Re: [boost] Concept Traits Library

Subject: Re: [boost] Concept Traits Library
From: Jeffrey Lee Hellrung, Jr. (jhellrung_at_[hidden])
Date: 2010-07-10 18:31:41

On 7/10/2010 12:49 PM, Frédéric Bron wrote:
[...]
> What I can say is that there is some differences between the traits I
> proposed and the one of the CTL: in CTL, there is no check of return
> type. We had quite a long dicussion about this and ended to a nice
> solution which is to check or not (optional) for the return type to be
> convertible to a given type.

I've found all 3 of the following options to be useful in my own (C++03) 
implementation of operator traits:

(i) ignore the return type of the operator entirely
(ii) check that the return type of the operator is convertible to a 
given type
(iii) check that the return type of the operator satisfies a given MPL 
metafunction class or MPL lambda expression

As a result, the trait's signature looks something like

template<
     class T,
     class Result = void,
     class ResultCond = mpl::always< true_type >
 >
struct my_trait;

The trait evaluates to true if the result type of the operator
- is convertible to Result or Result is void; and
- satisfies ResultCond.

Of course, option (iii) is a generalization of (ii), but (ii) (along 
with (i)) would be the common case (and hence providing an interface 
specifically for (ii) would be more client-friendly).  A use case for 
(iii) might be to check that the return type is *precisely* the given 
type, or if the return type is one of many 
compatible-but-not-necessarily-interconvertible types.

> If we agree to add only the operator traits to the type_traits library
> separatly from the CTL, I think that my proposal is nearly ready and I
> can do the extra work. If we want to add the CTL, it is better if
> someone else does it as it would be too much work for me.

Is your proposal available for inspection at the moment?  I don't recall 
recently running across a link that you've provided.  Sorry if I missed 
it :/

- Jeff