On Tue, Mar 13, 2012 at 11:13 PM, Peter Nyssen wrote:

Hi all,

I'm trying to understand how to use mpl::fold to transform a vector like this: vector< pair pair pair

...

into the following: pair< vector vector<B>

...

Following test program fails to compile and I don't understand why. If you could explain me what I'm missing here, you will gain my eternal respect.

Kind regards, Peter

//--------------------------------------------

#include #include #include #include #include #include

namespace mpl = boost::mpl;

struct tag1 {}; struct tag2 {};

template <typename T> struct is_tag1 : boost::false_type {};

template <> struct is_tag1<tag1> : boost::true_type {};

int main() { typedef mpl::vector< mpl::pair >, mpl::pair >, mpl::pair > > initial_vector;

typedef typename mpl::fold< initial_vector, mpl::pair, mpl::vector<> >, mpl::if_< is_tag1mpl::_2::type >, mpl::push_back< mpl::firstmpl::_1::type, mpl::secondmpl::_2::type >, mpl::push_back< mpl::secondmpl::_1::type, mpl::secondmpl::_2::type > > >::type sorted_pair;

Try removing all the ::type's in the fold operation and rely on lazy evaluation. mpl:first< mpl::_1 >::type is equivalent to mpl::_1::first, which of course doesn't exist and isn't what you want; but mpl::first< mpl::_1 > is a placeholder expression equivalent to the mapping ( T0, ... ) -> T0::first, which looks like what you want.

return 0; }

//--------------------------------------------

*$* g++ -Wall -I ../boost/ -o test test.cpp In file included from test.cpp:5:0: ../boost/boost/mpl/pair.hpp: In instantiation of ‘boost::mpl::first >’: test.cpp:32:40: instantiated from here ../boost/boost/mpl/pair.hpp:43:31: fout: no type named ‘first’ in ‘struct mpl_::arg<2>’

:: cough ::

test.cpp: In functie ‘int main()’: test.cpp:32:47: fout: template argument 1 is invalid In file included from test.cpp:5:0: ../boost/boost/mpl/pair.hpp: At global scope: ../boost/boost/mpl/pair.hpp: In instantiation of ‘boost::mpl::first >’: test.cpp:34:36: instantiated from here ../boost/boost/mpl/pair.hpp:43:31: fout: no type named ‘first’ in ‘struct mpl_::arg<1>’ ../boost/boost/mpl/pair.hpp: In instantiation of ‘boost::mpl::second >’: test.cpp:35:37: instantiated from here ../boost/boost/mpl/pair.hpp:56:32: fout: no type named ‘second’ in ‘struct mpl_::arg<2>’ test.cpp: In functie ‘int main()’: test.cpp:36:13: fout: template argument 1 is invalid test.cpp:36:13: fout: template argument 2 is invalid In file included from test.cpp:5:0: ../boost/boost/mpl/pair.hpp: At global scope: ../boost/boost/mpl/pair.hpp: In instantiation of ‘boost::mpl::second >’: test.cpp:38:37: instantiated from here ../boost/boost/mpl/pair.hpp:56:32: fout: no type named ‘second’ in ‘struct mpl_::arg<1>’ test.cpp: In functie ‘int main()’: test.cpp:40:13: fout: template argument 1 is invalid test.cpp:40:13: fout: template argument 2 is invalid test.cpp:41:9: fout: template argument 1 is invalid test.cpp:41:9: fout: template argument 2 is invalid test.cpp:41:9: fout: template argument 3 is invalid test.cpp:42:5: fout: template argument 3 is invalid test.cpp:42:24: fout: invalid type in declaration before ‘;’ token

See if that helps. (Too lazy to try it on my own.) - Jeff