Re: [boost] Egg 2nd request for reviews: Some comments

From: shunsuke (pstade.mb_at_[hidden])
Date: 2008-04-06 14:55:02

vicente.botet wrote:
> Could you tell me if the following is equivalent to what appears in the 
> quick start. I'm not very confortable with
> foreach.
> 
> </code>
> template<class Range, class Predicate>
> struct mono_make_filtered
> {
>     typedef
>         boost::filter_iterator<
>             typename boost::remove_cv<Predicate>::type,
>             typename boost::range_result_iterator<Range>::type
>         >
>     iter_t;
> 
>     typedef
>         boost::iterator_range<iter_t>
>     result_type;
> 
>     result_type operator()(Range &rng, Predicate &pred) const
>     {
>         return result_type(
>             iter_t(pred, boost::begin(rng), boost::end(rng)),
>             iter_t(pred, boost::end(rng), boost::end(rng))
>         );
>     }
> };
> 
> bool is_not_X(char ch) { return ch != 'X'; }
> bool is_not_Y(char ch) { return ch != 'Y'; }
> bool is_lower(char ch) { return std::islower(ch, std::locale()); }
> 
> void quick_start_make_filtered()
> {
>     std::string src("abXcYdXefXgYhY");
> 
>     mono_make_filtered::result_type lowers = make_filtered(src, &is_lower);
> 
>     foreach (char ch, lowers) {
>         std::cout << ch;
>     }
> 
>     foreach (char ch, make_filtered(make_filtered(src, &is_not_X), 
> &is_not_Y)) {
>         std::cout << ch;
>     }
> }
> </code>
> 
> If this is the case could you show which is really the advantage to use Egg. 
> I 'm sure there are a lot of, but I think the example do not show them.

No, it's not equivalent.
mono_make_filtered<std::string, bool(*)(char)>::result_type lowers = ...
is equivalent, though.
A problem of this form is that
users of `make_filtered` have to remember the usage of `mono_make_filtered`
instead of the standardized metafunction `result_of`.

Regards,

-- 
Shunsuke Sogame