration. ``` ``` int j = g(); // Good -- declaration has initialisation. ```Note that gcc implements for (int i = 0; i < 10; ++i) correctly (the scope of i is
only the scope of the for loop), so you can then
reuse i in another for loop in the
same scope. It also correctly scopes declarations in
if and while statements, e.g.
while (const char* p = strchr(str, '/'))
str = p + 1;There is one caveat: if the variable is an object, it's constructor is invoked every time it enters scope and is created and its destructor is invoked every time it goes out of scope:
```C++ // Inefficient implementation: for (int i = 0; i < 1000000; ++i) { Foo f; // My ctor and dtor get called 1000000 times each. f.DoSomething(i); } ```It may be more efficient to declare such a variable used in a loop outside that loop:
Foo f; // My ctor and dtor get called once each.
for (int i = 0; i < 1000000; ++i) {
f.DoSomething(i);
}Static or global variables of class type are forbidden. They cause hard-to-find bugs due to indeterminate order of construction and destruction.
Objects with static storage duration, including global variables, static variables, static class member variables and function static variables, must be Plain Old Data (POD): only ints, chars, floats, or pointers, or arrays/structs of POD.
The order in which class constructors and initialisers for static variables are called is only partially specified in C++ and can even change from build to build, this can cause bugs that are difficult to find. Therefore, in addition to banning globals of class type, we do not allow static POD variables to be initialised with the result of a function, unless that function (such as getenv(), or getpid()) does not itself depend on any other globals.
Likewise, the order in which destructors are called is defined to be the reverse of the order in which the constructors were called. Since constructor order is indeterminate, so is destructor order. For example, at program-end time a static variable might have been destroyed, but code still running, perhaps in another thread, tries to access it and fails. Alternatively, the destructor for a static 'string' variable might be run prior to the destructor for another variable that contains a reference to that string.
As a result we only allow static variables to contain POD data. This
rule completely disallows vector (use C arrays instead), or
string (use const char []).
If you need a static or global variable of a class type, consider initialising a pointer (which will never be freed), from either your main() function or from pthread_once(). Note that this must be a raw pointer, not a "smart" pointer, as the smart pointer's destructor will have the order-of-destructor issue that we are trying to avoid.
Classes are the fundamental unit of code in C++. Naturally, we use them extensively. This section lists the main dos and don'ts you should follow when writing a class.
In general, constructors should merely set member variables to their
initial values. Any complex initialisation should go in an explicit
Init() method.
It is possible to perform initialisation in the body of the constructor.
Convenience in typing. No need to worry about whether the class has been initialised or not.
The problems with doing work in constructors are:
-
There is no easy way for constructors to signal errors, short of using Exceptions.
-
If the work fails, we now have an object whose initialisation code failed, so it may be an indeterminate state.
-
If the work calls virtual functions, these calls will not get dispatched to the subclass implementations. Future modification to your class can quietly introduce this problem even if your class is not currently subclassed, causing much confusion.
-
If someone creates a global variable of this type (which is against the rules), the constructor code will be called before
main(), possibly breaking some implicit assumptions in the constructor code. For instance, gflags will not yet have been initialised.
If your object requires non-trivial initialisation, consider
having an explicit Init() method. In particular,
constructors should not call virtual functions, attempt to raise
errors, access potentially uninitialised global variables...etc...
You must define a default constructor if your class defines member variables and has no other constructors. Otherwise the compiler will do it for you, badly.
The default constructor is called when we new a
class object with no arguments. It is always called when
calling new[] (for arrays).
Initialising structures by default, to hold 'impossible' values, makes debugging much easier.
Extra work for you, the code writer.
If your class defines member variables and has no other constructors you must define a default constructor (one that takes no arguments). It should preferably initialise the object in such a way that its internal state is consistent and valid.
The reason for this is that if you have no other constructors and do not define a default constructor, the compiler will generate one for you. This compiler generated constructor may not initialise your object sensibly.
If your class inherits from an existing class but you add no new member variables, you are not required to have a default constructor.
Use the C++ keyword explicit for constructors with
one argument.
Normally, if a constructor takes one argument, it can be used
as a conversion. For instance, if you define
Foo::Foo(string name) and then pass a string to a
function that expects a Foo, the constructor will
be called to convert the string into a Foo and
will pass the Foo to your function for you. This
can be convenient but is also a source of trouble when things
get converted and new objects created without you meaning them
to. Declaring a constructor explicit prevents it
from being invoked implicitly as a conversion.
Avoids undesirable conversions.
None.
We require all single argument constructors to be
explicit. Always put explicit in front of
one-argument constructors in the class definition:
explicit Foo(string name);
The exception is copy constructors, which, in the rare
cases when we allow them, should probably not be
explicit.
Classes that are intended to be transparent wrappers around other classes are also exceptions. Such exceptions should be clearly marked with comments.
Provide a copy constructor and assignment operator only when necessary. Otherwise, disable them with DISALLOW_COPY_AND_ASSIGN.
The copy constructor and assignment operator are used to create copies of objects. The copy constructor is implicitly invoked by the compiler in some situations, e.g. passing objects by value.
Copy constructors make it easy to copy objects. STL containers require that all contents be copyable and assignable. Copy constructors can be more efficient than CopyFrom()-style workarounds because they combine construction with copying, the compiler can elide them in some contexts, and they make it easier to avoid heap allocation.
Implicit copying of objects in C++ is a rich source of bugs and of performance problems. It also reduces readability, as it becomes hard to track which objects are being passed around by value as opposed to by reference and therefore where changes to an object are reflected.
Few classes need to be copyable. Most should have neither a copy constructor nor an assignment operator. In many situations, a pointer or reference will work just as well as a copied value, with better performance. For example, you can pass function parameters by reference or pointer instead of by value and you can store pointers rather than objects in an STL container.
If your class needs to be copyable, prefer providing a copy method, such as CopyFrom() or Clone(), rather than a copy constructor, because such methods cannot be invoked implicitly. If a copy method is insufficient in your situation (e.g. for performance reasons, or because your class needs to be stored by value in an STL container), provide both a copy constructor and assignment operator.
If your class does not need a copy constructor or assignment operator, you must explicitly disable them.
To do so, add dummy declarations for the copy constructor and assignment operator in the private: section of your class, but do not provide any corresponding definition (so that any attempt to use them results in a link error).
Use a struct only for passive objects that carry data; everything else is a class.
The struct and class keywords behave almost identically in C++. We add our own semantic meanings to each keyword, so you should use the appropriate keyword for the data-type you're defining.
structs should be used for passive objects that carry data and may have associated constants, but lack any functionality other than access/setting the data members. The accessing/setting of fields is done by directly accessing the fields rather than through method invocations. Methods should not provide behaviour but should only be used to set up the data members, e.g., constructor, destructor, Initialise(), Reset(), Validate().
If more functionality is required, a class is more appropriate. If in doubt, make it a class.
For consistency with STL, you can use struct instead of class for functors and traits.
Note that member variables in structs and classes have different naming rules.
Composition is often more appropriate than inheritance. When using inheritance, make it public.
When a sub-class inherits from a base class, it includes the definitions of all the data and operations that the parent base class defines. In practice, inheritance is used in two major ways in C++: implementation inheritance, in which actual code is inherited by the child and Interfaces inheritance, in which only method names are inherited.
Implementation inheritance reduces code size by re-using the base class code as it specialises an existing type. Because inheritance is a compile-time declaration, you and the compiler can understand the operation and detect errors. Interface inheritance can be used to programmatically enforce that a class expose a particular API. Again, the compiler can detect errors, in this case, when a class does not define a necessary method of the API.
For implementation inheritance, as the code implementing a sub-class is spread between the base and the sub-class, it can be more difficult to understand an implementation. The sub-class cannot override functions that are not virtual, so the sub-class cannot change implementation. The base class may also define some data members, so that specifies physical layout of the base class.
