Re: [boost] Most needed/desired features in C++

From: Douglas Gregor (gregod_at_[hidden])
Date: 2002-02-18 15:06:48

On Monday 18 February 2002 02:09 pm, you wrote:
[snip]
> Tagging types can be totally done with traits classes.
> The key of this proposal is that tags defined this way participate in
> overload resolution, much like base classes does, except that unlike base
> classes they are used to sort out types in the last phase instead of the
> first.
> Their main purpose is to resolve ambiguities and to group specializations.
>
> template<class GPoint T> double distance ( T const& lhs, T const& rhs ) ;
> // won't clash with std::distance

As I see it, tags provide a way to control when a template type parameter can 
be bound to a particular type. This, I believe, is an extremely important 
ability and in some form it should be supported in C++0x. However, if tags 
are semantically equivalent to traits, why the new name and syntax? 

If I were to counter-propose something along these lines, I would propose 
that template parameters can optionally be followed by a restriction of the 
form:
  IF '(' expression ')'

  template parameters may only be bound to types such that 'expression' 
evaluates true. Just to compare on your example, the following declaration 
using tags:

template<class GPoint T> double distance ( T const& lhs, T const& rhs ) ;

would be written:

template<class T if (is_gpoint<T>::value)> 
double distance ( T const& lhs, T const& rhs ) ;

I personally would prefer that the extension be formulated in this way 
because:
  1) it's a pure extension, that will not break any existing code
  2) it builds on existing knowledge and implementations of traits
  3) it allows arbitrary expressions to determine binding (perhaps tags can 
do this also?)
  4) it's utterly trivial to implement

        Doug