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

David Greene wrote:

David Abrahams wrote:

...

So, IIUC, your ForwardOp is trying to use the copy algorithm with an inserter to insert each element of the sequence returned by the above lambda expression. One other problem is that insert<_1, _2> will have its placeholders substituted immediately as the "fold" is evaluated. So for each step of the copy, the same state and element will be passed. You need to protect that placeholder expression from early substitution.

That makes sense (mostly). I don't quite grok the lambda stuff, _ and how it gets replaced with the "right" stuff but I guess that's part of the learning curve.

insert<_,_> is equivalent to insert<_1,_2>

I had tried writing the get_types metafunction before but it didn't help. The lambda part is what I was missing. Thanks!

[But see below]

...

The easiest way is to transform it into a metafunction class using the lambda metafunction:

copy< get_types< // The ::types member of value_type< // The "value" component map_types // (when in map_types) , _2 // of the current element being processed > > , inserter< // an inserter starting with the outer state _1 // (the sequence being built) as its state. , lambda< // [preventing early substitution] insert<_,_> // and inserting each element into > // the inner state >

...

I tried this (replacing get_types with get_entries to reduce confusion):

struct entry { typedef map > entries; };

template<typename Entry> struct get_entries { typedef typename Entry::entries type; };

typedef map > map_types;

typedef copy< get_entries< // The ::types member of value_type< // The "value" component map_types // (when in map_types) , _2 // of the current element being processed > > , inserter< // an inserter starting with the outer state _1 // (the sequence being built) as its state. , lambda< // [preventing early substitution] insert<_,_> // and inserting each element into > // the inner state >

...

func;

typedef fold::type result;

I believe this is equivalent to what you wrote. I also tried the nested fold version as well.

Now g++ complains about apply for both versions:

boost-1_32/boost/mpl/aux_/preprocessed/gcc/fold_impl.hpp: In instantiation of `boost::mpl::apply_wrap2 >':

[Loads of "instatiated from" messages]

You left out the line that says "error:" and gives the error message. BTW, I suggest you get STLFilt, which will move this error to the top. But try using protect instead. I'm not sure that lambda<X> is derived from lambda<X>::type, and even if you used lambda

::type I'm not positive that would work.

boost-1_32/boost/mpl/aux_/preprocessed/gcc/fold_impl.hpp:49: instantiated from `boost::mpl::apply2 >' boost-1_32/boost/mpl/aux_/preprocessed/gcc/fold_impl.hpp:49: instantiated from `boost::mpl::aux::fold_impl<1, boost::mpl::m_iter, boost::mpl::m_iter, map_types, func>' boost-1_32/boost/mpl/fold.hpp:39: instantiated from `boost::mpl::fold' fold2.cc:51: instantiated from here boost-1_32/boost/mpl/aux_/preprocessed/gcc/fold_impl.hpp:49: error: no class template named `apply' in `struct map_types'

There are other errors but I'm not sure how useful they are.

Thanks for your patience. I'm just getting back into MPL after about a year after being off-list.

HTH, -- Dave Abrahams Boost Consulting http://www.boost-consulting.com