Standard library header <functional>

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

Standard Library header files

Language Support
<cstddef> <limits> <climits> <contract>	<cfloat> <cstdint> <new> <version>	<typeinfo> <exception> <initializer_list>	<csignal> <csetjmp> <cstdarg>
Concepts
<concepts>
Diagnostics
<stdexcept>	<cassert>	<cerrno>	<system_error>
General utilities
<utility> <memory> <memory_resource> <scoped_allocator> <compare>	<bitset> <tuple> <optional> <any> <variant>	<type_traits> <ratio> <chrono> <typeindex> <functional>	<ctime> <cstdlib>
Strings
<string> <string_view>	<cctype> <cwctype>	<cwchar> <cuchar>	<cstring> <charconv>
Localization
<locale>	<codecvt>	<clocale>
Containers
<span> <array> <vector>	<deque> <forward_list> <list>	<map> <set> <queue>	<unordered_map> <unordered_set> <stack>
Iterators
<iterator>
Algorithms
<algorithm>	<execution>
Numerics
<complex> <random>	<valarray> <numeric>	<cfenv> <cmath>	<bit>
Input/Output
<iosfwd> <ios> <iomanip> <streambuf>	<istream> <ostream> <iostream>	<sstream> <fstream> <syncstream>	<cstdio> <cinttypes> <strstream>
Regular expressions
<regex>
Filesystem support
<filesystem>
Thread support
<thread> <atomic>	<mutex> <shared_mutex>	<condition_variable> <future>
C compatibility
<ciso646> <cstdalign>	<cstdbool> <ccomplex>	<ctgmath>

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

Namespaces

placeholders	Defines placeholders for the unbound arguments in a std::bind expression
Constants
Defined in namespace std::placeholders
_1, _2, _3, _4, ... (C++11)	placeholders for the unbound arguments in a std::bind expression (constant)

Classes

function (C++11)	wraps callable object of any type with specified function call signature (class template)
mem_fn (C++11)	creates a function object out of a pointer to a member (function template)
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)
reference_wrapper (C++11)	CopyConstructible and CopyAssignable reference wrapper (class template)
Hashing
hash (C++11)	hash function object (class template)
template<> struct hash<bool>; template<> struct hash<char>; template<> struct hash<signed char>; template<> struct hash<unsigned char>; template<> struct hash<char16_t>; template<> struct hash<char32_t>; template<> struct hash<wchar_t>; template<> struct hash<short>; template<> struct hash<unsigned short>; template<> struct hash<int>; template<> struct hash<unsigned int>; template<> struct hash<long>; template<> struct hash<long long>; template<> struct hash<unsigned long>; template<> struct hash<unsigned long long>; template<> struct hash<float>; template<> struct hash<double>; template<> struct hash<long double>; template< class T > struct hash<T*>;	std::hash specializations for built-in types (class template specialization)

Functions

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)
invoke (C++17)	invokes any Callable object with given arguments (function template)

Function Objects

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)
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)
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)

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>
  invoke_result_t<F, Args...> invoke(F&& f, Args&&... args);

  // reference_wrapper:
  template <class T> class reference_wrapper;
  template <class T> reference_wrapper<T> ref(T&) noexcept;
  template <class T> 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> reference_wrapper<T> ref(reference_wrapper<T>) noexcept;
  template <class T> reference_wrapper<const T> cref(reference_wrapper<T>) noexcept;

  // 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>;

  // function template not_fn:
  template <class F> /*unspecified*/ not_fn(F&& f);

  // bind:
  template<class T> struct is_bind_expression;
  template<class T> struct is_placeholder;
  template<class F, class... BoundArgs>
  /*unspecified*/ bind(F&&, BoundArgs&&...);
  template<class R, class F, class... BoundArgs>
  /*unspecified*/ bind(F&&, BoundArgs&&...);
  namespace placeholders {
    // M is the implementation-defined number of placeholders
    /* implementation-defined */ _1;
    /* implementation-defined */ _2;
    ...
    /* implementation-defined */ _M;
  }

  // member function adaptors:
  template<class R, class T> /*unspecified*/ mem_fn(R T::*) noexcept;

