Re: [boost] [Proto][Phoenix] Use default transform with SFINAE for expressions?

Subject: Re: [boost] [Proto][Phoenix] Use default transform with SFINAE for expressions?
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2010-09-13 15:03:39

On 13/09/2010 18:03, Daniel Walker wrote:

> What I had in mind for the example is to illustrate the feasibility of
> a concept checking library that issues substitution failures rather
> than compilation errors when a type doesn't model a required concept.
> This would provide an API to restrict the compiler such that it
> considers only overloads for which the argument types meet the
> requirements of the function body. It might be possible to approximate
> the functionality of the "requires" keyword from ConceptC++ without
> requiring language extensions to C++0x. But this would be a long-term,
> general solution. In the mean time, I'm not sure what to do about your
> immediate problem.

Right, I've been saying that for a while.
Concepts being dropped from C++0x is not such a big deal since SFINAE 
can do the job anyway.

I've had plans for a while to rewrite Boost.ConceptCheck to do that, but 
unfortunately the interface would need to change, because 
Boost.ConceptCheck allows arbitrary statements instead of just expressions.

However, SFINAE has a few caveats compared to concepts:
- public/private/protected access control is done after template 
substitution, so you can get a hard error because of these
- it doesn't provide structural subtyping, i.e., it doesn't order the 
overloads by quality of match. But in practice this isn't much of a 
problem because you want to define concepts nominally anyway, and when 
you do that you can specify what you refine.
- it doesn't support retroactive modelling, but that kind of thing can 
be done with traits.