NAME

std::unary_function - std::unary_function

Synopsis

Defined in header <functional>
template <typename ArgumentType, typename ResultType> (deprecated in C++11)
struct unary_function; (removed in C++17)

unary_function is a base class for creating function objects with one argument.

unary_function does not define operator(); it is expected that derived classes will
define this. unary_function provides only two types - argument_type and result_type
- defined by the template parameters.

Some standard library function object adaptors, such as std::not1, require the
function objects they adapt to have certain types defined; std::not1 requires the
function object being adapted to have a type named argument_type. Deriving function
objects that take one argument from unary_function is an easy way to make them
compatible with those adaptors.

unary_function is deprecated in C++11.

Member types

Type Definition
argument_type ArgumentType
result_type ResultType

Example

// Run this code

#include <algorithm>
#include <functional>
#include <iostream>
#include <vector>

struct less_than_7 : std::unary_function<int, bool>
{
bool operator()(int i) const { return i < 7; }
};

int main()
{
std::vector<int> v;
for (int i = 0; i < 10; ++i) v.push_back(i);

std::cout << std::count_if(v.begin(), v.end(), std::not1(less_than_7()));

/* C++11 solution:
// Cast to std::function<bool (int)> somehow - even with a lambda
std::cout << std::count_if(v.begin(), v.end(),
std::not1(std::function<bool (int)>([](int i){ return i < 7; }))
);
*/
}

Output:

3

See also

function wraps callable object of any copy constructible type with
(C++11) specified function call signature
(class template)
move_only_function wraps callable object of any type with specified function
(C++23) call signature
(class template)
ptr_fun creates an adaptor-compatible function object wrapper from
(deprecated in C++11) a pointer to function
(removed in C++17) (function template)
pointer_to_unary_function adaptor-compatible wrapper for a pointer to unary function
(deprecated in C++11) (class template)
(removed in C++17)
binary_function adaptor-compatible binary function base class
(deprecated in C++11) (class template)
(removed in C++17)