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

From: Eric Niebler (eric_at_[hidden])
Date: 2004-03-08 14:08:51

Michael Stevens wrote:
> 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

Yup, Dave anticipated this problem, and fixing it is on my To-Do list. 
The problem is due to the fact that max is overloaded for the interval 
type to return a non-reference:

template<class T, class Policies> inline
interval<T, Policies> max (const interval<T, Policies>& x,
                            const interval<T, Policies>& y)
{
   typedef interval<T, Policies> I;
   if (interval_lib::detail::test_input(x, y))
     return I::empty();
   return I(std_max(x.lower(), y.lower()), std_max(x.upper(), 
y.upper()), true);
}

The fix is to detect whether the unqualified call to max returns an 
lvalue or not and define the return type of std_max appropriately. Easy 
enough.

What strikes me as odd with this overload, though, is that max(a,b) in 
this case will return something that is not equal to either a or b. That 
seems counter-intuitive. Is this right?

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