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

Daniel Berlin
2002-04-06T06:30:29Z

> > > - 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'
> }
>
>
>
>

I forgot to say, in general, you can come up with enough crazy lookup
requirements for each language we support/want to support that it just
makes sense to have the lookup function (by lookup function I mean
whatever you call when you see the "." or "->" to try to get a symbol
out of the environment) be a function pointer, filled in by whatever
created the environment.

For GDB internal symbols (IE $a = 5), we have a simple lookup.
For the "global, all enclosing" environment, you probably want a lookup
function that figures out the environment of the symbol you are trying
to access, and then just looks there.
None of these should have to care about what language that environment
is, they should just get the "right" answer.

I'm saying whatever thing creates the environment and puts symbols in it
knows what language these symbols represent, and thus, can best set the
lookup function.

For the hypothetical mixed language environments (IE you find some way
to embed java and C++ in the *same* frame or something, such that you
need to install multiple language symbols in a single environment, they
really are in the same exact scope), you'd just have an environment with
two sub environments, and a lookup function that looked at the current
language setting/something else to determine which one to look in first.

--Dan