[boost] Re: win32.hpp and min/max macros

From: Eric Niebler (eric_at_[hidden])
Date: 2004-02-24 12:31:10

Guillaume Melquiond wrote:

> 
> Is the interval library the only Boost library allowing min and max
> functions to be used with user-defined types? How is this problem dealt
> with in the other libraries.
> 

Most of the boost librarys makes qualified calls to min/max, as in 
std::min(a,b), and so do not use Koenig lookup. I don't think you should 
change anything, though. ADL is working for you and your users, and I 
don't want to muck that up.

> 
>>And the regression test with VC7.1 didn't expose this 
>>problem. (Shortcoming of vc7.1 or the interval regression tests?)
> 
> 
> Yes it is a shortcoming of the interval regression tests (that doesn't
> mean it isn't also a shortcoming of VC7.1). I only test for things that
> could break in the current code, not for things somebody else could
> change afterward :-).
> 

IMO, I think you should add a test for this. It's a valid usage scenario 
which you support. You (and I, as a maintainer) want to know when it breaks.

> 
> Wasn't there the idea of adding prefix and suffix headers to Boost? If
> it was undef'd or pragma_push'd for all the libraries, it would avoid
> changing these 800 occurrences. Or am I missing something?
> 

Not all compilers support push_macro/pop_macro, and unconditionally 
removing users' macros is impolite, considering there are ways we can 
make boost code impervious to these macros.

>>Another thought just occured to me. Consider:
>>
>>#define BOOST_EMPTY
>>template< class T >
>>void f(T const &a, T const &b) {
>>   using std::min; min BOOST_EMPTY (a,b);
>>}
>>
>>This seems to be enough to prevent min() macro substitution, and it 
>>would preserve ADL. I checked with VC6, VC7.1, gcc (cygwin) and with 
>>Comeau Online, and they all like it. Will this work on other compilers too?
> 
> 
> It's ugly. But I suppose we could use a boost_min macro to avoid
> cluttering the code (if it still works).
> 

I agree it's ugly. But not as ugly as the current solution in win32.hpp, 
which can silently change the meaning of users' code. You don't need a 
boost_min macro -- you can define your own interval_min/interval_max 
templates as follows:

#define BOOST_EMPTY
template< class T >
inline void interval_min(T const &a, T const &b) {
    using std::min; min BOOST_EMPTY (a,b);
}

and now use interval_min instead of min everywhere. Perhaps we can do 
this once and call it boost_min, so everybody can use it. Opinions?

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com