112 lines
2.5 KiB
Markdown
112 lines
2.5 KiB
Markdown
# Friend and Friend Class
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## About Author：
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## 0.Summary
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Friends provide a mechanism for ordinary functions or class member functions to access private or protected members in another class.In other words, there are two forms of friends:
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（1）Friend Function：Ordinary functions access a private or protected member of a class.
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（2）Friend Class：Member functions in class a access private or protected members in class B
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Advantages: improve the efficiency of the program.
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Disadvantages: it destroys the encapsulation of classes and the transparency of data.
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Conclusion：
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- Access to private members
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- Breaking encapsulation
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- Friendship is not transitive
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- The unidirectionality of friend relationship
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- There are no restrictions on the form and number of friend declarations
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## 1.Friend function
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It is declared in any region of the class declaration, and the definition is outside the class.
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```
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friend <type><name>(<Parameter table>);
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```
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Note that the friend function is only a common function, not a class member function of this class. It can be called anywhere. In the friend function, private or protected members of the class can be accessed through the object name.
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Code：[friend_func.cpp](friend_func.cpp)
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```c++
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#include <iostream>
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using namespace std;
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class A
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{
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public:
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A(int _a):a(_a){};
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friend int geta(A &ca); ///< Friend function
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private:
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int a;
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};
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int geta(A &ca)
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{
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return ca.a;
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}
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int main()
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{
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A a(3);
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cout<<geta(a)<<endl;
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return 0;
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}
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```
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## 2.Friend Class
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The declaration of a friend class is in the declaration of the class, and the implementation is outside the class.
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```
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friend class <friend class name>;
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```
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Class B is a friend of class A, so class B can directly access private members of A.
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Code：[friend_class.cpp](friend_class.cpp)
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```c++
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#include <iostream>
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using namespace std;
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class A
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{
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public:
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A(int _a):a(_a){};
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friend class B;
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private:
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int a;
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};
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class B
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{
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public:
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int getb(A ca) {
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return ca.a;
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};
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};
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int main()
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{
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A a(3);
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B b;
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cout<<b.getb(a)<<endl;
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return 0;
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}
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```
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## 3.Attention
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- Friendship has no inheritance
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If class B is a friend of class A and class C inherits from Class A, then friend class B cannot directly access private or protected members of class C.
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- Friendship is not transitive
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If class B is a friend of class A and class C is a friend of class B, then friend class C cannot directly access private or protected members of class A, that is, there is no such relationship as "friend of friend".
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