Extending std

cppreference Extending the namespace std

NOTE: 原文的描述采用的枚举法；

Adding declarations to std

It is undefined behavior to add declarations or definitions to namespace std or to any namespace nested within std, with a few exceptions noted below

NOTE: 这说明我们不能直接向std中添加declaration。

Adding template specializations

NOTE: 关于template specializaiton，参见C++\Language-reference\Template\Specialization。

Class templates

It is allowed to add template specializations for any standard library class template to the namespace std only if the declaration depends on at least one program-defined type and the specialization satisfies all requirements for the original template, except where such specializations are prohibited.

Function templates and member functions of templates

	version
It is allowed to add template specializations for any standard library function template to the namespace std only if the declaration depends on at least one program-defined type and the specialization satisfies all requirements for the original template, except where such specializations are prohibited.	(until C++20)
It is undefined behavior to declare a full specialization of any standard library function template.	(since C++20)

Variable templates

NOTE: C++14引入的

补充

More C++ Idioms/Non-throwing_swap

Using non-throwing swap idiom for template classes (e.g., Matrix<T>) can be a subtle issue. As per the C++98 standard, only the full specialization of std::swap is allowed to be defined inside std namespace for the user-defined types. Partial specializations or function overloading is not allowed by the language. Trying to achieve the similar effect for template classes (e.g., Matrix<T>) results into overloading of std::swap in std namepspace, which is technically undefined behavior.