Standard library header <functional>

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Standard library headers

Language support
<cstddef> <cstdlib> <version> (C++20) <limits> <climits>	<cfloat> <cstdint> (C++11) <stdfloat> (C++23) <new> <typeinfo>	<source_location> (C++20) <exception> <initializer_list> (C++11) <compare> (C++20)	<coroutine> (C++20) <csignal> <csetjmp> <cstdarg>
Concepts
<concepts> (C++20)
Diagnostics
<stdexcept> <stacktrace> (C++23)	<cassert>	<cerrno>	<system_error> (C++11)
Memory management
<memory>	<memory_resource> (C++17)	<scoped_allocator> (C++11)
Metaprogramming
<type_traits> (C++11)	<ratio> (C++11)
General utilities
<utility> <tuple> (C++11) <optional> (C++17) <variant> (C++17)	<any> (C++17) <expected> (C++23) <bitset>	<functional> <typeindex> (C++11) <execution> (C++17)	<charconv> (C++17) <format> (C++20) <bit> (C++20)
Strings
<string_view> (C++17) <string>	<cctype> <cwctype>	<cstring> <cwchar>	<cuchar> (C++11)
Containers
<array> (C++11) <deque> <forward_list> (C++11) <list>	<vector> <map> <set> <unordered_map> (C++11)	<unordered_set> (C++11) <queue> <stack> <flat_map> (C++23)	<flat_set> (C++23) <span> (C++20) <mdspan> (C++23)
Iterators
<iterator>
Ranges
<ranges> (C++20)	<generator> (C++23)
Algorithms
<algorithm>	<numeric>
Numerics
<cfenv> (C++11) <complex>	<random> (C++11) <valarray>	<cmath>	<numbers> (C++20)
Time
<chrono> (C++11)	<ctime>
Localization
<locale>	<clocale>	<codecvt> (C++11/17*)
Input/output
<iosfwd> <iostream> <ios> <streambuf>	<istream> <ostream> <iomanip> <print> (C++23)	<sstream> <spanstream> (C++23) <fstream> <syncstream> (C++20)	<filesystem> (C++17) <cstdio> <cinttypes> (C++11) <strstream> (C++98*)
Regular expressions
<regex> (C++11)
Concurrency support
<stop_token> (C++20) <thread> (C++11) <atomic> (C++11)	<stdatomic.h> (C++23) <mutex> (C++11) <shared_mutex> (C++14)	<condition_variable> (C++11)   <semaphore> (C++20) <latch> (C++20)	<barrier> (C++20) <future> (C++11)
C compatibility
<cstdbool> (C++11/17/20*)   <ccomplex> (C++11/17/20*)	<ctgmath> (C++11/17/20*)	<cstdalign> (C++11/17/20*)	<ciso646> (until C++20)

 

This header is part of the function objects library and provides the standard hash function.

