Re: C++ nested classes, namespaces, structs, and compound statements

[Daniel Berlin <dan at dberlin dot org>]

On Sat, 6 Apr 2002, Michael Elizabeth Chastain wrote:

> I'll bite.
> 
> > In summary, the data structure GDB needs to represent C++ structs
> > (classes, unions, whatever) has a lot of similarities to the structure
> > GDB needs to represent the local variables of a compound statement.
> 
> Sounds reasonable to me.
> 
> It also sounds dangerous.  It's true that namespaces, structs,
> and compound statements are all identifier binding contexts.
> But if you start treating a struct as a type of compound statement
> you could get into a maze of twisty forced meanings.  You have to
> reach down and create a new paradigm and then port both structs
> and compound statements to it.
> 
> Think about how much context information an identifier-binding-object
> needs to do its job.  I think it would be difficult to come up with a
> universal context object that both structs and compound statements
> can use.  Each identifier-binding-object has its own specialized
> context requirements.
> 
> > - And what about ambiguous member names?
> 
> The C++ language spec says: if class A inherits from both class B and
> class C, and both B and C have a member "foo_", then an unqualified
> reference to a.foo_ is illegal.  The programmer has to say a::B.foo_
> or a::C.foo_.

Not quite.

Watch this cuteness, copied from the C++ draft standard:

struct U { static int i; };
struct V : U { };
struct W : U { using U::i; };
struct X : V, W { void foo(); };
void X::foo()
{
	i; //Finds U::i in two ways: as W::i and U::i
	   // but no ambiguity because U::i is static

}


"A static member, a nested type or an enumerator defined in a base class T 
can  unambiguously be found even if an object has more than one base class 
subobject of type T.  Two base class subobjects share the non-static 
member subobjects of their common virtual base classes"

In other words, it's not just statics.
Observe:

          class V { public: int v; };
          class A {
          public:
              int a;
              static int   s;
              enum { e };
          };
          class B : public A, public virtual V {};
          class C : public A, public virtual V {};
          class D : public B, public C { };

          void f(D* pd)
          {
              pd->v++;         // ok: only one `v' (virtual)
              pd->s++;         // ok: only one `s' (static)
              int i = pd->e;   // ok: only one `e' (enumerator)
              pd->a++;         // error, ambiguous: two `a's in `D'
          }