[boost] Re: Variant size bug?

From: David Abrahams (dave_at_[hidden])
Date: 2003-11-15 22:18:29

Eric Friedman <ebf_at_[hidden]> writes:

> Peter Dimov wrote:
>
>> I'm somewhat amused by the possibilities that enable_if offers. Perhaps one
>> day we'll think of
>> template<class V> enable_if< is_instance_of< std::vector<_, _>, V>
>> >::type
>> f( V & v);
>> as a natural alternative to
>> template<class T, class A> void f( std::vector<T, A> & v );
>> but I doubt it. ;-)
>
> If we ever do, we can use mpl::lambda_match, which is in the sandbox:
>
>    template <class V>
>      typename enable_if<
>          mpl::lambda_match< std::vector<_,_>, V >
>        >::type
>    f( V & v );
>
> Additionally, lambda_match supports more advanced functionality, such as
>
>    template <class Double>
>      typename enable_if<
>          mpl::lambda_match< std::pair<_1,_1>, Double >
>        >::type
>    f( Double & d );
>
> Perhaps I should rename it to mpl::is_instance_of?

I rather like 

  mpl::pattern_match

Of course, next you'll have to be able to return an integral constant
wrapper which rates matches for closeness.  IOW,

mpl::pattern_match<std::pair<int,int>, std::pair<int,int> >::value == 3
mpl::pattern_match<std::pair<int,_1>, std::pair<int,int> >::value == 2
mpl::pattern_match<std::pair<_1,_1>, std::pair<int,int> >::value == 2
mpl::pattern_match<std::pair<_1,_2>, std::pair<int,int> >::value == 1

or something ;-)

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com