Function objects

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C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
Function objects
Function invocation
(C++17) (C++23)
Identity function object
(C++20)
Old binders and adaptors
( until C++17* )
( until C++17* )
( until C++17* )
( until C++17* )
( until C++17* )
( until C++17* )
( until C++17* ) ( until C++17* ) ( until C++17* ) ( until C++17* )
( until C++20* )
( until C++20* )
( until C++17* ) ( until C++17* )
( until C++17* ) ( until C++17* )

( until C++17* )
( until C++17* ) ( until C++17* ) ( until C++17* ) ( until C++17* )
( until C++20* )
( until C++20* )

A function object is any object for which the function call operator is defined. C++ provides many built-in function objects as well as support for creation and manipulation of new function objects.

Function invocation

The exposition-only operation INVOKE ( f, arg_0, arg_1, arg_2, ..., arg_N ) is defined as follows:

Let type Obj be the unqualified type of arg_0 (i.e., std:: remove_cv < std:: remove_reference < decltype ( arg_0 ) > :: type > :: type )

  • ( obj. * f ) ( arg_1, arg_2, ..., arg_N ) (invoke the member function on the object).
  • ( obj. get ( ) . * f ) ( arg_1, arg_2, ..., arg_N ) (invoke the member function on the specially referred object).
  • Otherwise
  • ( ( * obj ) . * f ) ( arg_1, arg_2, ..., arg_N ) (invoke the member function on the dereferenced object).
  • obj. * mptr (access the data member of the object).
  • obj. get ( ) . * mptr (access the data member of the specially referred object).
  • Otherwise
  • ( * obj ) . * mptr (access the data member of the dereferenced object).
  • Otherwise
  • INVOKE ( f, arg_0, arg_1, arg_2, ..., arg_N ) is equivalent to f ( arg_0, arg_1, arg_2, ..., arg_N ) (invoke the callable).


The exposition-only operation INVOKE<R> ( f, arg_0, arg_1, arg_2, ..., arg_N ) is defined as follows:

  • If R is (possibly cv-qualified) void
  • static_cast < void > ( INVOKE ( f, arg_0, arg_1, arg_2, ..., arg_N ) ) .
  • Otherwise
  • INVOKE ( f, arg_0, arg_1, arg_2, ..., arg_N ) implicitly converted to R .

Let type Actual be decltype ( INVOKE ( f, arg_0, arg_1, arg_2, ..., arg_N ) )

  • INVOKE<R> ( f, arg_0, arg_1, arg_2, ..., arg_N ) is ill-formed.
(since C++23)
(since C++11)


std::invoke and std::invoke_r (since C++23) can invoke any Callable object with given arguments according to the rules of INVOKE and INVOKE<R> (since C++23) .

(C++17) (C++23)
invokes any Callable object with given arguments and possibility to specify return type (since C++23)
(function template)

Function wrappers

These polymorphic wrapper classes provide support for storing arbitrary function objects.

(C++11)
copyable wrapper of any copy constructible callable object
(class template)
(C++23)
move-only wrapper of any callable object that supports qualifiers in a given call signature
(class template)
(C++26)
copyable wrapper of any copy constructible callable object that supports qualifiers in a given call signature
(class template)
(C++26)
non-owning wrapper of any callable object
(class template)
(C++11)
the exception thrown when invoking an empty std::function
(class)
(C++11)
creates a function object out of a pointer to a member
(function template)

Identity

std::identity is the identity function object: it returns its argument unchanged.

(C++20)
function object that returns its argument unchanged
(class)

Partial function application

std::bind_front and std::bind provide support for partial function application , i.e. binding arguments to functions to produce new functions.

(C++20) (C++23)
bind a variable number of arguments, in order, to a function object
(function template)
(C++11)
binds one or more arguments to a function object
(function template)
(C++11)
indicates that an object is std::bind expression or can be used as one
(class template)
(C++11)
indicates that an object is a standard placeholder or can be used as one
(class template)
Defined in namespace std::placeholders
(C++11)
placeholders for the unbound arguments in a std::bind expression
(constant)

Negators

std::not_fn creates a function object that negates the result of the callable object passed to it.

