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From: João Abecasis (jpabecasis_at_[hidden])
Date: 2006-03-01 09:27:59

Lucio Flores wrote:
> So as an exercise, I'm trying to use a lambda expression that applies the first
> argument twice on it's second argument.
> typedef mpl::apply<_1, mpl::apply<_1, _2>::type>::type twice_type;
> Then I instance this lambda function on boost::add_pointer and int
> typedef mpl::apply<twice_type, boost::add_pointer<_1>, int>::type result_type;
> But using boost::is_same, I've found that result_type==int, not int** as
> expected. Can someone see why?

If you look closely, as it is defined, twice_type is actually the same
as mpl::_2. Once you "invoke" the ::type member typedef you're
evaluating the metafunction and no longer have a lambda.

Here's a simple function I find useful in debugging template meta-code:

     template <class type>
     inline void print()
         class _ { _() {} };
         _ printer;

use it thus:

     int main()
         typedef mpl::apply<mpl::_1, mpl::_2>::type result;


You'll get a nice error message with the resulting type in it. This
should help you better understand what's going on.

You'll should look into protect and lambda to defer evaluation of the



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