Re: [boost] Range-aware version of min and max

Subject: Re: [boost] Range-aware version of min and max
From: Niels Dekker - address until 2010-10-10 (niels_address_until_2010-10-10_at_[hidden])
Date: 2010-01-25 11:19:27

Zachary Turner wrote:
> I am often frustrated by the inability of std::min and std::max to
> deal with data types of different ranges.  For example:
>
> boost::uint64_t big = 9876543210ULL;
> boost::uint32_t small = 12345;
>
> boost::uint32_t smallest = std::min(big, small);
>
> will generate a compiler warning about possible loss of data due to
> converting from boost::uint64_t to boost::uint32_t.

It doesn't seem to compile: http://codepad.org/IiFba8MF  I guess you mean:

boost::uint32_t smallest = std::min<boost::uint32_t>(big, small);

> Previously there was a discussion on the list about adding operators
> such as is_addable, is_multipliable, is_less_comparable, etc.  Using
> such a class, one could re-write min and max as follows (...):
>
> namespace boost {
>
> template<class A, class B>
> struct min_result
> {
>   typedef mpl::if_c<(integer_traits<A>::const_max <
> integer_traits<B>::const_max), A, B>::type type;
> };
>
> template<class A, class B>
> struct max_result
> {
>   typedef mpl::if_c<(integer_traits<A>::const_min >
> integer_traits<B>::const_min), A, B>::type type;
> };
>
> template<class A, class B>
> min_result<A,B>::type
> min(const A& a, const B& b)
> {
>   return static_cast<min_result<A,B>::type>((a < b) ? a : b);
> }
>
> template<class A, class B>
> max_result<A,B>::type
> max(const A& a, const B& b)
> {
>   return static_cast<max_result<A,B>::type>((a > b) ? a : b);
> }

Excuse me, where do you use is_addable, is_multipliable, is_less_comparable?

> I'm sure there's some implementation details I haven't considered,
> but is there any fundamental reason why an approach like this would
> be flawed or undesirable?

I think your suggestion is quite reasonable, but I see two drawbacks:

 * It only seems to support numeric types. Why not support the minimum of 
two datetime objects, or the minimun of two std::string objects?

 * It returns by-value. In general, I prefer min/max to return a reference 
to the object itself, instead of a copy.  Like std::min and std::max 
(C++03).

Both drawbacks could be avoided by renaming your function, for example to 
numeric_min_value.

HTH,

  Niels

--
Niels Dekker
http://www.xs4all.nl/~nd/dekkerware
Scientific programmer at LKEB, Leiden University Medical Center