Re: [Boost-users] Re: [MPL] map question

David Abrahams writes:

Aleksey Gurtovoy wrote:

...

Answering the OP question: MPL lambda expressions do not support scopes (yet), so it's not possible to implement the above without an auxiliary metafunction incapsulating the nested scope. Which actually might be a good thing -- it's hard to imagine that the "inline" version would be shorter and easier to understand than this:

template< typename Map, typename Entry > struct copy_entires : copy< typename Entry::second::entries , inserter > > { };

typedef fold >::type result;

In that case, can you explain what protect<> is for?

From http://www.boost.org/libs/mpl/doc/refmanual/protect.html:

'protect' is an identity wrapper for a Metafunction Class that prevents its argument from being recognized as a bind expression. Actually, the above definition is incomplete; should be: 'protect' is an identity wrapper for a Metafunction Class that prevents its argument from being recognized as a bind/placeholder expression. The main usage for 'protect' is to prevent parametrized metafunction classes from being accidentally treated as a placeholder expression simply because their arguments happened to embed a placeholder. For instance: template< typename Predicate > struct next_if { template< typename N, typename T > struct apply : eval_if< typename apply1::type , next<N> , identity<N> > { }; }; template< typename Sequence , typename Predicate > struct count_if : fold< Sequence, int_<0>, protect< next_if<Predicate> > >::type { }; Without 'protect', we'd have to pass 'Predicate' through 'lambda' before using it to parametrize 'next_if', or the whole thing would break on something like typedef count_if > >::type r; P.S. Please ignore the 'protect' example at the end of the page -- it's a total garbage :(. -- Aleksey Gurtovoy MetaCommunications Engineering