"C + + Basics" Friend

Directory (?) [-]

1. Overview
2. Characteristics
3. Realize
1. 1 friend function
1. 11 Declarations and definitions
2. 12 Example
2. 2 Friend Class
1. 21 Declarations and definitions
2. 22 Example
3. Attention
1. 1 friend relationship has no inheritance
2. 2 friend relationships are not transitive

1. Overview

Friend provides a mechanism for accessing private or protected members of another class by a common function or class member function. In other words, there are two types of friends:

(1) Friend function: A normal function for a private or protected member that accesses a class.

(2) Friend class: A member function in Class A accesses a private or protected member in Class B.

2. Features

Advantages: Improve the operation efficiency of the program.

Disadvantage: It destroys the encapsulation of classes and the transparency of data.

3. Implement

3.1. Friend function 3.1.1. Declarations and definitions

Declared in any area of the class declaration, and the definition is outside the class.

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1. Friend < type >< friend function name > (< parameter table >);

Note that the friend function is just a normal function, not a class member function of the class, it can be called anywhere, and the friend function accesses the private or protected members of the class through the object name.

3.1.2. Example

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2. TestFriend.cpp
6. #include "stdafx.h"
10. Class A
12. {
14. Public
16. A (int _a): A (_a) {};
20. friend int geta_a (a &_classa); Friend function
24. Private
26. int A;
28. };
32. int Geta_a (a &_classa)
34. {
36. return _CLASSA.A; //Access private variables by object name
38. }
42. int _tmain (int argc, _tchar* argv[])
44. {
46. A _classa (3);
48. Std::cout<<geta_a (_classa); the//friend function is just a normal function and can be called anywhere
50. return 0;
52. }

3.2. Friend class 3.2.1. Declarations and definitions

The declaration of a friend class is in the declaration of the class and is implemented outside of that class.

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1. Friend class < friend category name >;

An instance of a friend class is defined in the main function.

3.2.2. Example

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2. TestFriend.cpp
6. #include "stdafx.h"
10. Class B
12. {
14. Public
16. B (int _b): b (_b) {};
20. friend class C; Declare friend Class C
24. Private
26. int b;
28. };
32. Class C//Implement friend Class C
34. {
36. Public
38. int Getb_b (b _classb) {
40. return _classb.b; //Access private members of friend Class B
42. };
44. };
50. int _tmain (int argc, _tchar* argv[])
52. {
54. B _CLASSB (3);
56. C _CLASSC; //Define friend class instances
58. Std::cout<<_classc.getb_b (_CLASSB);
60. return 0;
62. }

4. Note 4.1. There is no inheritance of the friend relationship

If Class B is a friend of Class A, Class C inherits from Class A, then the friend Class B is not able to directly access the private or protected members of Class C.

4.2. Friend relationships are not transitive

The addition of Class B is a friend of Class A, Class C is a friend of Class B, then the Friend Class C is not able to directly access the private or protected members of Class A, that is, there is no "friend of friends" this relationship.

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"C + + Basics" Friend