  // polymorphic function wrappers:
  class bad_function_call;
  template<class> class function; // undefined
  template<class R, class... ArgTypes> class function<R(ArgTypes...)>;
  template<class R, class... ArgTypes>
  void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&);
  template<class R, class... ArgTypes>
  bool operator==(const function<R(ArgTypes...)>&, nullptr_t) noexcept;
  template<class R, class... ArgTypes>
  bool operator==(nullptr_t, const function<R(ArgTypes...)>&) noexcept;
  template<class R, class... ArgTypes>
  bool operator!=(const function<R(ArgTypes...)>&, nullptr_t) noexcept;
  template<class R, class... ArgTypes>
  bool operator!=(nullptr_t, const function<R(ArgTypes...)>&) noexcept;

  // searchers:
  template<class ForwardIterator, class BinaryPredicate = equal_to<>>
  class default_searcher;
  template<class RandomAccessIterator,
           class Hash = hash<typename iterator_traits<RandomAccessIterator>::value_type>,
           class BinaryPredicate = equal_to<>>
  class boyer_moore_searcher;
  template<class RandomAccessIterator,
           class Hash = hash<typename iterator_traits<RandomAccessIterator>::value_type>,
           class BinaryPredicate = equal_to<>>
  class boyer_moore_horspool_searcher;
  template<class ForwardIterator, class BinaryPredicate = equal_to<>>
  default_searcher<ForwardIterator, BinaryPredicate>
  make_default_searcher(ForwardIterator pat_first, ForwardIterator pat_last,
                        BinaryPredicate pred = BinaryPredicate());
  template<class RandomAccessIterator,
           class Hash = hash<typename iterator_traits<RandomAccessIterator>::value_type>,
           class BinaryPredicate = equal_to<>>
  boyer_moore_searcher<RandomAccessIterator, Hash, BinaryPredicate>
  make_boyer_moore_searcher(RandomAccessIterator pat_first,
                            RandomAccessIterator pat_last,
                            Hash hf = Hash(), BinaryPredicate pred = BinaryPredicate());
  template<class RandomAccessIterator,
           class Hash = hash<typename iterator_traits<RandomAccessIterator>::value_type>,
           class BinaryPredicate = equal_to<>>
  boyer_moore_horspool_searcher<RandomAccessIterator, Hash, BinaryPredicate>
  make_boyer_moore_horspool_searcher(RandomAccessIterator pat_first,
                                     RandomAccessIterator pat_last,
                                     Hash hf = Hash(),
                                     BinaryPredicate pred = BinaryPredicate());

  // 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;
}

Class std::reference_wrapper

namespace std {
  template <class T> class reference_wrapper {
    public :
    // types
    using type = T;
    // construct/copy/destroy
    reference_wrapper(T&) noexcept;
    reference_wrapper(T&&) = delete; // do not bind to temporary objects
    reference_wrapper(const reference_wrapper& x) noexcept;
    // assignment
    reference_wrapper& operator=(const reference_wrapper& x) noexcept;
    // access
    operator T& () const noexcept;
    T& get() const noexcept;
    // invocation
    template <class... ArgTypes>
    invoke_result_t<T&, ArgTypes...> operator() (ArgTypes&&...) const;
  };
}

Class std::is_bind_expression

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

Class std::is_placeholder

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

Class std::bad_function_call

class bad_function_call : public std::exception {
public:
    // constructor:
    bad_function_call() noexcept;
};

Class std::function

namespace std {
  template<class> class function; // undefined
  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&&);
    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;
  };
  // Null pointer comparisons:
  template <class R, class... ArgTypes>
  bool operator==(const function<R(ArgTypes...)>&, nullptr_t) noexcept;
  template <class R, class... ArgTypes>
  bool operator==(nullptr_t, const function<R(ArgTypes...)>&) noexcept;
  template <class R, class... ArgTypes>
  bool operator!=(const function<R(ArgTypes...)>&, nullptr_t) noexcept;
  template <class R, class... ArgTypes>
  bool operator!=(nullptr_t, const function<R(ArgTypes...)>&) noexcept;
  // specialized algorithms:
  template <class R, class... ArgTypes>
  void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&);
}

See Also

<string>	Specializes std::hash for std::string, std::u16string, std::u32string, std::wstring
<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