The following code failed to compile because I cannot call b->check() from A. Is there any way to do it? Thanks! class B; class A { public: void fA(B* b) { b->check(); } void check() { cout<<"This is A."<<endl; } }; class B { public: void fB(A* a) { a->check(); } void check() { cout<<"This is B."<<endl; } }; int main(void) { A a; B b; a.fA(&b); b.fB(&a); getchar(); return 0; }

Can you repost the code in source tags? It will make it easier to read. Also, post the compiler errors.

Put the definition of one of the member functions in a source file. You can't call a member function on something that you only have a forward declaration for.

Certainly. However, you will have to place your implementation of the classes in seperate source files.

In order to call B::check from A::fA(), the compiler must have seen the definition of B. Likewise, in order to call A::check from B::fB(), the compiler must have seen the definition of A. A forward declaration is just a declaration, not a definition.

The only way you can have this work is to place the definition of A's member functions in their own file (say, A.cpp) and those of B in their own file (say, B.cpp). Both of the .cpp files can #include "A.h" and "B.h" (but A.h and B.h should only provide forward-declarations of B and A, respectively, and not try to include their headers).

The better solution, however, is probably to avoid having a design that leads you to require circular dependancies among classes.

Got it! Thank your for your help!

Circular dependencies generally indicate poor design. They're bug-prone, difficult to debug, and complex to maintain.

You should definitely try to see if you can refactor it out somehow; you'll run into lots of trouble, particularly when it comes time to clear memory or have other people maintain your code.