All inheritance should be public. If you want to do private inheritance, you should be including an instance of the base class as a member instead.
Do not overuse implementation inheritance. Composition is often more appropriate. Try to restrict use of inheritance to the 'is-a' case: Bar subclasses Foo if it can reasonably be said that Bar "is a kind of" Foo.
Make your destructor virtual if necessary. If your class has virtual methods, its destructor
should be virtual.
Limit the use of protected to those member functions that might need to be accessed from subclasses. Note that data members should be private.
When redefining an inherited virtual function, explicitly declare it virtual in the declaration of the derived class. Rationale: If virtual is omitted, the reader has to check all ancestors of the class in question to determine if the function is virtual
or not.
Only very rarely is multiple implementation inheritance actually
useful. We allow multiple inheritance only when at most one of
the base classes has an implementation; all other base classes
must be pure interface classes tagged
with the Interface suffix.
Multiple inheritance allows a sub-class to have more than one base class. We distinguish between base classes that are pure interfaces and those that have an implementation.
Multiple implementation inheritance may let you re-use even more code than single inheritance (see Inheritance).
Only very rarely is multiple implementation inheritance actually useful. When multiple implementation inheritance seems like the solution, you can usually find a different, more explicit, and cleaner solution.
Multiple inheritance is allowed only when all superclasses, with the
possible exception of the first one, are pure interfaces. In order to ensure that they remain pure interfaces,
they must end with the Interface suffix.
Note: There is an exception to this rule on Windows.
Classes that satisfy certain conditions are allowed, but not required, to
end with an Interface suffix.
A class is a pure interface if it meets the following requirements:
-
It has only public pure virtual ("
= 0") methods and static methods (but see below for destructor). -
It may not have non-static data members.
-
It need not have any constructors defined. If a constructor is provided, it must take no arguments and it must be protected.
-
If it is a subclass, it may only be derived from classes that satisfy these conditions and are tagged with the
Interfacesuffix.
An interface class can never be directly instantiated because of the pure virtual method(s) it declares. To make sure all implementations of the interface can be destroyed correctly, the interface must also declare a virtual destructor (in an exception to the first rule, this should not be pure). See Stroustrup, The C++ Programming Language, 3rd edition, section 12.4 for details.
Tagging a class with the Interface suffix lets
others know that they must not add implemented methods or non
static data members. This is particularly important in the case of
Multiple Inheritance.
Additionally, the interface concept is already well-understood by
Java programmers.
The Interface suffix lengthens the class name, which
can make it harder to read and understand. Also, the interface
property may be considered an implementation detail that shouldn't
be exposed to clients.
A class may end with Interface only if it meets the
above requirements. We do not require the converse, however:
classes that meet the above requirements are not required to end
with Interface.
Do not overload operators except in rare, special circumstances.
A class can define that operators such as + and
/ operate on the class as if it were a built-in
type.
Can make code appear more intuitive because a class will
behave in the same way as built-in types (such as
int). Overloaded operators are more playful
names for functions that are less colourfully named, such as
Equals() or Add(). For some
template functions to work correctly, you may need to define
operators.
While operator overloading can make code more intuitive, it has several drawbacks:
-
It can fool our intuition into thinking that expensive operations are cheap, built-in operations.
-
It is much harder to find the call sites for overloaded operators. Searching for
Equals()is much easier than searching for relevant invocations of==. -
Some operators work on pointers too, making it easy to introduce bugs.
Foo + 4may do one thing, while&Foo + 4does something totally different. The compiler does not complain for either of these, making this very hard to debug.
Overloading also has surprising ramifications. For instance,
if a class overloads unary operator&, it
cannot safely be forward-declared.
In general, do not overload operators. The assignment operator
(operator=) in particular is insidious and
should be avoided. You can define functions like
Equals() and CopyFrom() if you
need them. Likewise, avoid the dangerous
unary operator& at all costs, if there's
any possibility the class might be forward-declared.
However, there may be rare cases where you need to overload
an operator to interoperate with templates or "standard" C++
classes (such as operator<<(ostream&, const T&) for logging). These are acceptable if fully
justified, but you should try to avoid these whenever
possible. In particular, do not overload operator==
or operator< just so that your class can be
used as a key in an STL container; instead, you should
create equality and comparison functor types when declaring
the container.
Some of the STL algorithms do require you to overload
operator==, and you may do so in these cases,
provided you document why.
See also Copy Constructors and Function Overloading.
Make data members private and provide
access to them through accessor functions as needed (for
technical reasons, we allow data members of a test fixture class
to be protected when using Google Test ). Typically a variable would be
called foo_ and the accessor function
foo(). You may also want a mutator function
set_foo().
Exception: static const data members (typically
called kFoo) need not be private.
The definitions of accessors are usually inlined in the header file.
See also Inheritance and Function Names.
Use the specified order of declarations within a class:
public: before private:, methods
before data members (variables)...etc...
Your class definition should start with its public:
section, followed by its protected: section and
then its private: section. If any of these sections
are empty, omit them.
Within each section, the declarations generally should be in the following order:
- Typedefs and Enums
- Constants (
static constdata members) - Constructors
- Destructor
- Methods, including static methods
- Data Members (except
static constdata members)
Friend declarations should always be in the private section and
the DISALLOW_COPY_AND_ASSIGN macro invocation
should be at the end of the private: section. It
should be the last thing in the class. See Copy Constructors.
Method definitions in the corresponding .cc file
should be the same as the declaration order, as much as possible.
Do not put large method definitions inline in the class definition. Usually, only trivial or performance-critical and very short methods may be defined inline. See Inline Functions for more details.
Prefer small and focused functions.
We recognise that long functions are sometimes appropriate, so no hard limit is placed on functions length. If a function exceeds about 40 lines, think about whether it can be broken up without harming the structure of the program.
Even if your long function works perfectly now, someone modifying it in a few months may add new behaviour and this could result in bugs that are hard to find. Keeping your functions short and simple makes it easier for other people to read and modify your code.
You could find long and complicated functions when working with some code. Do not be intimidated by modifying existing code: if working with such a function proves to be difficult, you find that errors are hard to debug, or you want to use a piece of it in several different contexts, consider breaking up the function into smaller and more manageable pieces.
There are various tricks and utilities that we use to make C++ code more robust and various ways we use C++ that may differ from what you see elsewhere.
If you actually need pointer semantics, std::unique_ptr is great. You should only use std::shared_ptr with a non-const referent when it is truly necessary to share ownership of an object (e.g. inside an STL container). You should never use auto_ptr. Never use raw pointers unless cleared by a project leader.
"Smart" pointers are objects that act like pointers, but automate management of the underlying memory.
Smart pointers are extremely useful for preventing memory leaks, and are essential for writing exception-safe code. They also formalise and document the ownership of dynamically allocated memory.
We prefer designs in which objects have single, fixed owners. Smart pointers which enable sharing or transfer of ownership can act as a tempting alternative to a careful design of ownership semantics, leading to confusing code and even bugs in which memory is never deleted. The semantics of smart pointers (especially auto_ptr) can be unobvious and confusing.
std::unique_ptr
Straightforward and risk-free. Use wherever appropriate.
auto_ptr
Confusing and bug-prone ownership-transfer semantics. Do not use.
shared_ptr
Safe with const referents (i.e. shared_ptr const T&). Reference-counted pointers with non-const referents can occasionally be the best design, but try to rewrite with single
owners where possible.
Use
cpplint.py
to detect style errors.
cpplint.py
is a tool that reads a source file and identifies many style errors. It is not perfect, and has both false positives and false negatives, but it is still a valuable tool. False positives can be ignored by putting // NOLINT at the end of the line.
Some projects have instructions on how to run cpplint.py from their project tools. If the project you are contributing to does not, you can download cpplint.py separately.
