我该如何处理C ++中的循环组合？

Question

I'm trying to create a cyclic composition in C++ but I'm dealing with declaration problems. How could I solve them?

This is an example, class A contains a vector of B objects, but class B needs A to be declared first because it's needed in its constructor:

class A {
private:
    std::vector<B> sons;
public:
    void create_son() {
        B obj(this);
        sons.push_back(obj);
        obj.some_method();
    }
};

class B {
private:
    A* parent;
public:
    B (A* _parent) { parent = _parent; }
    void some_method() {}
};

Answer 1

In class A , you use object of class B , so the complete definition of class B is needed. To solve this, put class B definition above class A . At the same time, in class B you work only with pointer to A, so you don't need the complete definition of class A : declaration is enough there.

So, add forward declaration of class A above class B definition.

class A;

class B {
private:
    A* parent;
public:
    B (A* _parent) { parent = _parent; }
    void some_method() {}
};

class A {
private:
    std::vector<B> sons;
public:
    void create_son() {
        B obj(this);
        sons.push_back(obj);
        obj.some_method();
    }
};

Answer 2

You can forward declare pointer types:

class A;   // sufficient to fully determine B

class B {
private:
    A* parent;
public:
    B (A* _parent) { parent = _parent; }
    void some_method() {}
};

// now we have B defined, we can define A
class A {
private:
    std::vector<B> sons;
public:
    void create_son() {
        B obj(this);
        sons.push_back(obj);
        obj.some_method();
    }
};