Namespaces
placeholders (C++11)	Provides placeholders for the unbound arguments in a std::bind expression
Classes
Wrappers
function (C++11)	wraps callable object of any copy constructible type with specified function call signature (class template)
move_only_function (C++23)	wraps callable object of any type with specified function call signature (class template)
copyable_function (C++26)	refinement of std::move_only_function that wraps callable object of any copy constructible type (class template)
function_ref (C++26)	non-owning reference to any callable object with specified function call signature (class template)
mem_fn (C++11)	creates a function object out of a pointer to a member (function template)
reference_wrapper (C++11)	CopyConstructible and CopyAssignable reference wrapper (class template)
unwrap_referenceunwrap_ref_decay (C++20)(C++20)	get the reference type wrapped in std::reference_wrapper (class template)
Helper classes
bad_function_call (C++11)	the exception thrown when invoking an empty std::function (class)
is_bind_expression (C++11)	indicates that an object is std::bind expression or can be used as one (class template)
is_placeholder (C++11)	indicates that an object is a standard placeholder or can be used as one (class template)
Arithmetic operations
plus	function object implementing x + y (class template)
minus	function object implementing x - y (class template)
multiplies	function object implementing x * y (class template)
divides	function object implementing x / y (class template)
modulus	function object implementing x % y (class template)
negate	function object implementing -x (class template)
Comparisons
equal_to	function object implementing x == y (class template)
not_equal_to	function object implementing x != y (class template)
greater	function object implementing x > y (class template)
less	function object implementing x < y (class template)
greater_equal	function object implementing x >= y (class template)
less_equal	function object implementing x <= y (class template)
Concept-constrained comparisons
ranges::equal_to (C++20)	function object implementing x == y (class)
ranges::not_equal_to (C++20)	function object implementing x != y (class)
ranges::greater (C++20)	function object implementing x > y (class)
ranges::less (C++20)	function object implementing x < y (class)
ranges::greater_equal (C++20)	function object implementing x >= y (class)
ranges::less_equal (C++20)	function object implementing x <= y (class)
compare_three_way (C++20)	function object implementing x <=> y (class)
Logical operations
logical_and	function object implementing x && y (class template)
logical_or	function object implementing x || y (class template)
logical_not	function object implementing !x (class template)
Bitwise operations
bit_and	function object implementing x & y (class template)
bit_or	function object implementing x | y (class template)
bit_xor	function object implementing x ^ y (class template)
bit_not (C++14)	function object implementing ~x (class template)
Negators
not_fn (C++17)	creates a function object that returns the complement of the result of the function object it holds (function template)
Identities
identity (C++20)	function object that returns its argument unchanged (class)
Searchers
default_searcher (C++17)	standard C++ library search algorithm implementation (class template)
boyer_moore_searcher (C++17)	Boyer-Moore search algorithm implementation (class template)
boyer_moore_horspool_searcher (C++17)	Boyer-Moore-Horspool search algorithm implementation (class template)
Hashing
hash (C++11)	hash function object (class template)
std::hash<Arithmetic>std::hash<Enumeration>std::hash<std::nullptr_t>std::hash<T*> (C++11)	std::hash specializations for fundamental, enumeration, and pointer types (class template specialization)
Constants
Defined in namespace std::placeholders
_1, _2, _3, _4, ... (C++11)	placeholders for the unbound arguments in a std::bind expression (constant)
Functions
bind_frontbind_back (C++20)(C++23)	bind a variable number of arguments, in order, to a function object (function template)
bind (C++11)	binds one or more arguments to a function object (function template)
refcref (C++11)(C++11)	creates a std::reference_wrapper with a type deduced from its argument (function template)
invokeinvoke_r (C++17)(C++23)	invokes any Callable object with given arguments and possibility to specify return type (since C++23) (function template)

Deprecated in C++11 and removed in C++17

Base
unary_function (deprecated in C++11)(removed in C++17)	adaptor-compatible unary function base class (class template)
binary_function (deprecated in C++11)(removed in C++17)	adaptor-compatible binary function base class (class template)
Binders
binder1stbinder2nd (deprecated in C++11)(removed in C++17)	function object holding a binary function and one of its arguments (class template)
bind1stbind2nd (deprecated in C++11)(removed in C++17)	binds one argument to a binary function (function template)
Function adaptors
pointer_to_unary_function (deprecated in C++11)(removed in C++17)	adaptor-compatible wrapper for a pointer to unary function (class template)
pointer_to_binary_function (deprecated in C++11)(removed in C++17)	adaptor-compatible wrapper for a pointer to binary function (class template)
ptr_fun (deprecated in C++11)(removed in C++17)	creates an adaptor-compatible function object wrapper from a pointer to function (function template)
mem_fun_tmem_fun1_tconst_mem_fun_tconst_mem_fun1_t (deprecated in C++11)(removed in C++17)	wrapper for a pointer to nullary or unary member function, callable with a pointer to object (class template)
mem_fun (deprecated in C++11)(removed in C++17)	creates a wrapper from a pointer to member function, callable with a pointer to object (function template)
mem_fun_ref_tmem_fun1_ref_tconst_mem_fun_ref_tconst_mem_fun1_ref_t (deprecated in C++11)(removed in C++17)	wrapper for a pointer to nullary or unary member function, callable with a reference to object (class template)
mem_fun_ref (deprecated in C++11)(removed in C++17)	creates a wrapper from a pointer to member function, callable with a reference to object (function template)