All parameters passed by reference must be labeled const. This can be relaxed in cases where 3rd party code requires it.
In C, if a function needs to modify a variable, the parameter must use a pointer, eg int foo(int *pval). In C++, the function can alternatively declare a reference parameter: int foo(int &val).
Defining a parameter as reference avoids ugly code like (*pval)++. Necessary for some applications like copy constructors. Makes it clear, unlike with pointers, that NULL is not a possible value.
References can be confusing, as they have value syntax but pointer semantics.
Within function parameter lists, all references must be const &. This decision is under review as there is a strong argument for references not being able to be null and ptr requires checks in code.
void Foo(const string &in, string *out);However, there are some instances where using const T* is preferable to const T& for input parameters. For example: You want to pass in NULL. The function saves a pointer or reference to the input. Remember that most of the time input parameters are going to be specified as const T&. Using const T* instead communicates to the reader that the input is somehow treated differently. So if you choose const T* rather than const T&, do so for a concrete reason; otherwise it will likely confuse readers by making them look for an explanation that doesn't exist.
Use overloaded functions (including constructors) only if a reader looking at a call site can get a good idea of what is happening without having to first figure out exactly which overload is being called.
Do not confuse function overloading with default parameters. Default parameters can help to reduce code in some cases (carefully), overloading suggests the methods implementaiton has mostly changed, given the new signature.
You may write a function that takes a const string& and overload it with another that takes const char*.
class MyClass { public: void Analyze(const string &text); void Analyze(const char *text, size_t textlen); };
Overloading can make code more intuitive by allowing an identically-named function to take different arguments. It may be necessary for templatized code, and it can be convenient for Visitors.
If a function is overloaded by the argument types alone, a reader may have to understand C++'s complex matching rules in order to tell what's going on. Also many people are confused by the semantics of inheritance if a derived class overrides only some of the variants of a function.
If you want to overload a function, consider qualifying the name with some information about the arguments, e.g., AppendString(), AppendInt() rather than just Append().
We do now allow default function parameters.
Often you have a function that uses lots of default values, but occasionally you want to override the defaults. Default parameters allow an easy way to do this without having to define many functions for the rare exceptions.
People often figure out how to use an API by looking at existing code that uses it. Default parameters are more difficult to maintain because copy-and-paste from previous code may not reveal all the parameters. Copy-and-pasting of code segments can cause major problems when the default arguments are not appropriate for the new code.
We prefer all arguments to be explicitly specified, to force programmers to consider the API and the values they are passing for each argument rather than silently accepting defaults they may not be aware of.
Well thought out default arguments though are allowed, but liberal use is frowned upon.
When using default arguments DO NOT default boolians, instead use ENUMs to clearly identify the argument from another.
We do not allow variable-length arrays or alloca().
Variable-length arrays have natural-looking syntax. Both variable-length arrays and alloca() are very efficient.
Variable-length arrays and alloca are not part of Standard C++. More importantly, they allocate a data-dependent amount of stack space that can trigger difficult-to-find memory overwriting bugs: "It ran fine on my machine, but dies mysteriously in production".
Use a safe allocator instead, such as std::unique_ptr/scoped_array.
We allow use of friend classes and functions, within reason.
Friends should usually be defined in the same file so that the reader does not have to look in another file to find uses of the private members of a class. A common use of friend is to have a FooBuilder class be a friend of Foo so that it can construct the inner state of Foo correctly, without exposing this state to the world. In some cases it may be useful to make a unittest class a friend of the class it tests.
Friends extend, but do not break, the encapsulation boundary of a class. In some cases this is better than making a member public when you want to give only one other class access to it. However, most classes should interact with other classes solely through their public members.
We extensivly use C++ exceptions, this is our primary error-handling mechanism.
-
Exceptions allow higher levels of an application to decide how to handle "can't happen" failures in deeply nested functions, without the obscuring and error-prone bookkeeping of error codes.
-
Exceptions are used by most other modern languages. Using them in C++ would make it more consistent with Python, Java, and the C++ that others are familiar with.
-
Some third-party C++ libraries use exceptions, and turning them off internally makes it harder to integrate with those libraries.
-
Exceptions are the only way for a constructor to fail. We can simulate this with a factory function or an
Init()method, but these require heap allocation or a new "invalid" state, respectively. -
Exceptions are really handy in testing frameworks.
- When you add a
throwstatement to an existing function, you must examine all of its transitive callers. Either they must make at least the basic exception safety guarantee, or they must never catch the exception and be happy with the program terminating as a result. For instance, iff()callsg()callsh(), andhthrows an exception thatfcatches,ghas to be careful or it may not clean up properly.
Use MaidSafe-specific exceptions to signal errors.
We do not use Run Time Type Information (RTTI).
RTTI allows a programmer to query the C++ class of an object at run time.
It is useful in some unittests. For example, it is useful in tests of factory classes where the test has to verify that a newly created object has the expected dynamic type. In rare circumstances, it is useful even outside of tests.
A query of type during run-time typically means a design problem. If you need to know the type of an object at runtime, that is often an indication that you should reconsider the design of your class.
Do not use RTTI, except in unittests. If you find yourself in need of writing code that behaves differently based on the class of an object, consider one of the alternatives to querying the type.
Virtual methods are the preferred way of executing different code paths depending on a specific subclass type. This puts the work within the object itself.
If the work belongs outside the object and instead in some processing code, consider a double-dispatch solution, such as the Visitor design pattern. This allows a facility outside the object itself to determine the type of class using the built-in type system.
If you think you truly cannot use those ideas, you may use RTTI. But think twice about it. :-) Then think twice again. Do not hand-implement an RTTI-like workaround. The arguments against RTTI apply just as much to workarounds like class hierarchies with type tags.
Use C++ casts like static_cast<>(). Do not use other cast formats like int y = (int)x; or int y = int(x);.
C++ introduced a different cast system from C that distinguishes the types of cast operations.
The problem with C casts is the ambiguity of the operation; sometimes you are doing a conversion (e.g., (int)3.5) and sometimes you are doing a cast (e.g., (int)"hello"); C++ casts avoid this. Additionally C++ casts are more visible when searching for them.
The syntax is nasty.
Do not use C-style casts. Instead, use these C++-style casts.
-
Use
static_castas the equivalent of a C-style cast that does value conversion, or when you need to explicitly up-cast a pointer from a class to its superclass. -
Use
const_castto remove theconstqualifier (see Use of const ). -
Use
reinterpret_castto do unsafe conversions of pointer types to and from integer and other pointer types. Use this only if you know what you are doing and you understand the aliasing issues. -
Do not use
dynamic_castexcept in test code. If you need to know type information at runtime in this way outside of a unittest, you probably have a Run-Time Type Information (RTTI) design flaw.
Use prefix form (++i) of the increment and decrement operators with iterators and other template objects.
When a variable is incremented (++i or i++) or decremented (--i or i--) and the value of the expression is not used, one must decide whether to preincrement (decrement) or postincrement (decrement).
When the return value is ignored, the "pre" form (++i) is never less efficient than the "post" form (i++), and is often more efficient. This is because post-increment (or decrement) requires a copy of i to be made, which is the value of the expression. If i is an iterator or other non-scalar type, copying i could be expensive. Since the two types of increment behave the same when the value is ignored, why not just always pre-increment?
The tradition developed, in C, of using post-increment when the expression value is not used, especially in for loops. Some find post-increment easier to read, since the "subject" (i) precedes the "verb" (++), just like in English.
For simple scalar (non-object) values there is no reason to prefer one form and we allow either. For iterators and other template types, use pre-increment.
We strongly recommend that you use const whenever it makes sense to do so.
Declared variables and parameters can be preceded by the keyword const to indicate the variables are not changed (e.g., const int foo). Class functions can have the const qualifier to indicate the function does not change the state of the class member variables (e.g., class Foo { int Bar(char c) const; };).
