std::unary_function - cppreference.com

Original:

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

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Original:

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.

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Original:

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.

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Original:

unary_function is deprecated in C++11. Its functionality has been made obsolete by std::function.

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#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; }))
        );
    */
}