Solution for mutual reference between two classes in C ++

Solution for mutual reference between two classes in C ++

I. Problem Description

Now there are two classes A and B that need to be defined. B is required for defining A and A is required for defining B.

Ii. Analysis

Definitions and calls of A and B are definitely not allowed in A single file, which will lead to an endless loop of two loop calls.

The root cause is: when defining A, B exists in A, so you need to check the space occupied by B. However, you need to know the space occupied by, causes an endless loop.

Solution:

(1) write two header files A. h and B. h to declare Class A and B respectively;

(2) write two. cpp files to define classes A and B respectively;

(3) import the header file of B into the header file of;

(4) do not import the header file of A in the header file of B, but declare Class A in the form of extern. In addition, use A pointer when using A in B.

Principle:When calling A with A pointer in B, when A needs to know the size of space occupied by B, it will find the definition file of B, although the definition file of B does not import the header file of A and does not know the space occupied by A, the pointer format used when B calls A is, B only knows that the pointer occupies 4 bytes and does not need to know the actual space occupied by A. That is to say, A also knows the space occupied by B.

III. C ++ example

A header file A. h:

# Ifndef _ A # define _# Include "B. h" // the header file of A imports the header file of B.// Extern class B; class A {private: int a; B objectb; // the header file of class A imports the header file of class B. When calling class B, you do not need to use the public pointer: A (); int geta (); void handle () ;};# endif _

B header file B .h:

# Ifndef _ B # define _ B/// # Include "A. h" // the header file of B does not import the header file of A. There are three points to note! Extern class A; // Note 1: Use extern to declareClass B {private: int B; A * objecta; // NOTE 2: when calling A, use the public: B (); int getb (); void handle () ;};# endif _ B

Definition file A. cpp of:

# Include
 
  # Include "A. h"Using namespace std; A: A () {this-> a = 100;} int A: geta () {return a;} void A: handle () {cout <"in A, objectb. B = "<
  
   

   

   

   
Definition file B. cpp:
   

   
# Include
    
     # Include "B. h" # include "A. h" // Note 3: import the header file of A in B. cpp.Using namespace std; B: B () {this-> B = 200;} int B: getb () {return B;} void B: handle () {objecta = new A (); cout <"in B, objecta-> a =" <
     
      
Geta () <
      
       

       

       

       
Main. cpp:
       

       
# Include
        
         
# Include
         
          # Include "A. h" // # include "B. h" // because A. h already contains B. h, you do not need to import B. h here.Using namespace std; void main () {A; a. handle (); B B; B. handle (); system ("pause ");}
         
        
       
Running result:
       

       


       
4. Note
       
Compilation fails in the following cases:
       
A. h contains B. h and B. h contains A. h.