Re: [Boost-build] Python port development

Subject: Re: [Boost-build] Python port development
From: Artyom (artyomtnk_at_[hidden])
Date: 2010-07-09 16:08:06

> From: Steven Watanabe <watanabesj_at_[hidden]>
> To: Boost.Build developer's and user's list <boost-build_at_[hidden]>
> Sent: Fri, July 9, 2010 10:56:32 PM
> Subject: Re: [Boost-build] Python port development
> 
> 
> > How can we  check for example if sizeof(wchar_t) >= 4?
> >  
> 
> There are  many ways to trigger a compiler error in C++.
> In C, yeah, a negative array  size is probably the easiest.
> 
> >    char  test[sizeof(wchar_t)*2 - 7]
> > 
> > If sizeof(wchar_t) == 4 then 4*2-7  == 1 and the code is legal and 
>compilation succeeds,
> > If sizeof(wchar_t)  ==  2 then 2*2 - 7 == -3 and this code is not legal and 
>compilation  fails.
> > 
> > It is very tricky to implement this  correctly.
> 
> How is this particularly tricky?  I can see that it would  get tedious
> is the size is not known to be one of a few possibility (thus  requiring
> a binary search), but it seems like this is rather the obvious  solution
> when running a compiled program isn't possible.
> 

It is not tricky when you know the solution :-).
But as you can see neither your nor Gevorg, did this right.

BTW: why do you need there *2? In order to get negative value as some
     compilers accept 0 as valid array size.

Also it is simple to check one specific case sizeof >= KnownValue,
but lets say you want to know that is the size of some structure,
with unknown expected size.

So you need to write a loop that compiles this for several cases and then run it 
increasing each time
twice and then run binary search... Not so simple at all.

Regards,
  Artyom