std::unary_function(3)

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

See also
   function wraps callable object of any copy constructible type with
   (C++11) specified function call signature
                        (class template)
   move_only_function wraps callable object of any type with specified function
   (C++23) call signature
                        (class template)
   ptr_fun creates an adaptor-compatible function object wrapper from
   (deprecated in C++11) a pointer to function
   (removed in C++17) (function template)
   pointer_to_unary_function adaptor-compatible wrapper for a pointer to unary function
   (deprecated in C++11) (class template)
   (removed in C++17)
   binary_function adaptor-compatible binary function base class
   (deprecated in C++11) (class template)
   (removed in C++17)