Deprecated in C++17 and removed in C++20

Negators
unary_negate (deprecated in C++17)(removed in C++20)	wrapper function object returning the complement of the unary predicate it holds (class template)
binary_negate (deprecated in C++17)(removed in C++20)	wrapper function object returning the complement of the binary predicate it holds (class template)
not1 (deprecated in C++17)(removed in C++20)	constructs custom std::unary_negate object (function template)
not2 (deprecated in C++17)(removed in C++20)	constructs custom std::binary_negate object (function template)

Synopsis

namespace std {
  // invoke
  template<class F, class... Args>
    constexpr invoke_result_t<F, Args...> invoke(F&& f, Args&&... args)
      noexcept(is_nothrow_invocable_v<F, Args...>);
  template<class R, class F, class... Args>
    constexpr R invoke_r(F&& f, Args&&... args)
      noexcept(is_nothrow_invocable_r_v<R, F, Args...>);
 
  // reference_wrapper
  template<class T> class reference_wrapper;
 
  template<class T> constexpr reference_wrapper<T> ref(T&) noexcept;
  template<class T> constexpr reference_wrapper<const T> cref(const T&) noexcept;
  template<class T> void ref(const T&&) = delete;
  template<class T> void cref(const T&&) = delete;
 
  template<class T>
    constexpr reference_wrapper<T> ref(reference_wrapper<T>) noexcept;
  template<class T>
    constexpr reference_wrapper<const T> cref(reference_wrapper<T>) noexcept;
 
  template<class T> struct unwrap_reference;
  template<class T> using unwrap_reference_t = typename unwrap_reference<T>::type;
  template<class T> struct unwrap_ref_decay;
  template<class T> using unwrap_ref_decay_t = typename unwrap_ref_decay<T>::type;
 
  // arithmetic operations
  template<class T = void> struct plus;
  template<class T = void> struct minus;
  template<class T = void> struct multiplies;
  template<class T = void> struct divides;
  template<class T = void> struct modulus;
  template<class T = void> struct negate;
  template<> struct plus<void>;
  template<> struct minus<void>;
  template<> struct multiplies<void>;
  template<> struct divides<void>;
  template<> struct modulus<void>;
  template<> struct negate<void>;
 
  // comparisons
  template<class T = void> struct equal_to;
  template<class T = void> struct not_equal_to;
  template<class T = void> struct greater;
  template<class T = void> struct less;
  template<class T = void> struct greater_equal;
  template<class T = void> struct less_equal;
  template<> struct equal_to<void>;
  template<> struct not_equal_to<void>;
  template<> struct greater<void>;
  template<> struct less<void>;
  template<> struct greater_equal<void>;
  template<> struct less_equal<void>;
 
  // logical operations
  template<class T = void> struct logical_and;
  template<class T = void> struct logical_or;
  template<class T = void> struct logical_not;
  template<> struct logical_and<void>;
  template<> struct logical_or<void>;
  template<> struct logical_not<void>;
 
  // bitwise operations
  template<class T = void> struct bit_and;
  template<class T = void> struct bit_or;
  template<class T = void> struct bit_xor;
  template<class T = void> struct bit_not;
  template<> struct bit_and<void>;
  template<> struct bit_or<void>;
  template<> struct bit_xor<void>;
  template<> struct bit_not<void>;
 
  // identity
  struct identity;
 
  // function template not_fn
  template<class F> constexpr /* unspecified */ not_fn(F&& f);
 
  // function templates bind_front and bind_back
  template<class F, class... Args> constexpr /* unspecified */ bind_front(F&&, Args&&...);
  template<class F, class... Args> constexpr /* unspecified */ bind_back(F&&, Args&&...);
 
