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


#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; }))



See also

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

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