Re: std::max(unsigned, size_t), amd64 and C++0x

Hi!

I took the time to think about how a C++0x solution might look like
and I'd like to share my thoughts on this. The rvalue reference
proposal makes the following possible:

     template<typename T1, typename T2>
     void do_something(T1 && x, T2 && y);

where template parameter type deduction will result in:

     int a = 42;
     const b = 23;
     do_something(a, a+3); // T1 = int&, T2 = int
     do_something(b, a); // T1 = const int&, T2 = int&

This enables us to perform some type transformation magic to deduce
the optimal return type of functions like min and max. Consider the
following examples:

     T1 = int&, T2 = int --> int
     T1 = const int&, T2 = int& --> const int &
     T1 = const int, T2 = int& --> const int
     T1 = int&, T2 = int& --> int &

It's clear that the return type must not be a reference unless both
parameters are lvalue references. Regarding cv qualifiers the return
type should merge the cv qualifiers of both arguments to get the most
cv-qualified version.

Suppose this kind of type transformation magic is hidden behind a
templated concept_map for the concept "Selectable":

     template<typename T1, typename T2>
     concept Selectable
     {
         typename result_type;
         requires ExplicitlyConvertible<T1,result_type>;
         requires ExplicitlyConvertible<T2,result_type>;
     };

Then, we'd be able to implement the function 'max' like this:

     template<typename T1, typename T2>
         requires Selectable<T1,T2>
         && LessThanComparable<T2,T1>
     inline Selectable<T1,T2>::result_type max (T1 && x, T2 && y)
     {
         typedef Selectable<T1,T2>::result_type rt;
         return (y<x) ? rt(x) : rt(y);
     }

The explicit conversion of 'x' and 'y' seems necessary because
ConceptGCC complains about returning references to temporaries in
cases where the cv qualifiers differ (ie T1=cost int&, T2=int&).

If you want some more freedom (ie mixing different integral types) it
should be possible with some more concept_maps for Selectable. For
example: a concept map for Selectable<int, long> ... etc ...

Comments?

Cheers,
SG

--
      [ See http://www.gotw.ca/resources/clcm.htm for info about ]
      [ comp.lang.c++.moderated. First time posters: Do this! ]