Guru of the Week (GotW) Archive

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Guru of the Week (GotW) Archive



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Guru of the Week is a regular series of C++ programming problems created and written by Herb Sutter. Since 1997, it has been a regular feature of the Internet newsgroup comp.lang.c++.moderated, where you can find each issue's questions and answers (and a lot of interesting discussion). For quick links to the most current GotW issues, watch the News & Events page. For revised and (sometimes greatly) expanded solutions for GotW issues #1 to #30, along with additional material never published in GotW form, see the book Exceptional C++. For revised and (sometimes greatly) expanded solutions for GotW issues #31 to #62, along with additional material never published in GotW form, see the book More Exceptional C++. GotW Archive (Most Recent First) Issue # Title and Description #88 (January 1, 2008) A Candidate For the "Most Important const." When you bind a temporary object to a reference, what are the semantics and consequences... exactly? #87 (May 17, 2003) Two-Phase or Not Two-Phase, Part 1: The Story of Dependent Names in Templates. In the land of C++, there are two towns: The village of traditional nontemplate C++ code, and the hamlet of templates. The two are tantalizingly similar, and so many things are the same in both places that visitors to the templates hamlet might be forgiven for thinking they're right at home, that the template programming environment is the same as it was in their familiar nontemplate surroundings. That is a natural mistake… but some of the local laws are different, often in subtle ways, and some classic assumptions must be unlearned before one can learn to write template code correctly and portably. This articles explores these small but important differences, and how they will soon increasingly affect your code. #86 (December 30, 2002) Slight Typos? Graphic Language and Other Curiosities. Sometimes even small and hard-to-see typos can accidentally have a significant effect on code. To illustrate how hard typos can be to see, and how easy phantom typos are to see accidentally even when they're not there, consider these examples. #85 (August 7, 2002) Style Case Study #3: Construction Unions. No, this issue isn't about organizing carpenters and bricklayers. Rather, it's about deciding between what's cool and what's not, good motivations gone astray, and the consequences of subversive activities carried on under the covers. It's about trying to get around the C++ rule of using constructed objects as members of unions... #84 (February 24, 2002) Monoliths "Unstrung." "All for one, and one for all" may work for Musketeers, but it doesn't work nearly as well for class designers. Here's an example that is not altogether exemplary, and it illustrates just how badly you can go wrong when design turns into overdesign. The example is, unfortunately, taken from a standard library near you... #83 (February 13, 2002) Style Case Study #2: Generic Callbacks. Part of the allure of generic code is its usability and reusability in as many kinds of situations as reasonably possible. How can the simple facility presented in the cited article be stylistically improved, and how can it be made more useful than it is and really qualify as generic and widely-usable code? #82 (June 30, 2001) Debate #1: Exception Safety and Specifications -- Are They Worth It? Is it worth the effort to write exception-safe code? Are exception specifications worthwhile? It may surprise you that these are still disputed and debated points, and ones where even experts may sometimes disagree. #81 (May 10, 2001) Constant Optimization? Does const-correctness help the compiler to optimize code? Most programmers' reaction is that, yes, it probably does. Which brings us to the interesting thing... #80 (Mar 31, 2001) Order, Order! Programmers learning C++ often come up with interesting misconceptions of what can and can't be done in C++. In this example, contributed by Jan Christiaan van Winkel, a student makes a basic mistake -- but one that many compilers allow to pass with no warnings at all. #79 (Feb 18, 2001) Template Typedef. This GotW exists to answer a recurring question about C++ syntax: When, and how, could you make use of a template typedef? #78 (Jan 31, 2001) Operators, Operators Everywhere. How many operators can you put together, when you really put your mind to it? This issue takes a break from production coding to get some fun C++ exercise. #77 (Dec 25, 2000) #Definition. What can and can't macros do? Not all compilers agree. #76 (Dec 8, 2000) Uses and Abuses of Access Rights. Who really has access to your class's internals? This issue is about liars, cheats, pickpockets, and thieves, and how to recognize and avoid them. #75 (Oct 15, 2000) Istream Initialization? Most people know the famous quote: "What if they gave a war and no one came?" This time, we consider the question: "What if we initialized an object and nothing happened?" As Scarlett might say in such a situation: "This isn't right, I do declare!" #74 (Sep 30, 2000) Uses and Abuses of Vector. Almost everybody uses std::vector, and that's good. Unfortunately, many people misunderstand some of its semantics and end up unwittingly using it in surprising and dangerous ways. How many of the subtle problems illustrated in this issue might be lurking in your current program? #73 (Aug 27, 2000) Style Case Study #1: Index Tables. This GotW introduces a new theme that we'll see again from time to time in future Style