[Boost-users] [MPL] transform and unpack_args

From: David Greene (greened_at_[hidden])
Date: 2004-12-27 14:21:19

All right, I'm still trying to figure out the MPL lambda stuff.

I had expected this to work:

template<typename Arg>
some func {
    typedef ... type;
};

typedef transform<
    sequence,
    some_func<_>
 >::type result;

But I get a compiler error about a missing apply<> in some_func.

This works:

typedef transform<
    sequence,
    quote1<some_func>
 >::type result;

I don't understand why the placeholder doesn't convert some_func
to an appropriate metafunction class.

Similarly, I tried to use unpack args:

template<typename Arg1, typename Arg2>
some_func2 {
    typedef ... type;
};

typedef transform<
    zipped_sequence,
    unpack_args<some_func2<_,_>
 >::type result;

The compiler complained about a missing apply<> in some_func2.

Again, this worked:

typedef transform<
    zipped_sequence,
    unpack_args<quote2<some_func2> >
 >::type result;

Is it because I'm using _ rather than _1 and _2?  The examples
in the MPL manual using is_same, etc. work fine for me.

Learning lots.  Hope to learn more.  :)

                       -Dave