  // bind
  template<class T> struct is_bind_expression;
  template<class T> struct is_placeholder;
 
  template<class F, class... BoundArgs>
    constexpr /* unspecified */ bind(F&&, BoundArgs&&...);
  template<class R, class F, class... BoundArgs>
    constexpr /* unspecified */ bind(F&&, BoundArgs&&...);
 
  namespace placeholders {
    // M is the implementation-defined number of placeholders
    /* see description */ _1;
    /* see description */ _2;
               .
               .
               .
    /* see description */ _M;
  }
 
  // member function adaptors
  template<class R, class T>
    constexpr /* unspecified */ mem_fn(R T::*) noexcept;
 
  // polymorphic function wrappers
  class bad_function_call;
 
  template<class> class function; // not defined
  template<class R, class... ArgTypes> class function<R(ArgTypes...)>;
 
  template<class R, class... ArgTypes>
    void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept;
 
  template<class R, class... ArgTypes>
    bool operator==(const function<R(ArgTypes...)>&, nullptr_t) noexcept;
 
  template<class...> class move_only_function; // not defined
 
  template<class R, class... ArgTypes>
    class move_only_function<R(ArgTypes...) /*cv ref*/ noexcept(/*noex*/)>;
 
  // searchers
  template<class ForwardIter, class BinaryPredicate = equal_to<>>
    class default_searcher;
 
  template<class RandomAccessIter,
           class Hash = hash<typename iterator_traits<RandomAccessIter>::value_type>,
           class BinaryPredicate = equal_to<>>
    class boyer_moore_searcher;
 
  template<class RandomAccessIter,
           class Hash = hash<typename iterator_traits<RandomAccessIter>::value_type>,
           class BinaryPredicate = equal_to<>>
    class boyer_moore_horspool_searcher;
 
  // hash function primary template
  template<class T>
    struct hash;
 
  // function object binders
  template<class T>
    inline constexpr bool is_bind_expression_v = is_bind_expression<T>::value;
  template<class T>
    inline constexpr int is_placeholder_v = is_placeholder<T>::value;
 
  namespace ranges {
    // concept-constrained comparisons
    struct equal_to;
    struct not_equal_to;
    struct greater;
    struct less;
    struct greater_equal;
    struct less_equal;
  }
}

Class template std::reference_wrapper

namespace std {
  template<class T> class reference_wrapper {
  public:
    // types
    using type = T;
 
    // construct/copy/destroy
    template<class U>
      constexpr reference_wrapper(U&&) noexcept(see below);
    constexpr reference_wrapper(const reference_wrapper& x) noexcept;
 
    // assignment
    constexpr reference_wrapper& operator=(const reference_wrapper& x) noexcept;
 
    // access
    constexpr operator T& () const noexcept;
    constexpr T& get() const noexcept;
 
    // invocation
    template<class... ArgTypes>
      constexpr invoke_result_t<T&, ArgTypes...> operator()(ArgTypes&&...) const;
  };
  template<class T>
    reference_wrapper(T&) -> reference_wrapper<T>;
}

Class template std::unwrap_reference

namespace std {
  template<class T>
    struct unwrap_reference;
}

Class template std::unwrap_ref_decay

namespace std {
  template<class T>
    struct unwrap_ref_decay;
}

Class template std::plus

namespace std {
  template<class T = void> struct plus {
    constexpr T operator()(const T& x, const T& y) const;
  };
 
  template<> struct plus<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      -> decltype(std::forward<T>(t) + std::forward<U>(u));
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::minus

namespace std {
  template<class T = void> struct minus {
    constexpr T operator()(const T& x, const T& y) const;
  };
 
  template<> struct minus<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      -> decltype(std::forward<T>(t) - std::forward<U>(u));
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::multiplies

namespace std {
  template<class T = void> struct multiplies {
    constexpr T operator()(const T& x, const T& y) const;
  };
 
  template<> struct multiplies<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      -> decltype(std::forward<T>(t) * std::forward<U>(u));
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::divides

namespace std {
  template<class T = void> struct divides {
    constexpr T operator()(const T& x, const T& y) const;
  };
 
