Re: [boost] [Review:Algorithms] Order of args to clamp

Subject: Re: [boost] [Review:Algorithms] Order of args to clamp
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2011-09-25 18:15:01

AMDG

On 09/25/2011 02:56 PM, Jeffrey Lee Hellrung, Jr. wrote:
> On Sun, Sep 25, 2011 at 10:48 AM, Dave Abrahams <dave_at_[hidden]> wrote:
> 
>> So, do the exercise.  And, BTW, which of the types involved is "the same
>> type" in this case?
>>
> 
> Okay, I'll try.
> 
> clamp(T x, L lo, H hi) -> common_type<T,L,H>::type
> 
> Let U = common_type<T,L,H>::type.

I think I prefer the signature in your earlier post:

template< class T, class L, class U >
typename common_type< T const &, L const &, U const & >::type
clamp(T const & x, L const & lower, U const & upper);

It would be better to end up with an l-value and
avoid copying when possible.

> Precondition:  operator< defines an ordering on objects of type U.   For any
> 2 objects a and b of types A and B, respectively, where A and B are each one
> of {T,L,H}, a < b is equivalent to (U)a < (U)b.  !(hi < lo).

This should be !(U(hi) < U(lo)).
Your restrictions on the comparison are
too strict.  You don't want to constrain
(lo < lo), (hi < hi), (hi < lo), or (lo < hi),
because that would exclude T = std::string,
L = H = const char*.

> [Note: I'm not sure yet precisely what "ordering" would be desired here
> (probably a "strict weak ordering" is sufficient, as for many other STL
> algorithms, but I confess I'm a bit rusty on various ordering properties),
> and it should certainly be specified if you want to be precise, but it's the
> same ordering as would be required for clamp(T,T,T), so it's somewhat
> tangential to this exercise.]
> Returns:  If x < lo or hi < x (these are mutually exclusive assuming an
> appropriate precise ordering), returns (U)lo or (U)hi, respectively.
> Otherwise, returns (U)x.
> [Note: I believe the above requirements yield at least 2 nontrivially
> different implementations: (x < lo ? lo : hi < x : hi ? x) or (hi < x ? hi :
> x < lo ? lo : x).]
> 
> Is this what you had in mind?
> 

In Christ,
Steven Watanabe