• 1Adding declarations to std
• 2Adding template specializations
  • 2.1Class templates
  • 2.2Function templates and member functions of templates
  • 2.3Variable templates
• 3Explicit instantiation of templates
• 4Other restrictions
  • 4.1Addressing restriction
  • 4.2Designated addressable functions
• 5Defect reports

• None of the templates defined in<type_traits> may be specialized for aprogram-defined type, except forstd::common_type andstd::basic_common_reference(since C++20). This includes thetype traits and the class templatestd::integral_constant.

• Specializations ofstd::hash for program-defined types must satisfyHash requirements.

• Specializations ofstd::atomic must have a deleted copy constructor, a deleted copy assignment operator, and a constexpr value constructor.

• Specializations ofstd::shared_ptr andstd::weak_ptr must beCopyConstructible andCopyAssignable. In addition, specializations ofstd::shared_ptr must beLessThanComparable, and convertible tobool.

• Specializations ofstd::istreambuf_iterator must have a trivial copy constructor, a constexpr default constructor, and a trivial destructor.

• std::unary_function andstd::binary_function may not be specialized.

It is allowed to add template specializations for any standard library function template to the namespacestd only if the declaration depends on at least oneprogram-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 standard library function template.

It is undefined behavior to declare a full or partial specialization of any standard library variable template, except where explicitly allowed.

Addressing restriction

The behavior of a C++ program is unspecified (possibly ill-formed) if it explicitly or implicitly attempts to form a pointer, reference (for free functions and static member functions) or pointer-to-member (for non-static member functions) to a standard library function or an instantiation of a standard library function template, unless it is designated anaddressable function (see below).

Following code was well-defined in C++17, but leads to unspecified behaviors and possibly fails to compile since C++20:

#include <cmath>#include <memory> int main(){// by unary operator&auto fptr0=&static_cast<float(&)(float,float)>(std::betaf); // by std::addressofauto fptr1=std::addressof(static_cast<float(&)(float,float)>(std::betaf)); // by function-to-pointer implicit conversionauto fptr2=static_cast<float(&)(float)>(std::riemann_zetaf); // forming a referenceauto& fref=static_cast<float(&)(float)>(std::riemann_zetaf);}

Designated addressable functions

• I/O manipulators:
  • fmtflags manipulators:
    • std::boolalpha
    • std::noboolalpha
    • std::showbase
    • std::noshowbase
    • std::showpoint
    • std::noshowpoint
    • std::showpos
    • std::noshowpos
    • std::skipws
    • std::noskipws
    • std::uppercase
    • std::nouppercase
    • std::unitbuf
    • std::nounitbuf
  • adjustfield manipulators:
    • std::internal
    • std::left
    • std::right
  • basefield manipulators:
    • std::dec
    • std::hex
    • std::oct
  • floatfield manipulators:
    • std::fixed
    • std::scientific
    • std::hexfloat
    • std::defaultfloat
  • basic_istream manipulators:
    • std::ws
  • basic_ostream manipulators:
    • std::endl
    • std::ends
    • std::flush
    • std::emit_on_flush
    • std::noemit_on_flush
    • std::flush_emit