Re: [boost] concepts and template template parameters (was: [Proto] Review)

From: Douglas Gregor (dgregor_at_[hidden])
Date: 2008-03-31 10:29:27

On Mon, 2008-03-31 at 15:22 +0100, Giovanni Piero Deretta wrote:
> but in general it might reopen the problem of deferred typechecking of
> template parameters.
> Are constrained template template parameters legal?

Yes.

> i.e.:
> 
>   // Takes a constrained template:
>   template<template<Value> class S> // is this syntax even legal?
>   struct foo {};
> 
>   // A constrained template
>   template<Regular Value>
>   struct vector
>  {};
> 
>   foo<vector> f; // OK?

Yes, this is fine, assuming that the template template parameter S was
meant to be written as:

  template<Regular> class S

You could also have more constraints on the template argument, e.g.,

  template<Regular Value> requires LessThanComparable<Value>
  struct set;

  foo<set> f2; // okay

> [ If it is not legal, I guess you could fake it with concept achetypes
> and some kind of rebind trick.]

Ick.

> Not useful for metaprogramming, but for more mundane things, you might
> not want to lose the benefits of
> concept typechecking.

It might be useful for metaprogramming; I don't actually know. Some
metaprogramming is likely to change when using concepts, but we're doing
our best not to hurt metaprogramming in the process of adding concepts.

  - Doug