Easier for people to understand how variables are being used. Allows the compiler to do better type checking, and, conceivably, generate better code. Helps people convince themselves of program correctness because they know the functions they call are limited in how they can modify your variables. Helps people know what functions are safe to use without locks in multi-threaded programs.
const is viral: if you pass a const variable to a function, that function must have const in its prototype (or the variable will need a const_cast). This can be a particular problem when calling library functions.
const variables, data members, methods and arguments add a level of compile-time type checking; it is better to detect errors as soon as possible. Therefore we strongly recommend that you use const whenever it makes sense to do so:
-
If a function does not modify an argument passed by reference or by pointer, that argument should be
const. -
Declare methods to be
constwhenever possible. Accessors should almost always beconst. Other methods should be const if they do not modify any data members, do not call any non-constmethods, and do not return a non-constpointer or non-constreference to a data member. -
Consider making data members
constwhenever they do not need to be modified after construction.
However, do not go crazy with const. Something like const int * const * const x; is likely overkill, even if it accurately describes how const x is. Focus on what's really useful to know: in this case, const int** x is probably sufficient.
Some people favor the form int const *foo to const int* foo. They argue that this is more readable because it's more consistent: it keeps the rule that const always follows the object it's describing. However, this consistency argument doesn't apply in this case, because the "don't go crazy" dictum eliminates most of the uses you'd have to be consistent with.
Putting the const first is arguably more readable, since it follows English in putting the "adjective" (const) before the "noun" (int).
That said, while we encourage putting const first, we do not require it. But be consistent with the code around you!
Of the built-in C++ integer types, the only one used
is int. If a program needs a variable of a different size, use a precise-width integer type from <stdint.h>, such as int16_t.
C++ does not specify the sizes of its integer types. Typically people assume that short is 16 bits, int is 32 bits, long is 32 bits and long long is 64 bits.
Uniformity of declaration.
The sizes of integral types in C++ can vary based on compiler and architecture.
<stdint.h> defines types like int16_t, uint32_t, int64_t, etc. You should always use those in preference to short, unsigned long long and the like, when you need a guarantee on the size of an integer. Of the C integer types, only int should be used. When appropriate, you are welcome to use standard types like size_t and ptrdiff_t.
We use int very often, for integers we know are not going to be too big, e.g., loop counters. Use plain old int for such things. You should assume that an int is at least 32 bits, but don't assume that it has more than 32 bits. If you need a 64-bit integer type, use int64_t or uint64_t.
For integers we know can be "big", use int64_t.
You should not use the unsigned integer types such as uint32_t, unless the quantity you are representing is really a bit pattern rather than a number, or unless you need defined twos-complement overflow. In particular, do not use unsigned types to say a number will never be negative. Instead, use assertions for this.
Some people, including some textbook authors, recommend using unsigned types to represent numbers that are never negative. This is intended as a form of self-documentation. However, in C, the advantages of such documentation are outweighed by the real bugs it can introduce. Consider:
for (unsigned int i = foo.Length()-1; i >= 0; --i) ...This code will never terminate! Sometimes gcc will notice this bug and warn you, but often it will not. Equally bad bugs can occur when comparing signed and unsigned variables. Basically, C's type-promotion scheme causes unsigned types to behave differently than one might expect. So, document that a variable is non-negative using assertions. Don't use an unsigned type.
Code should be 64-bit and 32-bit friendly. Bear in mind problems of printing, comparisons, and structure alignment.
Use 0 for integers, 0.0 for reals, NULL for pointers, and '\0' for chars.
Use 0 for integers and 0.0 for reals. This is not controversial.
For pointers (address values), do not use 0 or NULL. To represent null use nullptr.
Use '\0' for chars. This is the correct type and also makes code more readable.
Use sizeof(<var>varname</var>) instead of sizeof(<var>type</var>) whenever possible.
Use sizeof(<var>varname</var>) because it will update appropriately if the type of the variable changes. sizeof(<var>type</var>) may make sense in some cases, but should generally be avoided because it can fall out of sync if the variable's type changes.
Struct data;
memset(&data, 0, sizeof(data));Use only approved libraries from the Boost library collection.
Boost is a popular collection of peer-reviewed, free, open-source C++ libraries.
Boost code is generally very high-quality, is widely portable, and fills many important gaps in the C++ standard library, such as type traits, better binders, and better smart pointers. It also provides an implementation of the TR1 extension to the standard library.
Some Boost libraries are not required since c++11 and can cause confision. Boost is ratrher large to install.
In order to maintain a high level of readability for all contributors who might read and maintain code, we only allow an approved subset of Boost features. Currently, the following libraries are permitted:
-
ASIO Extremely usefull library for asnchronous operations.
-
Filesystem Well abstracted cross platform filesystem utilities
-
Function Function object wrappers for deferred calls or callbacks.
-
Multi Index Provides a class template named multi_index_container which enables the construction of containers maintaining one or more indices with different sorting and access semantics.
-
Random A complete system for random number generation.
-
Signals 2 Managed signals and slots callback implementation (thread-safe version 2).
-
Thread Portable C++ multi-threading.
-
Random A complete system for random number generation.
-
Static Assert Static assertions (compile time assertions).
-
Program Options Allows program developers to obtain program options, that is (name, value) pairs from the user, via conventional methods such as command line and config file.
We are actively considering adding other Boost features to the list, so this rule may be relaxed in the future.
Use only approved libraries and language extensions from C++11 (formerly known as C++0x).
C++11 is the latest ISO C++ standard. It contains significant changes both to the language and libraries.
C++11 has become the official standard, and eventually will be supported by most C++ compilers. It standardizes some common C++ extensions that we use already, allows shorthands for some operations, and has some performance and safety improvements.
The C++11 standard is substantially more complex than its predecessor (1,300 pages versus 800 pages), and is unfamilar to many developers. The long-term effects of some features on code readability and maintenance are unknown.
As with Boost, some C++11 extensions encourage coding practices that hamper readability—for example by removing checked redundancy (such as type names) that may be helpful to readers. In general the c++11 standard is extremely powerful with very useful features such as rvalue references and move scemantics. We track this closely.
Use only C++11 libraries and language features that have been approved for use.
- std::shared_ptr, std::unique_ptr
- lambda
- std::move
- Rvalue references
The most important consistency rules are those that govern naming. The style of a name immediately informs us what sort of thing the named entity is: a type, a variable, a function, a constant, a macro, etc., without requiring us to search for the declaration of that entity. The pattern-matching engine in our brains relies a great deal on these naming rules.
Naming rules are pretty arbitrary, but
we feel that consistency is more important than individual preferences in this area, so regardless of whether you find them sensible or not, the rules are the rules.
Function names, variable names, and filenames should be descriptive; eschew abbreviation. Types and variables should be nouns, while functions should be "command" verbs.
Give as descriptive a name as possible, within reason. Do not worry about saving horizontal space as it is far more important to make your code immediately understandable by a new reader. Examples of well-chosen names:
int num_errors; // Good.
int num_completed_connections; // Good.Poorly-chosen names use ambiguous abbreviations or arbitrary characters that do not convey meaning:
``` int n; // Bad - meaningless. int nerr; // Bad - ambiguous abbreviation. int n_comp_conns; // Bad - ambiguous abbreviation. ```Type and variable names should typically be nouns: e.g.,
FileOpener,
num_errors.
Function names should typically be imperative (that is they
should be commands): e.g., OpenFile(),
set_num_errors(). There is an exception for
accessors, which, described more completely in Function Names, should be named
the same as the variable they access.
Do not use abbreviations unless they are extremely well known outside your project. For example:
// Good
// These show proper names with no abbreviations.
int num_dns_connections; // Most people know what "DNS" stands for.
int price_count_reader; // OK, price count. Makes sense.Never abbreviate by leaving out letters:
int error_count; // Good.Filenames should be all lowercase and can include underscores
(_) or dashes (-). Follow the
convention that your
project uses. If there is no consistent local pattern to follow, prefer "_".
