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Extending the namespace std

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[edit] 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

#include <utility>
namespace std {
    // a function definition added to namespace std: undefined behavior
    pair<int, int> operator+(pair<int, int> a, pair<int, int> b) {
        return {a.first+b.first, a.second+b.second};
    }
}

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

  • It is undefined behavior to declare a full specialization of any member function of a standard library class template
  • It is undefined behavior to declare a full specialization of any member function template of a standard library class or class template
  • It is undefined behavior to declare a full or partial specialization of any member class template of a standard library class or class template.
  • Specializing the template std::complex for any type other than float, double, and long double is unspecified.
  • Specializations of std::hash for user-defined types must satisfy Hash requirements.
  • Specializations of std::atomic must have a deleted copy constructor, a deleted copy assignment operator, and a constexpr value constructor.
  • Specializations of std::istreambuf_iterator must have a trivial copy constructor, a constexpr default constructor, and a trivial destructor.

It is allowed to 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.

[edit] Other restrictions

The namespace std may not be declared as an inline namespace.