🚀 go-pugleaf

steve@taumet.com (Steve Clamage) recently wrote about forward declaration
of nested classes:

> You do it the same way you handle any other forward declaration.
> You have to be careful about scopes.
> 
> 	class A {
> 	  public:
> 	    class C;	// forward declaration of A::C
> 	    class B {
> 		C *p;	// pointer to A::C
> 	    };
> 	    class C {
> 		B *p;	// pointer to A::B
> 	    };
> 	};
> 

How about incomplete types and nested classes? What happens if we skip
the first declaration of class C? I ran into the problem when compiling
some old C code.

Section 9.1, paragraph 2 of the draft says: "A class declaration
introduces the class name into the scope where it is declared and hides
any class, object, function or other declaration of that name in an
enclosing scope."

Paragraph 3: "An elaborated-type-specifier can also be used in the
declarations of objects and functions. It differs from a class
declaration in that if a class of the elaborated name is in scope the
elaborated name will refer to it." 

What happens if no class with that name is found in scope???

Paragraph 4 has the following example: "A name declaration takes effect
immediately after the identifier is seen. For example,

	class A * A;

first specifies A to be the name of a class and the redefines it as
the name of a pointer to an object of that class."

I guess this means that class A is introduced into the current scope if
not defined earlier.

Example:

 	class A {
 	  public:
 	    class B *bp; // pointer to A::B or B?
	    f(void);
 	};

 	class B {
	  public:
 	    int memb;
 	};

	A::f()
	{
	    bp->memb = 4711;	// Is this legal?
	}

When the elaborated class specifier for B is seen, should the name be
introduced in the scope of class A (as in a nested class declaration)
or the scope enclosing A?

I think that the declaration 'class B *bp;' should introduce (a nested)
class B in the scope of A. The declaration of class B will introduce a
new type in the file/global scope. When 'bp' is used in A::f() it is a
pointer to the local class B in A's scope, which still is incomplete.
Since both Cfront and gcc accept the above declarations without complaint
I assume I've misinterpreted the standard. Could someone, please,
straighten things out for me?

> 
> Steve Clamage, TauMetric Corp, steve@taumet.com
> Vice Chair, ANSI C++ Committee, X3J16

							/Jan-Erik Dahlin
--
"Sometimes the magic works and sometimes it doesn't."
 Chief Dan George of the Sioux Indian tribe, after an unsuccessful rain dance.