In a VC dll project, when we export a class which contains a not exported class variable, VC will have a warning(C4251).

For example,

1. class A  
2. {  
3. private:  
4.     int m_i;  
5. };  
6.  
7. class __declspec(dllexport) B  
8. {  
9. private:  
10.     A m_a;  
11. }; 

VC will complain:

warning C4251: 'B::m_a' : class 'A' needs to have dll-interface to be used by clients of class 'B'

To work around this problem, we can change the member variable definition like below:

1. class A  
2. {  
3. private:  
4.     int m_i;  
5. };  
6.  
7.  
8. class __declspec(dllexport) B  
9. {  
10. private:  
11.     A* m_pa;  
12. };