David Abrahams writes:
Oh, I see it now; you're trying to get the "types" member out of the result of invoking value_type. Okay, to do that you need a metafunction that gets the types member:
template <class Value> struct get_types { typedef Value::types type; };
now you can write:
get_types< // The ::types member of value_type< // The "value" component map_types // (when in map_types) , _2 // of the current element being processed >
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. 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 >
Won't work. The 'lambda' metafunction doesn't have any special status here.
For instance, 'lambda<_1>' is a regular, well, lambda expression, not
different from, let's say, 'first<_1>'. It's all documented in the
reference manual, and illustrated by the corresponding tests
(e.g. http://cvs.sourceforge.net/viewcvs.py/boost/boost/libs/mpl/test/apply.cpp?vi...).
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