[boost] Re: [mpl] need help with 'extended template parametermatching'

From: Jonathan Turkanis (technews_at_[hidden])
Date: 2004-10-27 14:05:20

"Daniel Wallin" <dalwan01_at_[hidden]> wrote in message:

> I don't know if it's better, but you can get rid of the SFINAE by doing
> something like:
>
>    struct _ {};
>
>    template<class T, int N = 3>
>    struct rebind : rebind<T, N - 1>
>    {};
>
>    template<class T>
>    struct rebind<T, 0>
>    {
>        typedef T type;
>    };
>
>    template<template<class> class T, class P1>
>    struct rebind<T<P1>, 1>
>    {
>        typedef T<_> type;
>    };
>
>    template<template<class, class> class T, class P1, class P2>
>    struct rebind<T<P1, P2>, 2>
>    {
>        typedef T<_, _> type;
>    };
>
>    template<
>        template<class, class, class> class T
>      , class P1, class P2, class P3
>    >
>    struct rebind<T<P1, P2, P3>, 3>
>    {
>        typedef T<_, _, _> type;
>    };
>
> Or you could just use template_arity to pass the arity to the
> specialized template, instead of recursively trying every arity. I
> suspect template_arity has to do something equivalent to this though, so
> maybe this saves a few instantiations.

Thanks for the suggestions. I was hoping there might be something radically
simpler. I think I"m happy to stick with enable_if and template arity (on
platforms which require it), since it states the intent very clearly. It can
also be encapsulated better than the example I posted, e.g., enable_by_arity<
T<P>, 1 > ... .

>
> HTH,
> -- 
> Daniel Wallin
>

Jonathan