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From: taw@pb220.legnica.sdi.tpnet.pl (Tomasz Wegrzanowski)
Newsgroups: comp.lang.ada
Subject: Re: type Foo_ptr in new void*;
Date: 31 Jul 2001 01:21:50 GMT
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In article <3B64C26F.C195B4E0@worldnet.att.net>, James Rogers wrote:
> Yes, you are correct. C does allow you to forward declare a structure
> or a pointer without ever completing the definition. This is one of
> the nasty capabilities "provided" by C. You can even create an array
> of Foo *. However, pointer arithmetic from the start of this array
> produces unspecified behavior. What is the sizeof Foo? Since it is
> undefined, there is no correct answer to that question. Without a
> proper definition of the sizeof Foo, there can be no reliable pointer
> arithmetic.

Correct answer is compilation error.
It's illegal to do pointer arith on void* or incomplete types.

> In other words, you can declare some wonderful data structures in C
> based on incomplete type definitions. Unfortunately, a program built
> upon such a structure is neither reliable nor portable.

It's both portable and reliable.

> Ada does not allow you to compile a program with an incomplete type
> definition. There is no equivalent because the existence of an
> equivalent to this C capability would only introduce errors in your
> program. There is nothing positive that can be achieved by allowing
> a program to be incompletely specified.

Sure, if your favourite language has no feature X,
then feature X is useless.

Incomplete types warrant opacity.