Examples of acceptable file names:
my_useful_class.cc<br></br>
my-useful-class.cc<br></br>
myusefulclass.cc<br></br>
myusefulclass_test.cc // _unittest and _regtest are deprecated.<br></br>
C++ files should end in .cc and header files
should end in .h.
Do not use filenames that already exist
in /usr/include, such as db.h.
In general, make your filenames very specific. For example,
use http_server_logs.h rather
than logs.h. A very common case is to have a
pair of files called, e.g., foo_bar.h
and foo_bar.cc, defining a class
called FooBar.
Inline functions must be in a .h file. If your
inline functions are very short, they should go directly into your
.h file. However, if your inline functions
include a lot of code, they may go into a third file that
ends in -inl.h. In a class with a lot of inline
code, your class could have three files:
url_table.h // The class declaration.
url_table.cc // The class definition.
url_table-inl.h // Inline functions that include lots of code.
See also the section The -inl.h Files
Type names start with a capital letter and have a capital
letter for each new word, with no underscores:
MyExcitingClass, MyExcitingEnum.
The names of all types — classes, structs, typedefs, and enums — have the same naming convention. Type names should start with a capital letter and have a capital letter for each new word. No underscores. For example:
// classes and structs
class UrlTable { ...
class UrlTableTester { ...
struct UrlTableProperties { ...
// typedefs
typedef hash_map<UrlTableProperties *, string> PropertiesMap;
// enums
enum UrlTableErrors { ...Variable names are all lowercase, with underscores between
words. Class member variables have trailing underscores. For
instance: my_exciting_local_variable,
my_exciting_member_variable_.
For example:
string table_name; // OK - uses underscore.
string tablename; // OK - all lowercase.Data members (also called instance variables or member variables) are lowercase with optional underscores like regular variable names, but always end with a trailing underscore.
string table_name_; // OK - underscore at end.
string tablename_; // OK.Data members in structs should be named like regular variables without the trailing underscores that data members in classes have.
struct UrlTableProperties {
string name;
int num_entries;
}See Structs vs. Classes for a discussion of when to use a struct versus a class.
There are no special requirements for global variables,
which should be rare in any case, but if you use one,
consider prefixing it with g_ or some other
marker to easily distinguish it from local variables.
Use a k followed by mixed case:
kDaysInAWeek.
All compile-time constants, whether they are declared locally,
globally, or as part of a class, follow a slightly different
naming convention from other variables. Use a k
followed by words with uppercase first letters:
const int kDaysInAWeek = 7;Regular functions have mixed case; accessors and mutators match
the name of the variable: MyExcitingFunction(),
MyExcitingMethod(),
my_exciting_member_variable(),
set_my_exciting_member_variable().
Functions should start with a capital letter and have a capital letter for each new word. No underscores.
If your function crashes upon an error, you should append OrDie to the function name. This only applies to functions which could be used by production code and to errors that are reasonably likely to occur during normal operation.
AddTableEntry()
DeleteUrl()
OpenFileOrDie()Accessors and mutators (get and set functions) should match
the name of the variable they are getting and setting. This
shows an excerpt of a class whose instance variable is
num_entries_.
class MyClass {
public:
...
int num_entries() const { return num_entries_; }
void set_num_entries(int num_entries) { num_entries_ = num_entries; }
private:
int num_entries_;
};You may also use lowercase letters for other very short inlined functions. For example if a function were so cheap you would not cache the value if you were calling it in a loop, then lowercase naming would be acceptable.
Namespace names are all lower-case, and based on project names and
possibly their directory structure:
awesome_project.
See Namespaces for a discussion of namespaces and how to name them.
Enumerators should be named either like
Constant Names or like
Macro Names: either kEnumName
or ENUM_NAME.
Preferably, the individual enumerators should be named like
Constant Names. However, it is also
acceptable to name them like Macro Names. The enumeration name,
UrlTableErrors (and
AlternateUrlTableErrors), is a type, and
therefore mixed case.
enum UrlTableErrors {
kOK = 0,
kErrorOutOfMemory,
kErrorMalformedInput,
};
enum AlternateUrlTableErrors {
OK = 0,
OUT_OF_MEMORY = 1,
MALFORMED_INPUT = 2,
};Until January 2009, the style was to name enum values like Macro Names. This caused problems with name collisions between enum values and macros. Hence, the change to prefer constant-style naming was put in place. New code should prefer constant-style naming if possible. However, there is no reason to change old code to use constant-style names, unless the old names are actually causing a compile-time problem.
You're not really going to Preprocessor Macros, are you? If you do, they're like this:
MY_MACRO_THAT_SCARES_SMALL_CHILDREN.
Please see the Preprocessor Macros; in general macros should not be used. However, if they are absolutely needed, then they should be named with all capitals and underscores.
#define ROUND(x) ...
#define PI_ROUNDED 3.0If you are naming something that is analogous to an existing C or C++ entity then you can follow the existing naming convention scheme.
- `bigopen()`
- function name, follows form of `open()`
- `uint`
- `typedef`
- `bigpos`
- `struct` or `class`, follows form of `pos`
- `sparse_hash_map`
- STL-like entity; follows STL naming conventions
- `LONGLONG_MAX`
- a constant, as in `INT_MAX`
While comments are very important, the best code is self-documenting. Giving sensible names to types and variables is much better than using obscure names that you must then explain through comments.
When writing your comments, write for your audience: the next
contributor who will need to understand your code. Comment ** only ** when you code soemthing that is dificult to understand without comments (and consider if your naming is up to standard).
Use either the // or /* */ syntax, as long
as you are consistent.
You can use either the // or the /* */
syntax; however, // is much more common.
Be consistent with how you comment and what style you use where.
Start each file with a copyright notice, followed by a description of the contents of the file.
Every file should contain the following items, in order:
- a copyright statement (for example,
Copyright 2012 MaidSafe) - a license boilerplate. Choose the appropriate boilerplate for the license used by the project (for example, Apache 2.0, BSD, LGPL, GPL)
- an author line to identify the original author of the file
If you make significant changes to a file that someone else originally wrote, add yourself to the author line. This can be very helpful when another
contributor has questions about the file and needs to know whom to contact about it.
Every file should have a comment at the top, below the copyright notice and author line, that describes the contents of the file.
Generally a .h file will describe the classes
that are declared in the file with an overview of what they
are for and how they are used.
Do not duplicate comments in both the .h and
the .cc. Favour header file comments only.
Every class definition should have an accompanying comment that describes what it is for and how it should be used.
// Iterates over the contents of a GargantuanTable. Sample usage:
// GargantuanTableIterator* iter = table->NewIterator();
// for (iter->Seek("foo"); !iter->done(); iter->Next()) {
// process(iter->key(), iter->value());
// }
// delete iter;
class GargantuanTableIterator {
...
};If you have already described a class in detail in the comments at the top of your file feel free to simply state "See comment at top of file for a complete description", but be sure to have some sort of comment.
Document the synchronization assumptions the class makes, if any. If an instance of the class can be accessed by multiple threads, take extra care to document the rules and invariants surrounding multithreaded use.
Declaration comments describe use of the function; comments at the definition of a function describe operation.
These comments (if required) should be descriptive ("Opens the file") rather than imperative ("Open the file"); the comment describes the function, it does not tell the function what to do. In general, these comments do not describe how the function performs its task. Instead, that should be left to comments in the function definition.
Types of things to mention in comments at the function declaration:
-
What the inputs and outputs are.
-
For class member functions: whether the object remembers reference arguments beyond the duration of the method call, and whether it will free them or not.
-
If the function allocates memory that the caller must free.
-
Whether any of the arguments can be
NULL. -
If there are any performance implications of how a function is used.
