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

From: David Abrahams (dave_at_[hidden])
Date: 2004-12-20 16:47:55

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<pair<key2, int> > entries;
> };
> 
> template<typename Entry>
> struct get_entries {
>     typedef typename Entry::entries type;
> };
> 
> typedef map<pair<key1, entry> > 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<map_types, map_types, func>::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<map_types, map_types, 
> boost::mpl::pair<key2, int> >':
> 
> [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<insert<_,_> instead.  I'm not sure that lambda<X>
is derived from lambda<X>::type, and even if you used lambda<insert<_,_>
>::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<func, map_types, 
> boost::mpl::pair<key1, entry> >'
> boost-1_32/boost/mpl/aux_/preprocessed/gcc/fold_impl.hpp:49: 
> instantiated from `boost::mpl::aux::fold_impl<1, 
> boost::mpl::m_iter<map_types, 2, 3>, boost::mpl::m_iter<map_types, 3, 
> 3>, map_types, func>'
> boost-1_32/boost/mpl/fold.hpp:39:   instantiated from 
> `boost::mpl::fold<map_types, map_types, func>'
> 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