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Showing content from https://www.tutorialspoint.com/cpp_standard_library/cpp_functional_operator_non_member.htm below:

C++ Functional Operators

It compares a std::function with a null pointer. Empty functions (that is, functions without a callable target) compare equal, non-empty functions compare non-equal.

Following is the declaration for std::function.

template< class R, class... ArgTypes >
bool operator==( const std::function<R(ArgTypes...)>& f, std::nullptr_t )

template< class R, class... ArgTypes >
bool operator==( const std::function<R(ArgTypes...)>& f, std::nullptr_t )

f − It is used to compare between functions.

none

noexcep − It doesn't throw any exceptions.

In below example explains about std::function.

#include <functional>
#include <iostream>

using SomeVoidFunc = std::function<void(int)>;

class C {
   public:
      C(SomeVoidFunc void_func = nullptr) :
         void_func_(void_func) {
            if (void_func_ == nullptr) { 
               void_func_ = std::bind(&C::default_func, this, std::placeholders::_1);
            }
            void_func_(9);
         }
 
         void default_func(int i) { std::cout << i << '\n'; };
 
   private:
      SomeVoidFunc void_func_;
};
 
void user_func(int i) {
   std::cout << (i + 1) << '\n';
}

int main() {
   C c1;
   C c2(user_func);
}

Let us compile and run the above program, this will produce the following result −

9
10

functional.htm

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