Forward declaration of classes

Forward declaration of class

Although using forward reference declarations can solve some problems , But it's not everything . It should be noted that , Despite the use of forward reference declarations , But before providing a complete class declaration , Cannot define objects of this class , You cannot use objects of this class in inline member functions .

#include<bits/stdc++.h>
using namespace std;
class A;   
// The statement here can only state  A  Is a class name  
// Instead of giving a complete definition of this class , Therefore, it cannot be accessed A Internal members of 

class B
{
    
    int m_B=0;
    A *Point_A;  // The definition type is A*  The pointer to 
    void Add()
    {
    
        m_B+=Point_A->m_A;
    }
};

class  A
{
    
public:
    int m_A=10;
};
int main()
{
    

}

At this time, the run will error（field ‘member_A’ has incomplete type ‘A’: Field “member_A” The type of “A” Incomplete ）.

resolvent

1. Will need to be called to class A Functions defined outside the class , And define to class A Behind

#include<bits/stdc++.h>
using namespace std;
class A;   // The statement here can only state  A  Is a class ,  Instead of accessing A Internal members of 
class B
{
    
    int m_B;
    A *Point_A;  // The definition type is A*  The pointer to 
    void Add();
};

class  A
{
    
public:
    int m_A=10;
};
void B::Add()  // Definition Add  function 
{
    
    m_B+=Point_A->m_A;
}

int main()
{
    
    
}

2. Direct will class A Define the class B Before

#include<bits/stdc++.h>
using namespace std; 


class  A
{
    
public:
    int m_A=10;
};


class B
{
    
    int m_B=0;
    A *Point_A;
    void Add()
    {
    
        m_B+=Point_A->m_A;
    }
};

int main()
{
    

}

summary ：

C++ Class can be declared forward . however , Classes that are only forward declared but not defined are incomplete , Such a class , Can only be used to define pointers 、 quote 、 And pointers and references for function parameters , Objects cannot be defined （ Because the compiler only knows that this is a class , I don't know how big this class is ）, And you can't access class objects in any form （ Because the compiler doesn't know class A What kind of members does it contain ）.