-
If the function is re-entrant. What are its synchronization assumptions?
Here is an example:
// Returns an iterator for this table. It is the client's
// responsibility to delete the iterator when it is done with it,
// and it must not use the iterator once the GargantuanTable object
// on which the iterator was created has been deleted.
//
// The iterator is initially positioned at the beginning of the table.
//
// This method is equivalent to:
// Iterator* iter = table->NewIterator();
// iter->Seek("");
// return iter;
// If you are going to immediately seek to another place in the
// returned iterator, it will be faster to use NewIterator()
// and avoid the extra seek.
Iterator* GetIterator() const;However, ** do not be unnecessarily verbose **or state the completely obvious. Notice below that it is not necessary to say "returns false otherwise" because this is implied.
// Returns true if the table cannot hold any more entries.
bool IsTableFull();When commenting constructors and destructors, remember that the person reading your code knows what constructors and destructors are for, so comments that just say something like "destroys this object" are not useful. Document what constructors do with their arguments (for example, if they take ownership of pointers), and what cleanup the destructor does. If this is trivial, just skip the comment. It is quite common for destructors not to have a header comment.
Each function definition should have a comment describing what the function does if there's anything tricky about how it does its job. For example, in the definition comment you might describe any coding tricks you use, give an overview of the steps you go through, or explain why you chose to implement the function in the way you did rather than using a viable alternative. For instance, you might mention why it must acquire a lock for the first half of the function but why it is not needed for the second half.
Note you should not just repeat the comments given
with the function declaration, in the .h file or
wherever. It's okay to recapitulate briefly what the function
does, but the focus of the comments should be on how it does it.
In general the actual name of the variable should be descriptive enough to give a good idea of what the variable is used for. In rare cases, more comments are required.
Each class data member (also called an instance variable or
member variable) could have a comment describing what it is
used for. If the variable can take sentinel values with
special meanings, such as NULL or -1, document this.
For example:
private:
// Keeps track of the total number of entries in the table.
// Used to ensure we do not go over the limit. -1 means
// that we don't yet know how many entries the table has.
int num_total_entries_;As with data members, all global variables should have a comment describing what they are and what they are used for. For example:
// The total number of tests cases that we run through in this regression test.
const int kNumTestCases = 6;In your implementation you should have comments only in tricky, non-obvious, interesting, or important parts of your code.
Tricky or complicated code blocks should have comments before them. Example:
// Divide result by two, taking into account that x
// contains the carry from the add.
for (int i = 0; i < result->size(); i++) {
x = (x << 8) + (*result)[i];
(*result)[i] = x >> 1;
x &= 1;
}Also, lines that are non-obvious should get a comment at the end of the line. These end-of-line comments should be separated from the code by 2 spaces. Example:
// If we have enough memory, mmap the data portion too.
mmap_budget = max<int64>(0, mmap_budget - index_->length());
if (mmap_budget >= data_size_ && !MmapData(mmap_chunk_bytes, mlock))
return; // Error already logged.Note that there are both comments that describe what the code is doing, and comments that mention that an error has already been logged when the function returns.
If you have several comments on subsequent lines, it can often be more readable to line them up:
DoSomething(); // Comment here so the comments line up.
DoSomethingElseThatIsLonger(); // Comment here so there are two spaces between
// the code and the comment.
{ // One space before comment when opening a new scope is allowed,
// thus the comment lines up with the following comments and code.
DoSomethingElse(); // Two spaces before line comments normally.
}When you pass in NULL, boolean, or literal integer
values to functions, you should consider adding a comment about
what they are, or make your code self-documenting by using
constants. For example, compare:
versus:
bool success = CalculateSomething(interesting_value,
10, // Default base value.
false, // Not the first time we're calling this.
NULL); // No callback.Or alternatively, constants or self-describing variables:
const int kDefaultBaseValue = 10;
const bool kFirstTimeCalling = false;
Callback *null_callback = NULL;
bool success = CalculateSomething(interesting_value,
kDefaultBaseValue,
kFirstTimeCalling,
null_callback);Note that you should never describe the code itself. Assume that the person reading the code knows C++ better than you do, even though he or she does not know what you are trying to do:
``` // Now go through the b array and make sure that if i occurs, // the next element is i+1. ... // Geez. What a useless comment. ```Pay attention to punctuation, spelling, and grammar; it is easier to read well-written comments than badly written ones.
Comments should usually be written as complete sentences with proper capitalization and periods at the end. Shorter comments, such as comments at the end of a line of code, can sometimes be less formal, but you should be consistent with your style. Complete sentences are more readable, and they provide some assurance that the comment is complete and not an unfinished thought.
Although it can be frustrating to have a code reviewer point out that you are using a comma when you should be using a semicolon, it is very important that source code maintain a high level of clarity and readability. Proper punctuation, spelling, and grammar help with that goal.
Use TODO comments for code that is temporary, a
short-term solution, or good-enough but not perfect.
TODOs must include the string TODO in
all caps, followed by the
name, e-mail address, or other
identifier
of the person who can best provide context about the problem
referenced by the TODO. A colon is optional. The main
purpose is to have a consistent TODO format that can be
searched to find the person who can provide more details upon request.
A TODO is not a commitment that the person referenced
will fix the problem. Thus when you create a TODO, it is
almost always your
name that is given.
// TODO(kl@gmail.com): Use a "*" here for concatenation operator.
// TODO(Zeke) change this to use relations.If your TODO is of the form "At a future date do
something" make sure that you either include a very specific
date ("Fix by November 2005") or a very specific event
("Remove this code when all clients can handle XML responses.").
Mark deprecated interface points with DEPRECATED comments.
You can mark an interface as deprecated by writing a comment containing
the word DEPRECATED in all caps. The comment goes either
before the declaration of the interface or on the same line as the
declaration.
After the word DEPRECATED, write your name, e-mail address,
or other identifier in parentheses.
A deprecation comment must include simple, clear directions for people to fix their callsites. In C++, you can implement a deprecated function as an inline function that calls the new interface point.
Marking an interface point DEPRECATED will not magically
cause any callsites to change. If you want people to actually stop using
the deprecated facility, you will have to fix the callsites yourself or
recruit a crew to help you.
New code should not contain calls to deprecated interface points. Use the new interface point instead. If you cannot understand the directions, find the person who created the deprecation and ask them for help using the new interface point.
Coding style and formatting are pretty arbitrary, but a
project is much easier to follow if everyone uses the same style. Individuals may not agree with every aspect of the formatting rules, and some of the rules may take some getting used to, but it is important that all
project contributors follow the style rules so that
they can all read and understand everyone's code easily.
Each line of text in your code should be at most 100 characters long. Even you Fraser, yes you know who you are !
We recognize that this rule is controversial, but so much existing code already adheres to it, and we feel that consistency is important.
Those who favor
this rule argue that it is rude to force them to resize their windows and there is no need for anything longer. Some folks are used to having several code windows side-by-side, and thus don't have room to widen their windows in any case. People set up their work environment assuming a particular maximum window width, and 80 columns has been the traditional standard. Why change it?
Proponents of change argue that a wider line can make code more readable. The 80-column limit is an hidebound throwback to 1960s mainframes;
modern equipment has wide screens that can easily show longer lines.
100 characters is the maximum.
Exception: if a comment line contains an example command or a literal URL longer than 100 characters, that line may be longer than 100 characters for ease of cut and paste.
Exception: an #include statement with a long
path may exceed 100 columns. Try to avoid situations where this
becomes necessary.
Exception: you needn't be concerned about The #define Guard that exceed the maximum length.
Non-ASCII characters should be rare, and must use UTF-8 formatting.