Case Study issues: We examine a piece of published code, critique it to illustrate proper coding style, and develop an improved version. You may be amazed at just how much can be done even with code that has been written, vetted, and proofread by experts. #72 (Jul 31, 2000) Data Formats and Efficiency. How good are you at choosing highly compact and memory-efficient data formats? How good are you at writing bit-twiddling code? This GotW gives you ample opportunity to exercise both skills, as we consider efficient representations of chess games and a BitBuffer to hold them. #71 (Jun 30, 2000) Inheritance Traits? This issue reviews traits templates, and demonstrates some cool traits techniques. What can a template figure out about its type -- and then what can it do about it? The answers are nifty and illuminating, and not just for people who write C++ libraries. #70 (May 17, 2000) Encapsulation. What exactly is encapsulation as it applies to C++ programming? What does proper encapsulation and access control mean for member data -- should it ever be public or protected? This issue focuses on alternative answers to these questions, and shows how those answers can increase either the robustness or the fragility of your code. #69 (Apr 11, 2000) Enforcing Rules for Derived Classes. Too many times, just being at the top of the (inheritance) world doesn't mean that you can save programmers of derived classes from simple mistakes. But sometimes you can! This issue is about safe design of base classes, so that derived class writers have a more difficult time going wrong. #68 (Mar 26, 2000) Flavors of Genericity. How generic is a generic function, really? The answer can depend as much on its implementation as on its interface, and a perfectly generalized interface can be hobbled by simple -- and awkward-to-diagnose -- programming lapses. #67 (Feb 29, 2000) Double or Nothing. No, this issue isn't about gambling. It is, however, about a different kind of "float," so to speak, and lets you test your skills about basic floating-point operations in C and C++. #66 (Jan 29, 2000) Constructor Failures. What exactly happens when a constructor emits an exception? What if the exception comes from an attempt to construct a subobject or member object? This issue of GotW analyzes one aspect of C++ in detail, shows why it should work the way that it does, and demonstrates the implications for constructor exception specifications. #65 (Jan 3, 2000) Try and Catch Me. Is exception safety all about writing try and catch in the right places? If not, then what? And what kinds of things should you consider when developing an exception safety policy for your software? #64 (Dec 8, 1999) Standard Library Member Functions. Reuse is good, but can you always reuse the standard library with itself? Here is an example that might surprise you, where one feature of the standard library can be used portably with any of your code as much as you like, but it cannot be used portably with the standard library itself. #63 (Nov 10, 1999) Amok Code. Sometimes life hands you some debugging situations that seem just plain deeply weird. Try this one on for size, and see if you can reason about possible causes for the problem.   Note: The book More Exceptional C++ is the authoritative reference for GotW issues #31 to #62, #66, #71, and #77 plus much more. The solutions archived here, shown below, are the original GotW solutions substantially as posted to the newsgroup; these solutions were later revised (and some greatly expanded) in the book, to bring them up to the date with the published ISO/ANSI C++ standard and to add additional new material. Issue # Title and Description #62 (Oct 29, 1999) Smart Pointer Members. Most C++ programmers know they have to take special care for classes with pointer members. But what about classes with auto_ptr members? And can we make life safer for ourselves and our users by devising a smart pointer class designed specifically for class membership? #61 (Sep 18, 1999) CHALLENGE EDITION: ACID Programming. What are the similarities and differences in reasoning about exception safety in particular, and program safety in general? Is it possible to generalize an approach that encompasses general program correctness analysis, both in C++ and in other languages? And what guidelines can we extrapolate that will help improve our day-to-day programming? For answers, see this first-ever "10/10 difficult" issue of GotW. #60 (Aug 7, 1999) Exception-Safe Class Design, Part 2: Inheritance. What does IS-IMPLEMENTED-IN-TERMS-OF mean? It may surprise you to learn that there are definite exception-safety consequences when choosing between inheritance and delegation. Can you spot them? #59 (Jul 19, 1999) Exception-Safe Class Design, Part 1: Copy Assignment. Is it possible to make any C++ class strongly exception-safe, for example for its copy assignment operator? If so, how? What are the issues and consequences? #58 (Jul 10, 1999) Nested Functions. C++ has nested classes, but not nested functions. When might nested functions be useful, and can they be simulated in C++? #57 (Jun 26, 1999) Recursive Declarations. Can you write a function that returns a pointer to itself? If so, why would you want to? #56 (May 23, 1999) Exception-Safe Function Calls. Regular readers of GotW know that exception safety is anything but trivial. This puzzle points out an exception safety problem that was discovered only weeks before posting, and shows how best to avoid it in your own code. #55 (May 16, 1999) Equivalent Code? Can subtle code differences really matter, especially in something