Extending the namespace std

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.
• None of the type traits defined in <type_traits> may be specialized for a user-defined type, except for std::common_type.
• 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::shared_ptr and std::weak_ptr must be CopyConstructible and CopyAssignable. In addition, specializations of std::shared_ptr must be LessThanComparable, and convertible to bool.
• Specializations of std::istreambuf_iterator must have a trivial copy constructor, a constexpr default constructor, and a trivial destructor.
• Specializations of std::numeric_limits must define all members declared static constexpr in the primary template, in such a way that they are usable as integral constant expressions.
• std::unary_function and std::binary_function may not be specialized.

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.

Other restrictions

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