std::unary_function

 

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

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