as simple as postincrementing a function parameter? This issue explores an interesting interaction that becomes important in STL-style code. #54 (Apr 30, 1999) Using Vector and Deque. What is the difference between vector and deque? When should you use each one? And how can you properly shrink such containers when you no longer need their full capacity? These answers and more, as we consider news updates from the standards front. #53 (Mar 16, 1999) Migrating to Namespaces. Standard C++ includes support for namespaces and name visibility control with using declarations and directives. What's the best way to use these powerful facilities, and what's the most effective way to initially migrate your existing C++ code to a namespace-aware compiler and library? #52 (Feb 28, 1999) Extending the Standard Library - Part II. Following up from the introduction given in #51, we now examine "stateful" predicates. What are they? When are they useful? How compatible are they with standard containers and algorithms? #51 (Feb 7, 1999) Extending the Standard Library - Part I. This issue lets you test your standard algorithm skills. What does the standard library function remove() actually do, and how would you go about writing a generic function to remove only the third element in a container? #50 (Jan 9, 1999) Using Standard Containers. Oil and water just don't mix. Do pointers and standard containers mix any better? #49 (Dec 30, 1998) Template Specialization and Overloading. How do you specialize and overload templates? When you do, how do you determine which template gets called? Try your hand at analyzing these twelve examples. #48 (Nov 23, 1998) Switching Streams. What's the best way to switch between different stream sources and targets, including the standard console streams and files? #47 (Nov 9, 1998) Uncaught Exceptions. What is the standard function uncaught_exception(), and when should it be used? The answer given here isn't one that most people would expect. #46 (Oct 25, 1998) Typedefs. Why use typedef? Besides the many traditional reasons, we'll consider typedef techniques that make using the C++ standard library safer and easier. #45 (Oct 3, 1998) Reference Counting - Part III. In this final chapter of the miniseries, we consider the effects of thread safety on reference-counted strings. Is reference counting really an optimization? The answer will likely surprise you. #44 (Sep 8, 1998) Reference Counting - Part II. In the second of this three-part miniseries, we examine the effect of references and iterators into a reference-counted string. Can you spot the issues? #43 (Aug 30, 1998) Reference Counting - Part I. Reference counting is a common optimization (also called "lazy copy" and "copy on write"). Do you know how to implement it? #42 (Aug 23, 1998) Using auto_ptr. This issue illustrates a common pitfall with using auto_ptr. What is the problem, and how can you solve it? #41 (Aug 3, 1998) Using the Standard Library. How well do you know the standard library's algorithms? This puzzle requires a "master mind." #40 (Jul 27, 1998) Controlled Polymorphism. IS-A polymorphism is a very useful tool in OO modeling, but sometimes you may want to restrict which code can use certain classes polymorphically. This issue gives an example, and shows how to get the intended effect. #39 (Jul 20, 1998) Multiple Inheritance - Part III. Overriding inherited virtual functions is easy -- as long as you're not trying to override a virtual function that has the same signature in two base classes. This can happen even when the base classes don't come from different vendors! #38 (Jun 29, 1998) Multiple Inheritance - Part II. If you couldn't use multiple inheritance, how would you emulate it? Don't forget to emulate as natural a syntax as possible for the client code. #37 (Jun 13, 1998) Multiple Inheritance - Part I. Some languages, including the emerging SQL3 standard, continue to struggle with the decision of whether to support single or multiple inheritance. This GotW invites you to consider the issues. #36 (May 22, 1998) Initialization. What's the difference between direct initialization and copy initialization, and when are they used? #35 (Apr 24, 1998) Typename. "What's in a (type) name?" Here's an exercise that demonstrates why and how to use typename, using an idiom that's common in the standard library. #34 (Apr 3, 1998) Forward Declarations. Forward declarations are a great way to eliminate needless compile-time dependencies. But here's an example of a forward-declaration snare... how would you avoid it? #33 (Mar 22, 1998) Inline. Contrary to popular opinion, the keyword inline is not some sort of magic bullet. It is, however, a useful tool when employed properly. The question is, When should you use it? #32 (Feb 27, 1998) Preprocessor Macros. With all the type-safe features in C++, why would you ever use #define? #31 (Jan 31, 1998) (Im)pure Virtual Functions. Does it ever make sense to make a function pure virtual, but still provide a body?   