function object:
If a class defines the operator() operator function , the function can be called using the object name of the class .
A function object is an object, but the call form is the same as a normal function call , so it is called a function object
(Note that operator() has parentheses first, followed by parentheses (parameter list))
Friend function:
friend return_type function_name(parameter_type_list);
• Declaration method: Put the normally declared function inside the class and add the keyword friend in front
• Effect: Although this function does not belong to the class, it can access the private variables and private functions of the class
• Member functions of other classes can also be friend functions of this class, for example:
Friend class:
• Effect: A class A can declare another class B as its own friend , then all member functions of class B can access the private members of objects of class A
• Form: friend class B; (inside class A), eg:
