forward_declaration.cpp

class B;

template<class T>
class A {
    B* bp;
public:
    T x;
    A(T _x) {x = _x; bp = new B(); }
    T f() {
        return bp->f();
    }
};

class B {
    A<int>* ap;
public:
    int f() {
        return 0;
    }
};

int main() {
    A<int> a(0);
    return a->f();
}