std::unary_function (3) Linux Manual Page

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