Re: [boost] Submission: typelist

From: Peter Dimov (pdimov_at_[hidden])
Date: 2001-12-18 07:53:45

From: "Aleksey Gurtovoy" <alexy_at_[hidden]>
> Agree, current 'mpl::copy' is strange. The intention was to allow 'copy'
to
> add elements to an output sequence, as well as to create a "fresh copy" of
> them, and as I was sloppy at the time I wrote the function, instead of
> defining compile-time Output Iterator concept and implementing inserters,
I
> decided to just go with the current interface:
>
>     // (1) adds numbers [0..10] to a value_list<20,25,30,35>
>     typedef mpl::value_list<20,25,30,35> numbers;
>     typedef mpl::copy<
>           mpl::value_range<0,10>
>         , numbers // should support 'push_back'
>         >::type result;
>
>     BOOST_MPL_ASSERT_IS_SAME(
>           result
>         , mpl::value_list<0, 1, 2, 3,.., 9, 10, 20, 25, 30, 35>
>         );
>
>     // should be something like
>     //
>     // typedef mpl::value_list<20,25,30,35> numbers;
>     // typedef mpl::copy<
>     //      mpl::value_range<0,10>
>     //    , mpl::back_inserter<numbers>
>     //    >::type result;
>     //
>     // or (much more effective ;)
>     //
>     // typedef mpl::copy_backward<
>     //      mpl::value_range<0,10>
>     //    , mpl::front_inserter<numbers>
>     //    >::type result;

But why do you need an OutputIterator, or a Container in a pure functional
world? There is no place (or need) for these concepts as they imply
mutability. You can do everything you need without them.

std::copy is simply mpl::identity.

std::append, if there was one, would have looked like this:

template<class I, class J, class O> O append(I first, I last, J first2, J
last2, O out)
{
    return copy(first2, last2, copy(first, last, out));
}

--
Peter Dimov
Multi Media Ltd.