Correct way to forward declare structs? [duplicate]

Question

Let's suppose I want to declare structs A and B

struct A{
  B toB(){
    return B();
  }
};
struct B{
  A toA(){
    return A();
  }
};

I'll get an error that type B is undefined

main.cpp:2:2: error: unknown type name 'B'

What is the correct way to forward-declare B?

Doing the following

struct B;

struct A{
  B toB(){
    return B();
  }
};
struct B{
  A toA(){
    return A();
  }
};

results in

main.cpp:4:4: error: incomplete result type 'B' in function definition
main.cpp:1:8: note: forward declaration of 'B'
main.cpp:5:10: error: invalid use of incomplete type 'B'
main.cpp:1:8: note: forward declaration of 'B'

This forward declaration is fine, this has nothing to do with forward declarations. Unlike Java or C#, in C++ one does not have to define class constructors and class methods inline. The definition of the actual constructor can be moved to after both classes are defined, resolving the issue with the incomplete type. — Sam Varshavchik
– Sam Varshavchik, Commented Jan 3, 2024 at 14:24
this answer on the dupe target shows how to split this up into pieces that can be stitched together when compiling. — NathanOliver
– NathanOliver, Commented Jan 3, 2024 at 14:27

ChrisMM · Accepted Answer · 2024-01-03 14:26:53Z

2

The issue isn't with the forward declaration, the issue is that you're trying to use a B before its definition. A forward declaration only tells the compiler that there is, in your case, a struct named B, it doesn't define it.

You can separate the definition of the method from the class, like so:

struct B;

struct A{
  B toB(); // B is not defined yet
};

struct B{
  A toA(){
    return A();
  }
};


B A::toB() { // Now that B is defined, you can use it
    return B();
}

answered Jan 3, 2024 at 14:26

ChrisMM

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