TITLE: Type tags, scope, and forward declarations


PROBLEM: Alan Harper <aharper@dnai.com>

In a project I am working on we have tools which generate C ".h" files which we
then include in our project.  We are trying to switch over to compiling under
C++ (though we will not be using many of the features of C++), and I am finding
that at least one C++ compiler deals with forward "struct <tag> *" references
differently than C does.

The code looks like:
    /*
     *  If I include the following 2 lines, this will compile under Metrowerks
     *  C++, but won't w/o these lines.
     *
     * struct firstTypeTag;
     * struct secondTypeTag;
     */
    
    typedef struct twoPtrsTypeTag {
      struct  firstTypeTag       *ptr1;
      struct  secondTypeTag      *ptr2;
    } twoPtrsType;
    

RESPONSE: clamage@Eng.Sun.COM (Steve Clamage), 11 Jan 95

Right. This is a difference between C and C++. In addition, the
exact meaning of your declaration of struct twoPtrsTypeTag in C++
has been clarified by recent decisions of the C++ Committee. Thus,
it is possible to get different results from different C++ compilers.
Eventually, all C++ compilers will give the same result as the one
you don't like. :-)

Under current language rules, if the first appearance of an
"incomplete declaration", as in your example firstTypeTag and
secondTypeTag, is inside a class, it is a forward declaration
of a nested class name. That is, you have forward-declared the
type names twoPtrsTypeTag::firstTypeTag and twoPtrsTypeTag::secondTypeTag.
The members ptr1 and ptr2 have a type different from the file-scope
structs called firstTypeTag and secondTypeTag.

If you include the file-scope declarations in your comment, then
the declarations inside struct typePtrsTypeTag will refer to the
file-scope structs, as you want.

Happily, this rule is the same as in C.

Moral: In both C and C++, declare file-scope names at file scope,
not in any other scope. You will then not have any compatibility
problems.