Extending the namespace std

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

C++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for (C++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function declaration
inline specifier
Exception specifications (deprecated)
noexcept specifier (C++11)
Exceptions
throw-expression	try-catch block
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Compound types Union types
Specifiers
decltype (C++11) auto (C++11) alignas (C++11)	const/volatile constexpr (C++11)
Storage duration specifiers
Initialization
Default initialization Value initialization Zero initialization Copy initialization Direct initialization	Aggregate initialization List initialization (C++11) Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Implicit conversions - Explicit conversions
static_cast - dynamic_cast
const_cast - reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Initializer lists this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier (C++11) final specifier (C++11)	explicit (C++11) static
Special member functions
Default constructor Copy constructor Move constructor (C++11)	Copy assignment Move assignment (C++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs (C++11)
Miscellaneous
Inline assembly	History of C++

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};
    }
}

Adding template specializations

It is allowed to add template specializations for any standard library class (since C++20)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.

// Get the declaration of the primary std::hash template.
// We are not permitted to declare it ourselves.
// <typeindex> is guaranteed to provide such a declaration, 
// and is much cheaper to include than <functional>.

#include <typeindex> 

// Specialize std::hash so that MyType can be used as a key in 
// std::unordered_set and std::unordered_map
namespace std {
    template <> struct hash<MyType> {
      std::size_t operator()(const MyType& t) const { return t.hash(); }
    };
}

• 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.

• It is undefined behavior to declare a full or partial specialization of any standard library variable template.

• None of the templates defined in <type_traits> may be specialized for a program-defined type, except for std::common_type. This includes the type traits and the class template std::integral_constant.

• Specializing the template std::complex for any type other than float, double, and long double is unspecified.

• Specializations of std::hash for program-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.

(until C++17)

Explicit instantiation of templates

It is allowed to explicitly instantiate a class (since C++20)template defined in the standard library only if the declaration depends on the name of at least one program-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.