[boost] Re: std_min/std_max reference return value is problematic with boost::interval

From: Guillaume Melquiond (guillaume.melquiond_at_[hidden])
Date: 2004-03-08 14:13:00

Le lun 08/03/2004 à 18:53, Michael Stevens a écrit :
> Eric,
> 
> The recent changes to the new std_min/std_max caused some unexpected problems 
> in uBLAS. I have narrowed them down to support for boost::numeric::interval 
> in uBLAS. This is picked up by ublas/libs/test7. This test is however not at 
> present part of the regression run.
> 
> The new std_max (or std_min) is implemented by returning a constant reference 
> to the return value of std::max. This should be OK as std::max similarly 
> returns a const reference to one of its const reference parameters.  For gcc 
> this causes some problems when the type is a boost::numeric::interval. The 
> test case is very simple.
> 
> #include <boost/numeric/interval.hpp>
> #include <boost/minmax.hpp>
> 
> int main()
> {
> 	boost::numeric::interval<float> a,b,c;
> 	c = boost::std_max(a, a);
> }
> 
> gcc (2.95.3 and 3.3.1) is only warning "returning reference to temporary". I 
> am surprised this doesn't show up in more cases. Why gcc thinks it needs a 
> temporary here is beyond me

Hi,

The reason is: the return type of the function max(interval const &,
interval const &) is not a constant reference, it is a temporary (since
the maximum of two intervals is not one of these two intervals when they
overlap). Consequently, since std_max has been defined so that it
returns a reference, it can't be used when one argument is an interval
and GCC is right to complain.

Consequently, having std_min and std_max functions was not a good
solution. Is it possible to have macros that behave like them (that way
there would be no return type problem)?

However, in the particular case of uBLAS, the problem lies elsewhere
imho. At lines 620-621, 688-689, 753-754 of ublas/traits.hpp in the
equals function, uBLAS should not be using the norm_inf function. Indeed
norm_inf relies on abs(interval); and what is really needed is a
norm_inf_for_equals function that would rely on norm(interval). The same
problem will also appear later with complex intervals.

This solution should solve the issue with uBLAS. But the more general
issue of the return type of std_min and std_max is still here.

Regards,

Guillaume