You shouldn't hard-code user-facing text in source, even English, so use of non-ASCII characters should be rare. However, in certain cases it is appropriate to include such words in your code. For example, if your code parses data files from foreign sources, it may be appropriate to hard-code the non-ASCII string(s) used in those data files as delimiters. More commonly, unittest code (which does not
need to be localized) might contain non-ASCII strings. In such cases, you should use UTF-8, since that is
an encoding understood by most tools able
to handle more than just ASCII.
Hex encoding is also OK, and encouraged where it enhances
readability — for example, "\xEF\xBB\xBF" is the
Unicode zero-width no-break space character, which would be
invisible if included in the source as straight UTF-8.
Use only spaces, and indent 2 spaces at a time.
We use spaces for indentation. Do not use tabs in your code. You should set your editor to emit spaces when you hit the tab key.
Return type on the same line as function name, parameters on the same line if they fit.
Functions look like this:
ReturnType ClassName::FunctionName(Type par_name1, Type par_name2) {
DoSomething();
...
}If you have too much text to fit on one line:
ReturnType ClassName::ReallyLongFunctionName(Type par_name1, Type par_name2,
Type par_name3) {
DoSomething();
...
}or if you cannot fit even the first parameter:
ReturnType LongClassName::ReallyReallyReallyLongFunctionName(
Type par_name1, // 4 space indent
Type par_name2,
Type par_name3) {
DoSomething(); // 2 space indent
...
}Some points to note:
-
The return type is always on the same line as the function name.
-
The open parenthesis is always on the same line as the function name.
-
There is never a space between the function name and the open parenthesis.
-
There is never a space between the parentheses and the parameters.
-
The open curly brace is always at the end of the same line as the last parameter.
-
The close curly brace is either on the last line by itself or (if other style rules permit) on the same line as the open curly brace.
-
There should be a space between the close parenthesis and the open curly brace.
-
All parameters should be named, with identical names in the declaration and implementation.
-
All parameters should be aligned if possible.
-
Default indentation is 2 spaces.
-
Wrapped parameters have a 4 space indent.
If your function is const, the const
keyword should be on the same line as the last parameter:
// Everything in this function signature fits on a single line
ReturnType FunctionName(Type par) const {
...
}
// This function signature requires multiple lines, but
// the const keyword is on the line with the last parameter.
ReturnType ReallyLongFunctionName(Type par1,
Type par2) const {
...
}If some parameters are unused, comment out the variable name in the function definition:
// Always have named parameters in interfaces.
class Shape {
public:
virtual void Rotate(double radians) = 0;
}
// Always have named parameters in the declaration.
class Circle : public Shape {
public:
virtual void Rotate(double radians);
}
// Comment out unused named parameters in definitions.
void Circle::Rotate(double /*radians*/) {}On one line if it fits; otherwise, wrap arguments at the parenthesis.
Function calls have the following format:
bool retval = DoSomething(argument1, argument2, argument3);
If the arguments do not all fit on one line, they should be broken up onto multiple lines, with each subsequent line aligned with the first argument. Do not add spaces after the open paren or before the close paren:
bool retval = DoSomething(averyveryveryverylongargument1,
argument2, argument3);If the function has many arguments, consider having one per line if this makes the code more readable:
bool retval = DoSomething(argument1,
argument2,
argument3,
argument4);If the function signature is so long that it cannot fit within the maximum Line Length, you may place all arguments on subsequent lines:
if (...) {
...
...
if (...) {
DoSomethingThatRequiresALongFunctionName(
very_long_argument1, // 4 space indent
argument2,
argument3,
argument4);
}Prefer no spaces inside parentheses. The else
keyword belongs on a new line.
There are two acceptable formats for a basic conditional statement. One includes spaces between the parentheses and the condition, and one does not.
The most common form is without spaces. Either is fine, but be consistent. If you are modifying a file, use the format that is already present. If you are writing new code, use the format that the other files in that directory or project use. If in doubt and you have no personal preference, do not add the spaces.
if (condition) { // no spaces inside parentheses
... // 2 space indent.
} else if (...) { // The else goes on the same line as the closing brace.
...
} else {
...
}If you prefer you may add spaces inside the parentheses:
if ( condition ) { // spaces inside parentheses - rare
... // 2 space indent.
} else { // The else goes on the same line as the closing brace.
...
}Note that in all cases you must have a space between the
if and the open parenthesis. You must also have
a space between the close parenthesis and the curly brace, if
you're using one.
Short conditional statements may be written on one line if
this enhances readability. You may use this only when the
line is brief and the statement does not use the
else clause.
if (x == kFoo) return new Foo();
if (x == kBar) return new Bar();This is not allowed when the if statement has an
else:
In general, curly braces are not required for single-line statements, but they are allowed if you like them; conditional or loop statements with complex conditions or statements may be more readable with curly braces. Some
projects
require that an if must always always have an
accompanying brace.
if (condition)
DoSomething(); // 2 space indent.
if (condition) {
DoSomething(); // 2 space indent.
}However, if one part of an if-else
statement uses curly braces, the other part must too:
// Not allowed - curly on ELSE but not IF if (condition) foo; else { bar; }
</font>
```C++
// Curly braces around both IF and ELSE required because
// one of the clauses used braces.
if (condition) {
foo;
} else {
bar;
}
Switch statements may use braces for blocks. Empty loop bodies should use
{} or continue.
case blocks in switch statements can have
curly braces or not, depending on your preference. If you do
include curly braces they should be placed as shown below.
If not conditional on an enumerated value, switch statements
should always have a default case (in the case of
an enumerated value, the compiler will warn you if any values
are not handled). If the default case should never execute,
simply
assert:
switch (var) {
case 0: { // 2 space indent
... // 4 space indent
break;
}
case 1: {
...
break;
}
default: {
assert(false);
}
}Empty loop bodies should use {} or
continue, but not a single semicolon.
while (condition) {
// Repeat test until it returns false.
}
for (int i = 0; i < kSomeNumber; ++i) {} // Good - empty body.
while (condition) continue; // Good - continue indicates no logic.No spaces around period or arrow. Pointer operators do not have trailing spaces.
The following are examples of correctly-formatted pointer and reference expressions:
x = *p;
p = &x;
x = r.y;
x = r->y;Note that:
-
There are no spaces around the period or arrow when accessing a member.
-
Pointer operators have no space after the
*or&.
When declaring a pointer variable or argument, you may place the asterisk adjacent to either the type or to the variable name:
// These are fine, space preceding.
char *c;
const string &str;
// These are fine, space following.
char* c; // but remember to do "char* c, *d, *e, ...;"!
const string& str;You should do this consistently within a single file, so, when modifying an existing file, use the style in that file.
When you have a boolean expression that is longer than the Line Length, be consistent in how you break up the lines.
In this example, the logical AND operator is always at the end of the lines:
if (this_one_thing > this_other_thing &&
a_third_thing == a_fourth_thing &&
yet_another && last_one) {
...
}Note that when the code wraps in this example, both of
the && logical AND operators are at the
end of the line. This is more common in Google code, though
wrapping all operators at the beginning of the line is also
allowed. Feel free to insert extra parentheses judiciously
because they can be very helpful in increasing readability
when used appropriately. Also note that you should always use the
punctuation operators, such as && and
~, rather than the word operators, such as and
and compl.
Do not needlessly surround the return expression with
parentheses.
Use parentheses in return expr; only where you would use
them in x = expr;.
return result; // No parentheses in the simple case.
return (some_long_condition && // Parentheses ok to make a complex
another_condition); // expression more readable.Your choice of = or ().
You may choose between = and (); the
following are all correct:
int x = 3;
int x(3);
string name("Some Name");
string name = "Some Name";The hash mark that starts a preprocessor directive should always be at the beginning of the line.
Even when preprocessor directives are within the body of indented code, the directives should start at the beginning of the line.
