Re: Access to function returning class-wide type

Adam Beneschan wrote:

Others have posted discussions of how to accomplish what you want, or
what we think you might want, using a different construct.

I watched comp.lang.ada for little time, searched through posts which I got
and I belive I didn't see that. I'm sorry if I browsed to short.

I just wanted to point out that, from a low-level standpoint, there's
no way to have an access type that could refer to *both* a function
that returns Abstracted'Class *and* a function that returns the
specific type Derived. The reason is that an access-to-function is
basically just going to hold the address of the function (and some
other information). A function that returns T'Class can't work the
same as a function that returns a specific type, because the function
that returns T'Class won't know in advance how big the return object
is going to be, so some dynamic heap (or "secondary stack") allocation
must be involved. A function that returns Derived, though, knows how
big the return object is, so the result can be pre-allocated. This
means that both the function and the function's caller have to adopt
different mechanisms for passing the return object between them. If
you have an access-to-function which could point to either one, the
code that makes a call through this access-to-function pointer isn't
going to know which return object mechanism would be used, so the
result would be a mess.

If Ada were to provide a mechanism so that B.Proc'Access could be
used, the compiler would have to generate its own wrapper, same as
you're doing. This isn't necessarily a bad thing; there are already
other cases where Ada compilers have to generate wrappers (at least
ours does). But I think it's useful to understand the underlying
mechanisms involved, to get a better understanding of why the language
won't let you do certain things.

I think I understand what you want to show me. As I posted in other thread
[see news:g8jf3n$e00$1@xxxxxxxxxxxxxxxxxxxxxxxxx ], I've made a test with
type/subtype and it's failing as well. There is a more obvious collision
there, we expect a function which doesn't do dynamic range check on
returned object, and we pass a function which does.

By the way, if you decide that the other suggestions don't work and
you need to write a wrapper anyway, you shouldn't have to rewrite the
whole wrapper for every type; I think you can use a generic to make
the process of declaring the wrapper a lot simpler.

As I've shown in original post, the wrapper needs to consist only a return
statement from 'main' constructor. I'll try to do it with generic function
as you suggest, but in the worst case the difference will be... well...
small ;-)

--
Paweł Płazieński aka Nivertius
"In the end, there will be Ada, XML and gzip"
.

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