[Opencxx-users] Re: [Synopsis-devel] constant expression analyzer and 'sizeof'

[David A. <da...@bo...>]

Stefan Seefeld <sse...@ar...> writes:

> Hi there,
>
> I'm working on a constant expression analyzer
> to assist the C++ parser. It's working nicely
> so far. Here is what it already can deal with:
>
> http://synopsis.fresco.org/viewsvn/synopsis-Synopsis/trunk/tests/OpenCxx/Con
> stEvaluator/
>
> However, some cases are not as simple, and so
> I'd appreciate feedback: the 'sizeof' operator
> can be used in const expressions, and so I have
> to deal with it. 
> AFAIK each compiler has its own memory
> layout, and thus there's no standard algorithm
> to determine the size of an arbitrary type.
>
> On the other hand, there is no requirement for
> Synopsis / OpenC++ to be binary compatible with
> any other compiler, so may be this is a non-issue.

It's still an issue.  There are some things that are constant:

  sizeof(char) == 1
  sizeof(char[2]) == 2
  sizeof(char(&)[2]) == 2
  sizeof(char[3]) == 3
  sizeof(X[N]) == N * sizeof(X)

There are programs that rely on these relationships.

> Still, I need to provide some numbers when asked
> for the size of an object. Any ideas how to approach
> this ? Any references are highly appreciated !
>
> Regards,
> 		Stefan

You're going to need to build in full overload resolution and
template instantiation capability, which is no small job.

consider:

      template <class T>
      typename something<T>::type f(T&);
      
      char f(...);

      sizeof(f(g()));

If g returns a value, the result is 1.  If g returns a reference,
you'll call the first f, and you'll instantiate something<T> to find
out its return type.  Before you object, this sort of thing is very
common in the Boost libraries.

Even though it's a big job, it would be a great service to the C++
community.  I could probably find people to help you with it.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com