Interdependent struct declaration

Question

Possible Duplicate:
When to use forward declaration?
C++ classes with members referencing each other

I am pretty new to C++, and I have a question regarding two structures defined below as shown below. Each struct contains a pointer to the other:

    struct A{
    ...
        ...
    B *ptr;
}

    struct B{
    ...
    ...
    A* ptr;
};

However, since the 2nd structure is defined only after the first, I get a compilation error. Is there a solution for this? I tried to declare the struct separately in header files, but it didn't work. Any help is appreciated! Thanks.

Just forward declare `struct B` before `A`, meanwhile I will find the duplicate. — iammilind, Oct 11 '12 at 10:01

score 3 · Accepted Answer · edited Oct 11 '12 at 10:25

3

In C++ in order to have a pointer to a type you don't need complete definition of that type. You can just forward declare it.

struct B;
struct A {
    ...
    struct B* ptr;
};
struct B {
    ...
    struct A* ptr;
};

edited Oct 11 '12 at 10:25

John Dibling

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answered Oct 11 '12 at 10:03

BigBoss

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@JohnDibling "forward declaration" & "declaration" are equivalent in respect to classes. – Luchian Grigore Oct 11 '12 at 10:27
Turned out to be quite simple! Thank you all, appreciate your answers. – Oct 11 '12 at 10:50

score 2 · Answer 2 · answered Oct 11 '12 at 10:01

2

Yes, use a forward declaration:

struct B;  //forward declare B
struct A{
    ...
        ...
    B *ptr;
};

struct B{
    ...
    ...
    A* ptr;
};

Since the members are pointers, a full definition isn't required - a declaration is sufficient.

answered Oct 11 '12 at 10:01

Luchian Grigore

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Interdependent struct declaration

2 Answers2