Circular dependencies in C++ classes using references

Question

I have two objects of classes (A and B) which must be able to refer to each other, e.g.:

class A {
public:
    A(B& b);

private:
    B& b;
};

class B {
public:
    B(A& a);

private:
    A& a;
};

But I can't do this:

A a(b);
B b(a);

With pointers this would be easy, as a pointer can be NULL. How can I achieve the same result using references, or is it not possible?

Forward declarations and separating header + source or think your design over. — nada
– nada, Commented Sep 30, 2019 at 7:48
@nada that doesn't cover the actual construction of the objects, though. — Quentin
– Quentin, Commented Sep 30, 2019 at 7:49
Yes, the construction of the objects is point I can't get past. — Jason
– Jason, Commented Sep 30, 2019 at 7:50
Regarding the construction, it's not simply possible with your current design and implementation. — Some programmer dude
– Some programmer dude, Commented Sep 30, 2019 at 8:01

user7860670 · Accepted Answer · 2019-09-30 10:22:23Z

4

One solution is to pack both of them into third object and initialize at constructor:

class C
{
    public: A m_a;
    public: B m_b;

    public: C(void): m_a{m_b}, m_b{m_a} {}
};

Note that this approach requires class A not to access passed reference to class B at constructor because object B is not initialized at that point.

edited Sep 30, 2019 at 10:22

answered Sep 30, 2019 at 8:02

user7860670

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4 Comments

Quest Over a year ago

Isn't that an UB? passing uninitialized m_b to the constructor of m_a?

user7860670 Over a year ago

@Quest Accessing passed reference will be UB, but just passing a reference (or a pointer) is not.

Jason Over a year ago

This is an interesting approach, thanks, I'll give it a try.

Aconcagua Over a year ago

@Quest Actually, that has been asked in a separate question already...

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