ForwardDeclaration

#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){}