# Function objects

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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, t1, t2, ..., tN) is defined as follows, given the type `T1` as std::remove_cv<std::remove_reference<decltype(t1)>::type>::type: If f is a pointer to member function of class `T`, then `INVOKE`(f, t1, t2, ..., tN) is equivalent to: (t1.*f)(t2, ..., tN) if std::is_same<T, T1>::value || std::is_base_of<T, T1>::value is true. (t1.get().*f)(t2, ..., tN) if `T1` is a specialization of std::reference_wrapper. ((*t1).*f)(t2, ..., tN) if `T1` does not satisfy the previous two conditions. Otherwise, if `N == 1` and f is a pointer to data member of class `T`, then `INVOKE`(f, t1) is equivalent to: t1.*f if std::is_same<T, T1>::value || std::is_base_of<T, T1>::value is true. t1.get().*f if `T1` is a specialization of std::reference_wrapper. (*t1).*f if `T1` does not satisfy the previous two conditions. Otherwise, `INVOKE`(f, t1, t2, ..., tN) is equivalent to f(t1, t2, ..., tN) (that is, f is a FunctionObject). The exposition-only operation `INVOKE<R>`(f, t1, t2, ..., tN) is defined as static_cast<void>(`INVOKE`(f, t1, t2, ..., tN)) if `R` is (possibly cv-qualified) void, otherwise `INVOKE`(f, t1, t2, ..., tN) implicitly converted to `R`.
If `std::reference_converts_from_temporary_v` <R, decltype(`INVOKE`(f, t1, t2, ..., tN))> is true, `INVOKE<R>`(f, t1, t2, ..., tN) 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).

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

### Function wrappers

std::function provides support for storing arbitrary function objects.

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)
bad_function_call (C++11) | the exception thrown when invoking an empty std::function (class)
mem_fn (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.

identity (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.

bind_front bind_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)
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)
Defined in namespace `std::placeholders`
_1, _2, _3, _4, ... (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.

not_fn (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.

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)

### Reference wrappers

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

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

### Operator function objects

C++ defines several function objects that represent common arithmetic and logical operations:

##### 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)

### Constrained comparison function objects

C++20 defines a set of constrained comparison function objects. The equality operators (`ranges::equal_to` and `ranges::not_equal_to`) require the types of the arguments to model `equality_comparable_with`. The relational operators (`ranges::less`, `ranges::greater`, `ranges::less_equal`, and `ranges::greater_equal`) require the types of the arguments to model `totally_ordered_with`. The three-way comparison operator (`compare_three_way`) requires the type to model `three_way_comparable_with`.

ranges::equal_to (C++20) | function object implementing x == y (class)
ranges::not_equal_to (C++20) | function object implementing x != y (class)
ranges::less (C++20) | function object implementing x < y (class)
ranges::greater (C++20) | function object implementing x > y (class)
ranges::less_equal (C++20) | function object implementing x <= y (class)
ranges::greater_equal (C++20) | function object implementing x >= y (class)
compare_three_way (C++20) | function object implementing x <=> y (class)

### Old binders and adaptors

Several utilities that provided early functional support are deprecated in C++11 and removed in C++17 (old negators are deprecated in C++17 and removed in C++20):

##### 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
binder1st binder2nd (deprecated in C++11)(removed in C++17) | function object holding a binary function and one of its arguments (class template)
bind1st bind2nd (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_t mem_fun1_t const_mem_fun_t const_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_t mem_fun1_ref_t const_mem_fun_ref_t const_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)
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)

### 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 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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