  template<> struct divides<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      -> decltype(std::forward<T>(t) / std::forward<U>(u));
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::modulus

namespace std {
  template<class T = void> struct modulus {
    constexpr T operator()(const T& x, const T& y) const;
  };
 
  template<> struct modulus<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      -> decltype(std::forward<T>(t) % std::forward<U>(u));
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::negate

namespace std {
  template<class T = void> struct negate {
    constexpr T operator()(const T& x) const;
  };
 
  template<> struct negate<void> {
    template<class T> constexpr auto operator()(T&& t) const
      -> decltype(-std::forward<T>(t));
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::equal_to

namespace std {
  template<class T = void> struct equal_to {
    constexpr bool operator()(const T& x, const T& y) const;
  };
 
  template<> struct equal_to<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      -> decltype(std::forward<T>(t) == std::forward<U>(u));
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::not_equal_to

namespace std {
  template<class T = void> struct not_equal_to {
    constexpr bool operator()(const T& x, const T& y) const;
  };
 
  template<> struct not_equal_to<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      -> decltype(std::forward<T>(t) != std::forward<U>(u));
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::greater

namespace std {
  template<class T = void> struct greater {
    constexpr bool operator()(const T& x, const T& y) const;
  };
 
  template<> struct greater<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      -> decltype(std::forward<T>(t) > std::forward<U>(u));
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::less

namespace std {
  template<class T = void> struct less {
    constexpr bool operator()(const T& x, const T& y) const;
  };
 
  template<> struct less<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      -> decltype(std::forward<T>(t) < std::forward<U>(u));
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::greater_equal

namespace std {
  template<class T = void> struct greater_equal {
    constexpr bool operator()(const T& x, const T& y) const;
  };
 
  template<> struct greater_equal<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      -> decltype(std::forward<T>(t) >= std::forward<U>(u));
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::less_equal

namespace std {
  template<class T = void> struct less_equal {
    constexpr bool operator()(const T& x, const T& y) const;
  };
 
  template<> struct less_equal<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      -> decltype(std::forward<T>(t) <= std::forward<U>(u));
 
    using is_transparent = /* unspecified */;
  };
}

Class std::ranges::equal_to

namespace std::ranges {
  struct equal_to {
    template<class T, class U>
    constexpr bool operator()(T&& t, U&& u) const;
 
    using is_transparent = /* unspecified */;
  };
}

Class std::ranges::not_equal_to

namespace std::ranges {
  struct not_equal_to {
    template<class T, class U>
    constexpr bool operator()(T&& t, U&& u) const;
 
    using is_transparent = /* unspecified */;
  };
}

Class std::ranges::greater

namespace std::ranges {
  struct greater {
    template<class T, class U>
    constexpr bool operator()(T&& t, U&& u) const;
 
    using is_transparent = /* unspecified */;
  };
}

Class std::ranges::less

namespace std::ranges {
  struct less {
    template<class T, class U>
    constexpr bool operator()(T&& t, U&& u) const;
 
    using is_transparent = /* unspecified */;
  };
}

Class std::ranges::greater_equal

namespace std::ranges {
  struct greater_equal {
    template<class T, class U>
    constexpr bool operator()(T&& t, U&& u) const;
 
    using is_transparent = /* unspecified */;
  };
}

Class std::ranges::less_equal

namespace std::ranges {
  struct less_equal {
    template<class T, class U>
    constexpr bool operator()(T&& t, U&& u) const;
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::logical_and

namespace std {
  template<class T = void> struct logical_and {
    constexpr bool operator()(const T& x, const T& y) const;
  };
 
  template<> struct logical_and<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      -> decltype(std::forward<T>(t) && std::forward<U>(u));
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::logical_or

namespace std {
  template<class T = void> struct logical_or {
    constexpr bool operator()(const T& x, const T& y) const;
  };
 
  template<> struct logical_or<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      -> decltype(std::forward<T>(t) || std::forward<U>(u));
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::logical_not

