将参考更新为类成员

Question

I'm pretty sure I'm misunderstanding references.

In the code below, I'd like to initialize A to have a state, but at some time in the future, when I call init (registering a B object to A), I'd like stateB variable to show the value of the B state.

I can do this by making stateB a pointer, then initializing some memory in A's constructor and setting it to the default, then moving this pointer and deleting the memory when calling init.

Can I do this without stateB being a pointer by changing the reference of stateB to be b->state?

Class A {
public:
  int stateB = 0;
  B *b = nullptr;
  void init(B *b) {
     &state = &b->state;
  }
}

class B {
public:
  int state = 1;
}

Answer 1

First off, this line is invalid:

&state = &b->state;

You cannot assign to the address of state . All you can do is state = b->state , but that will not meet all your required semantics. That leaves you with a couple options. You can make stateB a pointer:

class A {
    int* stateB;

public:
    void init(B* b) {
        stateB = &b->state;
    }
};

But you cannot make stateB a reference. References must be initialized and can never be reassigned. So if you want your A to be reinitialized to multiple different B s, then you'll have to stick with a pointer. However, if you want to effectively bind A to a single B , then a reference is fine:

class A {
    int& stateB;
public:
    A(B& b) : stateB(b.state) { } // must be initialized
                                  // in the ctor
                                  // this A can only ever point to this
                                  // specific B
{ };

Note that both of these solutions can suffer from a dangling pointer/reference if the B goes out of scope while you still have the A that is referring to its state. One solution to that problem would be to simply use shared_ptr :

class A {
    std::shared_ptr<int> stateB;

public:
    A(std::shared_ptr<B> b)
    : stateB(b, &b->state)  // "aliasing" constructor
    { }
};

That makes sure that the B will stay alive at least as long as the A .