[C++-sig] Re: Boost.Python and GCC 3.4.2

[David Abrahams]

Patrick Hartling <patrick.hartling at gmail.com> writes:

> The code that GCC is complaining about is the type cast in the last
> term of the last statement in the following:
>
> template <class Data>
> struct additional_instance_size
> {
>     typedef instance<Data> instance_data;
>     typedef instance<char> instance_char;
>     static const std::size_t value = sizeof(instance_data) -
> ((size_t)(&((instance_char *)0)->storage));
>
>
> };
>
> I haven't run into this problem before, and I am not sure how to go
> about fixing it (and submitting a patch if appropriate).  I did find a
> semi-related discussion on another mailing list with a reference to
> the relevant part of the C++ Standard:
>
>    http://www.cygwin.com/ml/ecos-discuss/2003-02/msg00214.html
>
> I have seen reports from other people on this list saying that they
> are using GCC 3.4 to compile Boost.Python without problems, so I may
> be doing something wrong.  I have attached some simple test code that
> reproduces the compiler error.  Is there a way to work around this, or
> is it a GCC bug?

It is a GCC bug, but not where you think.  This expression:

   (&((instance_char *)0)->storage)

is generated by GCC's C library sizeof() macro.  sizeof is required to
return an integral constant, by whatever means.  So the GCC people
need to stick in some special compiler magic to make it happy.

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