CODEWITHSUNDEEP
c++data-structuresdependencies
Artaza Sameen
Artaza Sameen

Reputation: 787

What is the workaroud to Types that has a cyclic relationship?

I have three self defined data types.

- struct A

- struct B

- union U

And here is how they are related to each other.

union U; // This is a forward declaration

struct A{
    U u;
};

struct B{
    A a;
};

union U{
    B b;
    int integer;
};

And I end up getting the same type of error

error: field 'type_union' has incomplete type 'Union'

I've tried various types of forward type declarations. But, one of these types end up being a incomplete type.

There is a cyclic dependency between these types that seems impossible to work. How can I make them work. OR any other workaround.

Upvotes: 0

Views: 46

Answers (1)

robthebloke
robthebloke

Reputation: 9678

In this case, you cannot make this work. U literally contains an instance of itself!!

Other than that, the rules are:

union U;

struct A{
    U u; ///< this can't work. sizeof U needs to be known!
};

union U;

struct A{
    U* u; ///< this can work, pointers are of known size!
};

Forward declaration only really work in this context, if you are defining a pointer to the unknown object. Your case breaks down horribly here:

union U{
    B b; //< B contains A, which contains U, which you are defining!
    int integer;
};

In your case you'd have to break one of the dependencies via a pointer & forward declaration. However, something seems a little fishy with your design here? Possibly rethink your design instead?

struct A{
    U u; //< either this has to be a pointer
};

struct B{
    A a; //< or this
};

union U{
    B b; //< or this
    int integer;
};

Upvotes: 2

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