Re: general c++ question

[Jordi Gutierrez Hermoso]

On 30/06/07, Shai Ayal <address@hidden> wrote:

> I want to declare a function returning the figure::figure_properties
> class.

Ok.

> This function will have to be declared before
> figure::figure_properties is declared.

Why? Oh, never mind. I'm sure there are good reasons to do this.

> This is usually done using forward declarations -- i.e. I would
> expect he following to work:

> class figure;
> class figure::figure_properties;
> figure::figure_properties test();

This won't work unless you have already given the full definition of
figure and you specified in that definition that figure_properties is
a subclass of figure.

C++ doesn't allow partial class definitions, which in a sense is what
you're attempting to do, and the reason is that otherwise the compiler
wouldn't know how much space to allocate for a class when declared if
later on you could potentially add more stuff to that class.

Since the only reason you might want nested classes, besides for a
comfortable syntax, is to grant the nested class access to the private
members of the enclosing class[1], what you can do is put both figure
and figure_properties in the same namespace and declare friendships in
order to give you the same access.

    namespace fig{
        class figure;
        class properties;
    }

    // Later on...

    class fig::figure{
      // ...
      friend class properties;
    };

    // Class definition must come after figure::base in order to have
    // explicit friendship declared first by figure::base
    class fig::properties{
      // ...
    };

Then you can return fig::properties from your
function. Unfortunately, now the base class figure is called
fig::figure, but you can typedef that away if you want to.

HTH,
- Jordi G. H.

[1] I think some versions of the C++ standard deny nested classes this
kind of access privilege, but the GNU implementation of C++ allows it.
At any rate, it will be allowed for C++09.