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c++forward-declaration

Why incomplete type (forward declared) size calculation cannot be postponed when using type as a field?

In the following code:

class B;

struct A
{
   B* b; // <- why MUST be a pointer? Why size cannot be calculated later...?
}

struct B
{
   ...
}

As far as I understand, struct A must define b as B*, as the compiler cannot tell the size of B when calculating size of A.

What I do not understand, is why the compiler cannot postpone the calculation until it does find the full definition of B (as it was promised by the programmer by forwarding declare the class)

Solution

  • Imagine that you could actually define A as:

    class B;

struct A {
   B b; // not a pointer
}

    That is, b is a data member of incomplete type – the compiler doesn't know the size of B at this point.

    Then, if B were defined as:

    struct B {
   A a;
}

    This would require infinite memory.