C++ When is it OK to extend the `std` namespace?

by

The only case where it is OK to add a definition into the std namespace is specialization of a template that already exists in the namespace and to explicitly instantiate a template. However, only if they depend on a user defined type.

[namespace.std] (standard draft):

  1. The behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within namespace std unless otherwise specified. A program may add a template specialization for any standard library template to namespace std only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.

  2. The behavior of a C++ program is undefined if it declares

    (2.1) an explicit specialization of any member function of a standard library class template, or

    (2.2) an explicit specialization of any member function template of a standard library class or class template, or

    (2.3) an explicit or partial specialization of any member class template of a standard library class or class template.

    A program may explicitly instantiate a template defined in the standard library only if the declaration depends on the name of a user-defined type and the instantiation meets the standard library requirements for the original template.

As an example of standard templates that are explicitly designed to be extended for user defined types: std::hash and std::iterator_traits.

More Related Contents:

  1. What does the namespace std add? (C++) [closed]
  2. Why is “using namespace std;” considered bad practice?
  3. Converting std::__cxx11::string to std::string
  4. Sorting zipped (locked) containers in C++ using boost or the STL
  5. printf with std::string?
  6. What are inline namespaces for?
  7. How do I combine hash values in C++0x?
  8. Flattening iterator
  9. What can and can’t I specialize in the std namespace?
  10. How to convert std::chrono::time_point to calendar datetime string with fractional seconds?
  11. When is it necessary to use the flag -stdlib=libstdc++?
  12. How to use std::async on a member function?
  13. std::bind overload resolution
  14. std::put_time implementation status in GCC?
  15. C++ Add months to chrono::system_clock::time_point
  16. How can I use Standard Library (STL) classes in my dll interface or ABI?
  17. Why would I ever use push_back instead of emplace_back?
  18. Is there a range class in C++11 for use with range based for loops?
  19. Passing rvalues through std::bind
  20. How do I erase elements from STL containers?
  21. cc1plus: error: unrecognized command line option “-std=c++11” with g++
  22. error C2065: ‘cout’ : undeclared identifier
  23. std::thread error (thread not member of std)
  24. std::remove_const with const references
  25. std::forward_list and std::forward_list::push_back
  26. stoi and std::to_string on mingw 4.7.1 [duplicate]
  27. Why does unique_ptr take two template parameters when shared_ptr only takes one?
  28. C++ std::tuple order of destruction
  29. I want to kill a std::thread using its thread object? [duplicate]
  30. map with incomplete value type

Leave a Comment