Re: [boost] review request: addition to type_traits library of is_less_comparable<T, U> and others

Subject: Re: [boost] review request: addition to type_traits library of is_less_comparable<T, U> and others
From: Jeffrey Hellrung (jhellrung_at_[hidden])
Date: 2009-10-09 12:18:13

Frédéric Bron wrote:
>> What immediately comes to my mind are expression templates (e.g.,
>> Boost.Proto).  This is also a case where the return type is not necessarily
>> convertible to bool.  I don't think this library should decide whether it's
>> appropriate for a client of this library to test operator< for such types;
>> leave that decision for the client.
> 
> I did not think about expression templates but do you see any reason
> for asking if the expression template is comparable? Normally the user
> do not bother about expression templates (which is an implementation
> detail) but only about the result type which is a "normal" type? I may
> be wrong as I only know expression templates in the frame of uBlas.
> 
> Frédéric

First, I'd like to say I think this would be a useful addition to 
Boost.TypeTraits, and I appreciate your work Frederic ;)

There were 2 points that I raised in response to a previous message that 
I'd like to clearly distinguish:

1) You cannot generally express all comparison operators in terms of 
operator<, or even in terms of a combination of operator< and 
operator==.  Expression templates were one example I thought of (e.g., 
Boost.Proto), but it just seems wrong to me for a library to dictate the 
semantic relationship among the comparison operators.  operator< and 
operator== are enough *most* of the time to get the other comparison 
operators, but I don't think *all* of the time (though a good example 
escapes me at the moment).  I gather you don't take issue with this, as 
you provide metafunctions corresponding to all the comparison operators.

2) At the very least, boost::tribool is a legitimate return value (in 
addition to bool) for any of the comparison operators, which indicates 
to me that the interface should allow specification of the result type 
of a comparison operator.

- Jeff