#include <list> 

class A; 

class B
{
	B();
public:
	A* a; //only this use is allowed with only a forward declaration, without the actual declaration
	//A a2; // this would require the declaration of A
};


//actual declaration of class A:
class A
{ 
public: 
	A();
	int i;
	typedef std::list<int> AiLst;
};



int main()
{ 
	// using something else than declaring a pointer to A is allowed now.
	A::AiLst lst;
	A a;
	return 0; 
} 

// this is the definition and can come later in the same translation unit.
A::A():i(0){}