[boost] Re: Proposal for 'is_dereferenceable' and other templatemetafunctions

From: David Abrahams (dave_at_[hidden])
Date: 2004-09-11 06:57:27

Daniel Wallin <dalwan01_at_[hidden]> writes:

> David Abrahams wrote:
>> Daniel Wallin <dalwan01_at_[hidden]> writes:
>>>I have been toying with a more general solution to this problem, where
>>>one can do things like:
>>>
>>>   check_dereference<T, is_convertible<mpl::_, U&> >
>>>   check_add<T, T, is_convertible<mpl::_, T> >
>>>
>>> etc. 
>> That *might* be impressive if I could tell what those expressions
>> were
>> supposed to mean ;-)
>
> The metafunction class 

I don't see a metafunction class here.  Don't you mean "lambda expression?"

> is applied to the result type of the operation,
> and is required to have boolean result. For instance:
>
>    struct X
>    {
>        void operator+(X const&) const;
>        X operator+(int) const;
>    };
>
>    template<class T> struct Y : mpl::false_ {};
>    template<> struct Y<X> : mpl::true_ {};
>
>    check_add<X, X, Y<mpl::_> >::type // X + X, invokes Y<void> -> false
>    check_add<X, int, Y<mpl::_> >::type // X + int, invoked Y<X> -> true
>
> Using this we could express more complicated concept checks. For
> instance:
>
> "The Lvalue Iterator concept adds the requirement that the return type
>   of operator* type be a reference to the value type of the iterator."
>
>     template<class Iterator>
>     struct is_iterator_lvalue_dereferencable
>         : check_dereference<
>               Iterator
>             , is_same<
>                   mpl::_
>                 , typename iterator_value<Iterator>::type&
>               >
>           >
>     {
>     };

Oh, very nice!  Now it's impressive. :-)

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com