Mutually dependent constexpr constructors

Question

I have two classes, each constructible from the other.

Example:

class B;

class A{
 public:
  double val;
  constexpr A(B b): val(b.val){};
};

class B{
 public:
  double val;
  constexpr B(A a): val(a.val){};
};

I need to forward-declare class B, so A knows about it. When these constructors are not constexpr, I can move their definitions to a source file and it happily compiles.

However, to make it constexpr, they have to be defined in the header. B is ok to construct from A, because it sees the full definition of A. A cannot construct from B because it only sees a declaration, and therefore has no idea about B::val.

I'm left with only making class B constexpr. Is there a way to do it for both classes?

Answer 1

Using gcc I get the error (https://godbolt.org/z/qvP7absdr):

<source>:6:15: error: 'b' has incomplete type
    6 | constexpr A(B b) : val(b.val){};
      |             ~~^
<source>:1:7: note: forward declaration of 'class B'
    1 | class B;
      |       ^
<source>: In constructor 'constexpr A::A(B)':
<source>:6:11: error: invalid type for parameter 1 of 'constexpr' function 'constexpr A::A(B)'
    6 | constexpr A(B b) : val(b.val){};
      |           ^
Compiler returned: 1

So this fails because type B is incomplete when it is used it is the definition of the constructor A::A(B b).

In order to deal with this we can wait until we have declared B fully before we define the constructor and use B. Essentially, move the definition of the constructor out of class A and after class B

class B;

class A{
 public:
  double val;
  constexpr A(B b);
};

class B{
 public:
  double val;
  constexpr B(A a): val(a.val){};
};

constexpr A::A(B b) : val(b.val){};

See an example without compilations issues: https://godbolt.org/z/44fbcr8sh