Forward declarations of unnamed struct

Question

Bounty question: So, these two Foos aren't the same thing. Fine. The second form is given in a library. How do I forward-declare it given that I can't change it?

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I always thought C and C++ allowed repeated declarations provided that there were no repeated definitions. Then I came across this problem when trying to write C++ code which extends a C library.

struct Foo;
typedef struct {} Foo;

This gives the following error:

'struct Foo' has a previous declaration as 'struct Foo'

I want to forward-declare, darn it! What's wrong here?

Answer 1

typedef-ing anonymous struct is a practice that pre-dates C++03 and is mainly oriented to retain compatibility with pre-C99 compilers.

Given that this is 2011, and that both C++ and C are changed, I wonder why there is no more up-to-date version of such a library!

If it is not in development anymore, you cannot "leave", but just "survive" and change it is the way to do that. If still in deployment, submit the issue to the development team.

If you need a workaround, consider that struct can inherit. So, write a forward declaration like

struct MyFoo;

and define it as

#include "old_library.h"
struct MyFoo: public Foo {};

And in all your code, forget about Foo and always use MyFoo.

Answer 2

You're declaring two different entities with the same name. The first, struct Foo, is a struct named Foo. The second is an alias for an anonymous struct.

If you do instead:

struct Foo;
struct Foo {};

It works, because you're declaring a struct named Foo in both situations.

You cannot forward declare anonymous structs. You're left with two choices: include the whole definition, or change the header and name the struct.