(C++17)
creates a function object that returns the complement of the result of the function object it holds
(function template)

Searchers

Searchers implementing several string searching algorithms are provided and can be used either directly or with std::search .

(C++17)
standard C++ library search algorithm implementation
(class template)
(C++17)
Boyer-Moore search algorithm implementation
(class template)
(C++17)
Boyer-Moore-Horspool search algorithm implementation
(class template)

Reference wrappers

Reference wrappers allow reference arguments to be stored in copyable function objects:

(C++11)
CopyConstructible and CopyAssignable reference wrapper
(class template)
(C++11) (C++11)
creates a std::reference_wrapper with a type deduced from its argument
(function template)
(C++20) (C++20)
get the reference type wrapped in std::reference_wrapper
(class template)

Transparent function objects

Associative containers and unordered associative containers (since C++20) provide heterogeneous lookup and erasure (since C++23) operations, but they are only enabled if the supplied function object type T is transparent : the qualified identifier T::is_transparent is valid and denotes a type.

All transparent function object types in the standard library defines a nested type is_transparent . However, user-defined transparent function object types do not need to directly provide is_transparent as a nested type: it can be defined in a base class, as long as T::is_transparent satisfies the transparent requirement stated above.

(since C++14)

Operator function objects

C++ defines the following function objects that represent common arithmetic and logical operations.

The void specializations deduce their parameter types and return types from their arguments, they are all transparent .

(since C++14)
Arithmetic operations
function object implementing x + y
(class template)
(C++14)
function object implementing x + y deducing parameter and return types
(class template specialization)
function object implementing x - y
(class template)
(C++14)
function object implementing x - y deducing parameter and return types
(class template specialization)
function object implementing x * y
(class template)
(C++14)
function object implementing x * y deducing parameter and return types
(class template specialization)
function object implementing x / y
(class template)
(C++14)
function object implementing x / y deducing parameter and return types
(class template specialization)
function object implementing x % y
(class template)
(C++14)
function object implementing x % y deducing parameter and return types
(class template specialization)
function object implementing - x
(class template)
(C++14)
function object implementing - x deducing parameter and return types
(class template specialization)
Comparisons
function object implementing x == y
(class template)
(C++14)
function object implementing x == y deducing parameter and return types
(class template specialization)
function object implementing x ! = y
(class template)
(C++14)
function object implementing x ! = y deducing parameter and return types
(class template specialization)
function object implementing x > y
(class template)
(C++14)
function object implementing x > y deducing parameter and return types
(class template specialization)
function object implementing x < y
(class template)
(C++14)
function object implementing x < y deducing parameter and return types
(class template specialization)
function object implementing x >= y
(class template)
(C++14)
function object implementing x >= y deducing parameter and return types
(class template specialization)
function object implementing x <= y
(class template)
(C++14)
function object implementing x <= y deducing parameter and return types
(class template specialization)
Logical operations
function object implementing x && y
(class template)
(C++14)
function object implementing x && y deducing parameter and return types
(class template specialization)
function object implementing x || y
(class template)
(C++14)
function object implementing x || y deducing parameter and return types
(class template specialization)
function object implementing ! x
(class template)
(C++14)
function object implementing ! x deducing parameter and return types
(class template specialization)
Bitwise operations
function object implementing x & y
(class template)
(C++14)
function object implementing x & y deducing parameter and return types
(class template specialization)
function object implementing x | y
(class template)
(C++14)
function object implementing x | y deducing parameter and return types
(class template specialization)
function object implementing x ^ y
(class template)
(C++14)
function object implementing x ^ y deducing parameter and return types
(class template specialization)
(C++14)
function object implementing ~x
(class template)
(C++14)
function object implementing ~x deducing parameter and return types
(class template specialization)


Constrained comparison function objects

The following comparison function objects are constrained .

