doc: mention smart pointers in Cpp style guide

Add rule for smart pointers, i.e., std::unique_ptr and std::shared_ptr,
to the Cpp style guide. Mostly copied from the Google style guide.

PR-URL: #17055
Ref: #16970
Ref: #16974
Ref: #17000
Ref: #17012
Ref: #17020
Ref: #17030
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

CPP_STYLE_GUIDE.md

@@ -16,6 +16,7 @@
 * [`nullptr` instead of `NULL` or `0`](#nullptr-instead-of-null-or-0)
 * [Do not include `*.h` if `*-inl.h` has already been included](#do-not-include-h-if--inlh-has-already-been-included)
 * [Avoid throwing JavaScript errors in nested C++ methods](#avoid-throwing-javascript-errors-in-nested-c-methods)
+* [Ownership and Smart Pointers](#ownership-and-smart-pointers)
 
 Unfortunately, the C++ linter (based on
 [Google’s `cpplint`](https://github.com/google/styleguide)), which can be run
@@ -168,3 +169,27 @@ A lot of code inside Node.js is written so that typechecking etc. is performed
 in JavaScript.
 
 Using C++ `throw` is not allowed.
+
+## Ownership and Smart Pointers
+
+"Smart" pointers are classes that act like pointers, e.g.
+by overloading the `*` and `->` operators. Some smart pointer types can be
+used to automate ownership bookkeeping, to ensure these responsibilities are
+met. `std::unique_ptr` is a smart pointer type introduced in C++11, which
+expresses exclusive ownership of a dynamically allocated object; the object
+is deleted when the `std::unique_ptr` goes out of scope. It cannot be
+copied, but can be moved to represent ownership transfer.
+`std::shared_ptr` is a smart pointer type that expresses shared ownership of a
+dynamically allocated object. `std::shared_ptr`s can be copied; ownership
+of the object is shared among all copies, and the object
+is deleted when the last `std::shared_ptr` is destroyed.
+
+Prefer to use `std::unique_ptr` to make ownership
+transfer explicit. For example:
+
+```cpp
+std::unique_ptr<Foo> FooFactory();
+void FooConsumer(std::unique_ptr<Foo> ptr);
+```
+
+Never use `std::auto_ptr`. Instead, use `std::unique_ptr`.