C++ Provide Friends Mechanism , Allow external classes and functions to access private members of classes and helper methods that protect members , Declare them as... Of a given class Friend class ( Or friend function ), Make it have access to class member functions . But the friend itself is not a member of the class , It doesn't belong to any class .

advantage : Improve the reliability of the software 、 Reusability 、 Maintainability .

Insufficient : It increases the cost of calling functions when the program is running —— When member functions are called frequently , It will lead to the reduction of execution efficiency , Increase the running time of the program .
Friends can be ordinary functions , Or member functions of other classes that have been defined , Or entire class . Set a function as friend , be called Friend function , Set a class as a friend , be called Friend class . All member functions of a friend class can access the private members of the class that granted the friend relationship .

One 、 Friend function

Friend function : If a function is defined somewhere outside a class , Use... In the class definition friend Declare it , This function is called the of this class Friend function .
Definition form of friend function :

  friend  Return type   Function name ( parameter list );

        { … }
explain :
1） Friend function is not a member function of a class , Private members that can access objects , Just in the class Add... To the statement friend, Because it is not a member function , It is not necessary to add... Before the function name when defining “ Class name ::”.
2) A function can be a friend function of multiple classes , Just declare it separately in each class .
3） Friend function none this The pointer , therefore , Generally, there should be an entry parameter —— The object name passed through the entry parameter refers to the member of the object .
4) When a function needs to access two or more classes , Friend functions are very useful .
5)C++ It is not allowed to declare constructors and destructors as friend functions .

friend member function : It means that the member function of one class can be a friend function of another class, that is, the private members of another class can be accessed through the member function of one class .

Two 、 Friend class

Friend class Is an access mechanism established between multiple classes , When two or more classes are defined in the program , If you want one class to have access to private members of another class , You can declare this class as a friend of another class , All member functions of a friend class can be regarded as friend functions of that class , Can access private and protected members of this class .

Define the form of friend class :

friend  Class name ;

Such as : class A
        {···
                friend B; } ;

        class B {...} ;

3、 ... and 、 About friends :

1. Friends can access and modify the private or protected data of the class , Call the private or protected member function of the class , Make the class both encapsulated , And flexible .
2. The relationship between friends is one-way rather than two-way . If a class is declared B It's a class A Friends class , It's not equal to class A It's a class B Friends class , class A Member functions in cannot access classes B Private data in .

3. A friend's relationship cannot be passed or inherited , If a class B It's a class A Friends class , class C It's a class B Friends class , It's not equal to class C It's a class A Friends class . If you want class c It's a class A Friends class , In addition, it must be explicitly declared in the class .

Four 、 Class template

about Some functions with the same function but different data types , You can define a General function template To achieve . about Some classes with the same function but different data types , You can also define a general class template .

Class template definition form :

template <class Type parameter name >
class Class name
{

        ···

};
The form of object defined by class template is :

Class template name < Actual type name > Object name ;

If you define member functions outside the class template , The form of class template shall be written as :

        template <class Type parameter name >
Function type   Class template name < Type parameter name >：： Member function name ()

                { The body of the function }