namespace std {
  template<class T = void> struct logical_not {
    constexpr bool operator()(const T& x) const;
  };
 
  template<> struct logical_not<void> {
    template<class T> constexpr auto operator()(T&& t) const
      -> decltype(!std::forward<T>(t));
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::bit_and

namespace std {
  template<class T = void> struct bit_and {
    constexpr T operator()(const T& x, const T& y) const;
  };
 
  template<> struct bit_and<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      -> decltype(std::forward<T>(t) & std::forward<U>(u));
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::bit_or

namespace std {
  template<class T = void> struct bit_or {
    constexpr T operator()(const T& x, const T& y) const;
  };
 
  template<> struct bit_or<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      -> decltype(std::forward<T>(t) | std::forward<U>(u));
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::bit_xor

namespace std {
  template<class T = void> struct bit_xor {
    constexpr T operator()(const T& x, const T& y) const;
  };
 
  template<> struct bit_xor<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      -> decltype(std::forward<T>(t) ^ std::forward<U>(u));
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::bit_not

namespace std {
  template<class T = void> struct bit_not {
    constexpr T operator()(const T& x) const;
  };
 
  template<> struct bit_not<void> {
    template<class T> constexpr auto operator()(T&& t) const
      -> decltype(~std::forward<T>(t));
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::identity

namespace std {
  struct identity {
    template<class T>
      constexpr T&& operator()(T&& t) const noexcept;
 
    using is_transparent = /* unspecified */;
  };
}

Class template std::is_bind_expression

namespace std {
  template<class T> struct is_bind_expression;
}

Class template std::is_placeholder

namespace std {
  template<class T> struct is_placeholder;
}

Class std::bad_function_call

namespace std {
  class bad_function_call : public exception {
  public:
    // see [exception] for the specification of the special member functions
    const char* what() const noexcept override;
  };
}

Class template std::function

namespace std {
  template<class> class function; // not defined
 
  template<class R, class... ArgTypes>
  class function<R(ArgTypes...)> {
  public:
    using result_type = R;
 
    // construct/copy/destroy
    function() noexcept;
    function(nullptr_t) noexcept;
    function(const function&);
    function(function&&) noexcept;
    template<class F> function(F);
 
    function& operator=(const function&);
    function& operator=(function&&);
    function& operator=(nullptr_t) noexcept;
    template<class F> function& operator=(F&&);
    template<class F> function& operator=(reference_wrapper<F>) noexcept;
 
    ~function();
 
    // function modifiers
    void swap(function&) noexcept;
 
    // function capacity
    explicit operator bool() const noexcept;
 
    // function invocation
    R operator()(ArgTypes...) const;
 
    // function target access
    const type_info& target_type() const noexcept;
    template<class T>       T* target() noexcept;
    template<class T> const T* target() const noexcept;
  };
 
  template<class R, class... ArgTypes>
    function(R(*)(ArgTypes...)) -> function<R(ArgTypes...)>;
 
  template<class F> function(F) -> function</* see description */>;
 
  // null pointer comparison functions
  template<class R, class... ArgTypes>
    bool operator==(const function<R(ArgTypes...)>&, nullptr_t) noexcept;
 
  // specialized algorithms
  template<class R, class... ArgTypes>
    void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept;
}

Class template std::move_only_function

namespace std {
  template<class... S> class move_only_function; // not defined
 
  template<class R, class... ArgTypes>
  class move_only_function<R(ArgTypes...) /*cv-ref*/ noexcept(/*noex*/)> {
  public:
    using result_type = R;
 
    // construct/move/destroy
    move_only_function() noexcept;
    move_only_function(nullptr_t) noexcept;
    move_only_function(move_only_function&&) noexcept;
    template<class F> move_only_function(F&&);
 
    template<class T, class... Args>
      explicit move_only_function(in_place_type_t<T>, Args&&...);
    template<class T, class U, class... Args>
      explicit move_only_function(in_place_type_t<T>, initializer_list<U>, Args&&...);
 
    move_only_function& operator=(move_only_function&&);
    move_only_function& operator=(nullptr_t) noexcept;
    template<class F> move_only_function& operator=(F&&);
 
    ~move_only_function();
 
    // move_only_function invocation
    explicit operator bool() const noexcept;
 
    R operator()(ArgTypes...) /*cv-ref*/ noexcept(/*noex*/);
 
    // move_only_function utility
    void swap(move_only_function&) noexcept;
 
    friend void swap(move_only_function&, move_only_function&) noexcept;
 
    friend bool operator==(const move_only_function&, nullptr_t) noexcept;
 
  private:
    template<class VT>
      static constexpr bool /*is-callable-from*/ = /* see description */; // exposition-only
  };
}

Class template std::default_searcher

namespace std {
  template<class ForwardIter1, class BinaryPredicate = equal_to<>>
    class default_searcher {
    public:
      constexpr default_searcher(ForwardIter1 pat_first, ForwardIter1 pat_last,
                                 BinaryPredicate pred = BinaryPredicate());
 
      template<class ForwardIter2>
        constexpr pair<ForwardIter2, ForwardIter2>
          operator()(ForwardIter2 first, ForwardIter2 last) const;
 
    private:
      ForwardIter1 pat_first_;            // exposition only
      ForwardIter1 pat_last_;             // exposition only
      BinaryPredicate pred_;              // exposition only
  };
}

Class template std::boyer_moore_searcher

namespace std {
  template<class RandomAccessIter1,
           class Hash = hash<typename iterator_traits<RandomAccessIter1>::value_type>,
           class BinaryPredicate = equal_to<>>
    class boyer_moore_searcher {
    public:
      boyer_moore_searcher(RandomAccessIter1 pat_first,
                           RandomAccessIter1 pat_last,
                           Hash hf = Hash(),
                           BinaryPredicate pred = BinaryPredicate());
 
      template<class RandomAccessIter2>
        pair<RandomAccessIter2, RandomAccessIter2>
          operator()(RandomAccessIter2 first, RandomAccessIter2 last) const;
 
    private:
      RandomAccessIter1 pat_first_;       // exposition only
      RandomAccessIter1 pat_last_;        // exposition only
      Hash hash_;                         // exposition only
      BinaryPredicate pred_;              // exposition only
    };
}

Class template std::boyer_moore_horspool_searcher

namespace std {
  template<class RandomAccessIter1,
           class Hash = hash<typename iterator_traits<RandomAccessIter1>::value_type>,
           class BinaryPredicate = equal_to<>>
    class boyer_moore_horspool_searcher {
    public:
      boyer_moore_horspool_searcher(RandomAccessIter1 pat_first,
                                    RandomAccessIter1 pat_last,
                                    Hash hf = Hash(),
                                    BinaryPredicate pred = BinaryPredicate());
 
      template<class RandomAccessIter2>
        pair<RandomAccessIter2, RandomAccessIter2>
          operator()(RandomAccessIter2 first, RandomAccessIter2 last) const;
 
    private:
      RandomAccessIter1 pat_first_;       // exposition only
      RandomAccessIter1 pat_last_;        // exposition only
      Hash hash_;                         // exposition only
      BinaryPredicate pred_;              // exposition only
  };
}

See also

<string>	Specializes std::hash for std::string, std::u16string, std::u32string, std::wstring
<string_view>	Specializes std::hash for std::string_view, std::u16string_view, std::u32string_view, std::wstring_view
<system_error>	Specializes std::hash for std::error_code
<bitset>	Specializes std::hash for std::bitset
<memory>	Specializes std::hash for std::unique_ptr, std::shared_ptr
<typeindex>	Specializes std::hash for std::type_index
<vector>	Specializes std::hash for std::vector<bool>
<thread>	Specializes std::hash for std::thread::id
<optional>	Specializes std::hash for std::optional
<variant>	Specializes std::hash for std::variant
<coroutine>	Specializes std::hash for std::coroutine_handle
<stacktrace>	Specializes std::hash for std::stacktrace_entry and std::basic_stacktrace