  • The equality operators ( ranges::equal_to and ranges::not_equal_to ) require the types of the arguments to satisfy equality_comparable_with .
  • The relational operators ( ranges::less , ranges::greater , ranges::less_equal , and ranges::greater_equal ) require the types of the arguments to satisfy totally_ordered_with .
  • The three-way comparison operator ( compare_three_way ) requires the type to model three_way_comparable_with .

All these function objects are transparent .

(C++20)
constrained function object implementing x == y
(class)
(C++20)
constrained function object implementing x ! = y
(class)
(C++20)
constrained function object implementing x < y
(class)
(C++20)
constrained function object implementing x > y
(class)
(C++20)
constrained function object implementing x <= y
(class)
(C++20)
constrained function object implementing x >= y
(class)
(C++20)
constrained function object implementing x <=> y
(class)
(since C++20)


Helper items

Following exposition-only items are used for several components in the standard library but they are not part of the interface of the standard library.

template < class Fn, class ... Args >

concept /*callable*/ =
requires ( Fn && fn, Args && ... args ) {
std:: forward < Fn > ( fn ) ( std:: forward < Args > ( args ) ... ) ;

} ;
(1) ( exposition only* )
template < class Fn, class ... Args >

concept /*nothrow-callable*/ =
/*callable*/ < Fn, Args... > &&
requires ( Fn && fn, Args && ... args ) {
{ std:: forward < Fn > ( fn ) ( std:: forward < Args > ( args ) ... ) } noexcept ;

} ;
(2) ( exposition only* )
template < class Fn, class ... Args >
using /*call-result-t*/ = decltype ( std:: declval < Fn > ( ) ( std:: declval < Args > ( ) ... ) ) ;
(3) ( exposition only* )
template < const auto & T >
using /*decayed-typeof*/ = decltype ( auto ( T ) ) ;
(4) ( exposition only* )
(since C++26)


Old binders and adaptors

Several utilities that provided early functional support are deprecated and removed:

Base
(deprecated in C++11) (removed in C++17)
adaptor-compatible unary function base class
(class template)
(deprecated in C++11) (removed in C++17)
adaptor-compatible binary function base class
(class template)
Binders
(deprecated in C++11) (removed in C++17)
function object holding a binary function and one of its arguments
(class template)
(deprecated in C++11) (removed in C++17)
binds one argument to a binary function
(function template)
Function adaptors
(deprecated in C++11) (removed in C++17)
adaptor-compatible wrapper for a pointer to unary function
(class template)
(deprecated in C++11) (removed in C++17)
adaptor-compatible wrapper for a pointer to binary function
(class template)
(deprecated in C++11) (removed in C++17)
creates an adaptor-compatible function object wrapper from a pointer to function
(function template)
(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)
(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)
(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)
(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) (removed in C++20)
wrapper function object returning the complement of the unary predicate it holds
(class template)
(deprecated in C++17) (removed in C++20)
wrapper function object returning the complement of the binary predicate it holds
(class template)
(deprecated in C++17) (removed in C++20)
constructs custom std::unary_negate object
(function template)
(deprecated in C++17) (removed in C++20)
constructs custom std::binary_negate object
(function template)
(until C++20)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 185 C++98 using function objects improved the program efficiency removed the claim
LWG 660 C++98 function objects for bitwise operations are missing added
LWG 2149 C++98 function objects taking one or two arguments were required to
provide nested types to denote the argument and result types 		not required
LWG 2219 C++11 INVOKE did not handle std::reference_wrapper correctly handles correctly
LWG 2420 C++11 INVOKE<R> did not discard the return value if R is void discards the return value in this case
LWG 2926
( P0604R0 ) 		C++11 the syntax of the INVOKE operation with a return
type R was INVOKE ( f, t1, t2, ..., tN, R ) 		changed to
INVOKE<R> ( f, t1, t2, ..., tN )
LWG 3655 C++11 INVOKE did not handle unions correctly
due to the resolution of LWG issue 2219 		handles correctly
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