# 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 wrappers

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

function (C++11) | wraps callable object of any type 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)

### Function invocation

std::invoke can invoke any Callable object with given arguments.

invoke (C++17) | invokes any Callable object with given arguments (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 (C++20) | binds 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)
refcref (C++11)(C++11) | creates a std::reference_wrapper with a type deduced from its argument (function template)
unwrap_referenceunwrap_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)

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 `EqualityComparableWith`. The relational operators (`ranges::less`, `ranges::greater`, `ranges::less_equal`, and `ranges::greater_equal`) require the types of the arguments to model `StrictTotallyOrderedWith`.

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)

### 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

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)
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)

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