opencxx-users

[Opencxx-users] constant expression analyzer and 'sizeof'

[Stefan S. <sse...@ar...>]

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.

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

[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

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

[Grzegorz J. <ja...@ac...>]

Hi Stefan,

David Abrahams wrote:
> Stefan Seefeld <sse...@ar...> writes:
> 
> 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.

I have already made this plea, and we have already had a discussion on 
opencxx-users, but let me try again.

Stefan, please reconsider relicensing your C++ introspection code under 
license less restrictive than LGPL.

This code was derived from OpenC++, and as you and me agreed, it would 
benefit both projects if this code could be reused and, ideally, shared.
Unfortunately, due to the fact that your code is under LGPL, your 
contribution cannot be merged into OpenC++, without disturbing its less 
restrictive license.

LGPL has drawbacks, please consider the fact that Boost has rejected it.

Currently OpenC++Core subproject within OpenC++ is working towards goal 
of creating C++ introspection and refactoring library. We were 
discussing it a lot, and you know that both your efforts and OpenC++ 
Core could benefit from cooperation.

Please reconsider relicensing your C++ introspection code under license 
as lax as OpenC++ license or Boost Software License.

Still hoping for cooperation
Grzegorz