[map0 / insert] inserting into map0 does not work without including map10.hpp

[Jeffrey Hellrung <jhellrung@…>]

The following does not compile on MSVC9:

#include <boost/mpl/insert.hpp>
#include <boost/mpl/map/map0.hpp>
#include <boost/mpl/pair.hpp>

typedef boost::mpl::insert<
    boost::mpl::map0<>,
    boost::mpl::pair< void, void >
>::type map1_type;

typedef boost::mpl::insert<
    map1_type,
    boost::mpl::pair< int, int >
>::type map2_type;

int main() { }

The compiler output is

1>------ Build started: Project: test, Configuration: Debug Win32 ------
1>Compiling...
1>main.cpp
1>d:\boost_1_41_0\boost\mpl\insert.hpp(32) : error C2903: 'apply' : symbol is neither a class template nor a function template
1>        d:\test\main.cpp(13) : see reference to class template instantiation 'boost::mpl::insert<Sequence,Pos_or_T>' being compiled
1>        with
1>        [
1>            Sequence=map1_type,
1>            Pos_or_T=boost::mpl::pair<int,int>
1>        ]
1>d:\boost_1_41_0\boost\mpl\insert.hpp(32) : error C2039: 'apply' : is not a member of 'boost::mpl::insert_impl<boost::mpl::non_sequence_tag>'
1>        d:\boost_1_41_0\boost\mpl\aux_\insert_impl.hpp(64) : see declaration of 'boost::mpl::insert_impl<boost::mpl::non_sequence_tag>'
1>d:\boost_1_41_0\boost\mpl\insert.hpp(32) : error C2955: 'boost::mpl::apply' : use of class template requires template argument list
1>        d:\boost_1_41_0\boost\mpl\aux_\preprocessed\plain\apply.hpp(134) : see declaration of 'boost::mpl::apply'
1>d:\boost_1_41_0\boost\mpl\insert.hpp(32) : error C2143: syntax error : missing ',' before '<'
1>d:\test\main.cpp(13) : error C2039: 'type' : is not a member of 'boost::mpl::insert<Sequence,Pos_or_T>'
1>        with
1>        [
1>            Sequence=map1_type,
1>            Pos_or_T=boost::mpl::pair<int,int>
1>        ]
1>d:\test\main.cpp(13) : error C2146: syntax error : missing ';' before identifier 'map2_type'
1>d:\test\main.cpp(13) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>d:\test\main.cpp(13) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>Build log was saved at "file://d:\test\Debug\BuildLog.htm"
1>test - 8 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

A fix (which took me a while to figure out, since the above error message didn't give me any clues) is to include <boost/mpl/map/map10.hpp>. I think either a note should be made in the documentation for insert, the error message should be more helpful (maybe a comment in mpl/insert.hpp line 32???), or the above code should "just work".

[Steven Watanabe]

In C++0x, the code will just work, since the typeof based implementation of mpl::map has no fixed limits. Unfortunately, with the non-typeof implementation, there is no good way to make it just work.