// Good - directives at beginning of line
if (lopsided_score) {
#if DISASTER_PENDING // Correct -- Starts at beginning of line
DropEverything();
# if NOTIFY // OK but not required -- Spaces after #
NotifyClient();
# endif
#endif
BackToNormal();
}Sections in public, protected and
private order, each indented one space.
The basic format for a class declaration (lacking the comments, see Class Comments for a discussion of what comments are needed) is:
class MyClass : public OtherClass {
public: // Note the 1 space indent!
MyClass(); // Regular 2 space indent.
explicit MyClass(int var);
~MyClass() {}
void SomeFunction();
void SomeFunctionThatDoesNothing() {
}
void set_some_var(int var) { some_var_ = var; }
int some_var() const { return some_var_; }
private:
bool SomeInternalFunction();
int some_var_;
int some_other_var_;
DISALLOW_COPY_AND_ASSIGN(MyClass);
};Things to note:
-
Any base class name should be on the same line as the subclass name, subject to the 100-column limit.
-
The
public:,protected:, andprivate:keywords should be indented one space. -
Except for the first instance, these keywords should be preceded by a blank line. This rule is optional in small classes.
-
Do not leave a blank line after these keywords.
-
The
publicsection should be first, followed by theprotectedand finally theprivatesection. -
See Declaration Order for rules on ordering declarations within each of these sections.
Constructor initializer lists can be all on one line or with subsequent lines indented four spaces.
There are two acceptable formats for initializer lists:
// When it all fits on one line:
MyClass::MyClass(int var) : some_var_(var), some_other_var_(var + 1) {}or
// When it requires multiple lines, indent 4 spaces, putting the colon on
// the first initializer line:
MyClass::MyClass(int var)
: some_var_(var), // 4 space indent
some_other_var_(var + 1) { // lined up
...
DoSomething();
...
}The contents of namespaces are not indented.
Namespaces do not add an extra level of indentation. For example, use:
namespace {
void foo() { // Correct. No extra indentation within namespace.
...
}
} // namespaceDo not indent within a namespace:
``` namespace {// Wrong. Indented when it should not be. void foo() { ... }
} // namespace
</font>
When declaring nested namespaces, put each namespace on its own line.
```C++
namespace foo {
namespace bar {
Use of horizontal whitespace depends on location. Never put trailing whitespace at the end of a line.
void f(bool b) { // Open braces should always have a space before them.
...
int i = 0; // Semicolons usually have no space before them.
int x[] = { 0 }; // Spaces inside braces for array initialization are
int x[] = {0}; // optional. If you use them, put them on both sides!
// Spaces around the colon in inheritance and initializer lists.
class Foo : public Bar {
public:
// For inline function implementations, put spaces between the braces
// and the implementation itself.
Foo(int b) : Bar(), baz_(b) {} // No spaces inside empty braces.
void Reset() { baz_ = 0; } // Spaces separating braces from implementation.
...Adding trailing whitespace can cause extra work for others editing the same file, when they merge, as can removing existing trailing whitespace. So: Don't introduce trailing whitespace. Remove it if you're already changing that line, or do it in a separate clean-up
operation (preferably when no-one else is working on the file).
if (b) { // Space after the keyword in conditions and loops.
} else { // Spaces around else.
}
while (test) {} // There is usually no space inside parentheses.
switch (i) {
for (int i = 0; i < 5; ++i) {
switch ( i ) { // Loops and conditions may have spaces inside
if ( test ) { // parentheses, but this is rare. Be consistent.
for ( int i = 0; i < 5; ++i ) {
for ( ; i < 5 ; ++i) { // For loops always have a space after the
... // semicolon, and may have a space before the
// semicolon.
switch (i) {
case 1: // No space before colon in a switch case.
...
case 2: break; // Use a space after a colon if there's code after it.x = 0; // Assignment operators always have spaces around
// them.
x = -5; // No spaces separating unary operators and their
++x; // arguments.
if (x && !y)
...
v = w * x + y / z; // Binary operators usually have spaces around them,
v = w*x + y/z; // but it's okay to remove spaces around factors.
v = w * (x + z); // Parentheses should have no spaces inside them.vector<string> x; // No spaces inside the angle
y = static_cast<char*>(x); // brackets (< and >), before
// <, or between >( in a cast.
vector<char *> x; // Spaces between type and pointer are
// okay, but be consistent.
set<list<string> > x; // C++ requires a space in > >.
set< list<string> > x; // You may optionally use
// symmetric spacing in < <.Minimize use of vertical whitespace.
This is more a principle than a rule: don't use blank lines when you don't have to. In particular, don't put more than one or two blank lines between functions, resist starting functions with a blank line, don't end functions with a blank line, and be discriminating with your use of blank lines inside functions.
The basic principle is: The more code that fits on one screen, the easier it is to follow and understand the control flow of the program. Of course, readability can suffer from code being too dense as well as too spread out, so use your judgement. But in general, minimize use of vertical whitespace.
Some rules of thumb to help when blank lines may be useful:
-
Blank lines at the beginning or end of a function very rarely help readability.
-
Blank lines inside a chain of if-else blocks may well help readability.
The coding conventions described above are mandatory. However, like all good rules, these sometimes have exceptions, which we discuss here.
You may diverge from the rules when dealing with code that does not conform to this style guide.
If you find yourself modifying code that was written to specifications other than those presented by this guide, you may have to diverge from these rules in order to stay consistent with the local conventions in that code. If you are in doubt about how to do this, ask the original author or the person currently responsible for the code. Remember that consistency includes local consistency, too.
Windows programmers have developed their own set of coding conventions, mainly derived from the conventions in Windows headers and other Microsoft code. We want to make it easy for anyone to understand your code, so we have a single set of guidelines for everyone writing C++ on any platform.
It is worth reiterating a few of the guidelines that you might forget if you are used to the prevalent Windows style:
-
Do not use Hungarian notation (for example, naming an integer
iNum). Use the Google naming conventions, including the.ccextension for source files. -
Windows defines many of its own synonyms for primitive types, such as
DWORD,HANDLE, etc. It is perfectly acceptable, and encouraged, that you use these types when calling Windows API functions. Even so, keep as close as you can to the underlying C++ types. For example, useconst TCHAR *instead ofLPCTSTR. -
When compiling with Microsoft Visual C++, set the compiler to warning level 4 or higher, and treat all warnings as errors.
-
Do not use
#pragma once; instead use the standard Google include guards. The path in the include guards should be relative to the top of your project tree. -
In fact, do not use any nonstandard extensions, like
#pragmaand__declspec, unless you absolutely must. Using__declspec(dllimport)and__declspec(dllexport)is allowed; however, you must use them through macros such asDLLIMPORTandDLLEXPORT, so that someone can easily disable the extensions if they share the code.
However, there are just a few rules that we occasionally need to break on Windows:
-
Normally we forbid the use of Multiple Inheritance; however, it is required when using COM and some ATL/WTL classes. You may use multiple implementation inheritance to implement COM or ATL/WTL classes and interfaces.
-
The usual way of working with precompiled headers is to include a header file at the top of each source file, typically with a name like
StdAfx.horprecompile.h. To make your code easier to share with other projects, avoid including this file explicitly (except inprecompile.cc), and use the/FIcompiler option to include the file automatically. -
Resource headers, which are usually named
resource.hand contain only macros, do not need to conform to these style guidelines.
Use common sense and BE CONSISTENT.
If you are editing code, take a few minutes to look at the
code around you and determine its style. If they use spaces
around their if clauses, you should, too. If
their comments have little boxes of stars around them, make
your comments have little boxes of stars around them too.
The point of having style guidelines is to have a common vocabulary of coding so people can concentrate on what you are saying, rather than on how you are saying it. We present global style rules here so people know the vocabulary. But local style is also important. If code you add to a file looks drastically different from the existing code around it, the discontinuity throws readers out of their rhythm when they go to read it. Try to avoid this.
OK, enough writing about writing code; the code itself is much more interesting. Have fun!
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