std::unary_function (3) - Linux Manuals

std::unary_function: std::unary_function

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