Note: The book Exceptional C++ is the authoritative reference for GotW issues #1 to #30, plus much more. The solutions archived here, shown below, are the original GotW solutions substantially as posted to the newsgroup; these solutions were later revised (and some greatly expanded) in the book, to bring them up to the date with the published ISO/ANSI C++ standard and to add additional new material. Issue # Title and Description #30 (Jan 22, 1998) Name Lookup. When you call a function, which function do you call? The answer is determined by name lookup, but you're almost certain to find some of the details surprising . #29 (Jan 3, 1998) Strings. So you want a case-insensitive string class? Your mission, should you choose to accept it, is to write one. #28 (Dec 20, 1997) The Fast Pimpl Idiom. It's sometimes tempting to cut corners in the name of "reducing dependencies" or in the name of "efficiency," but it may not always be a good idea. Here's an excellent idiom to accomplish both objectives simultaneously and safely. #27 (Nov 29, 1997) Forwarding Functions. What's the best way to write a forwarding function? The basic answer is easy, but we'll also learn about a recent and subtle change to the language. #26 (Nov 22, 1997) Bool. Do we really need a builtin bool type? Why not just emulate it in the existing language? This GotW shows the answer. #25 (Nov 8, 1997) SPECIAL EDITION: auto_ptr. This GotW covers basics about how you can use the standard auto_ptr safely and effectively. (This GotW Special Edition was written in honor of the voting out of the Final Draft International Standard for Programming Language C++, which included a last-minute auto_ptr change.) #24 (Oct 31, 1997) Compilation Firewalls. Using the Pimpl Idiom can dramatically reduce code interdependencies and build times. But what should go into a pimpl_ object, and what is the safest way to use it? #23 (Oct 11, 1997) Object Lifetimes - Part II. Following up from #22, this issue considers a C++ idiom that's frequently recommended... but often dangerously wrong. #22 (Oct 4, 1997) Object Lifetimes - Part I. "To be, or not to be..." When does an object actually exist? This problem considers when an object is safe to use. #21 (Sept 21, 1997) Code Complexity - Part II. The challenge: Take the three-line function from GotW #20 and make it strongly exception-safe. This exercise illustrates some important lessons about exception safety. #20 (Sept 14, 1997) Code Complexity - Part I. This problem presents an interesting challenge: How many execution paths can there be in a simple three-line function? The answer will almost certainly surprise you. #19 (Aug 29, 1997) Automatic Conversions. Automatic conversions from one type to another can be extremely convenient. This GotW covers a typical example to illustrate why they're also extremely dangerous. #18 (Aug 18, 1997) Iterators. Every programmer who uses the standard library has to be aware of these common and not-so-common iterator mistakes. How many of them can you find? #17 (Aug 7, 1997) Casts. How well do you know C++'s casts? Using them well can greatly improve the reliability of your code. #16 (Jul 30, 1997) Maximally Reusable Generic Containers. How flexible can you make this simple container class? Hint: You'll learn more than a little about member templates along the way. #15 (Jul 21, 1997) Class Relationships - Part II. Design patterns are an important tool in writing reusable code. Do you recognize the patterns used in this GotW? If so, can you improve them? #14 (Jun 28, 1997) Class Relationships - Part I. How are your OO design skills? This GotW illustrates a common class design mistake that still catches many programmers. #13 (Jun 13, 1997) OOP. Is C++ an object-oriented language? It both is and is not, contrary to popular opinion. #12 (May 30, 1997) Control Flow. How well do you really know the order in which C++ code is executed? Test your knowledge against this problem. #11 (May 16, 1997) Object Identity. "Who am I, really?" This problem addresses how to decide whether two pointers really refer to the same object. #10 (Apr 30, 1997) Memory Management - Part II. Are you thinking about doing your own class-specific memory management, or even replacing C++'s global new and delete? First try this problem on for size. #9 (Apr 16, 1997) Memory Management - Part I. This GotW covers basics about C++'s main distinct memory stores. The following problem attacks some deeper memory-management questions in more detail. #8 (Apr 4, 1997) CHALLENGE EDITION: Exception Safety. Exceptions can be an elegant solution to some problems, but they introduce many hidden control flows and can be difficult to use correctly. Try your hand at implementing a very simple container (a stack that users can push and pop) and see the issues involved with making it exception-safe and exception-neutral. #7 (Mar 28, 1997) Compile-Time Dependencies. Most programmers #include many more headers than necessary. Do you? To find out, consider this issue's problem. #6 (Mar 21, 1997) Const-Correctness. Always use const as much as possible, but no more. Here are some obvious and not-so-obvious places where const should be used -- or shouldn't. #5 (Mar 14, 1997) Overriding Virtual Functions. Virtual functions are a pretty basic feature, right? If you can answer questions like this one, then you know them cold. #4 (Mar 7, 1997) Class Mechanics. How good are you at the details of writing classes? This GotW focuses not only on blatant errors, but even more so on professional style. #3 (Feb 28, 1997) Using the Standard Library (or, Temporaries Revisited). You're much better off using standard library algorithms than handcrafting your own. Here we reconsider the example in the last GotW to demonstrate how many of the problems would have been avoided by simply reusing what's already available in the standard library. #2 (Feb 21, 1997) Temporary Objects. Unnecessary temporaries are frequent culprits that can throw all your hard work - and your program's performance - right out the window. #1 (Feb 21, 1997) Variable Initialization. How many ways are there to initialize variables? Don't forget to watch out for bugs that look like variable initialization, but aren't.