使用boost transform_iterator有条件地转换std :: transform

Question

_pimpl->_connections is a std::map so its elements are std::pair<KeyType, gcl::SectionConnectionT*> I want to use a Predicate gcl::SectionConnectionT::NotConnectedTo(r) to filter out unnecessary values. but If I use

std::remove_copy_if(it_pair.first, it_pair.second, std::back_inserter(sectionConnections), gcl::SectionConnectionT::NotConnectedTo(r));

It tries to insert the pair into the vector. but the vector is of type <gcl::SectionConnectionT*> Some google search took me to transform_iterator which I cannot understand how to understand. I used it like this. but getting compilations errorS

std::pair<CollectionT::iterator, CollectionT::iterator> it_pair = _pimpl->_connections.equal_range(l);
std::vector<gcl::SectionConnectionT*> sectionConnections;
std::remove_copy_if(it_pair.first, it_pair.second, boost::make_transform_iterator(std::back_inserter(sectionConnections), util::shorthand::pair_second()), gcl::SectionConnectionT::NotConnectedTo(r));

Answer 1

The Boost.Iterator library is likely unable to deduce the return type of until::shorthand::pair_second::operator()(CollectionT::value_type&) const , so the returned transform_iterator is not modeling the Writable Lvalue Iterator concept, as is required to be used for the third parameter to std::remove_copy_if (the output iterator).

The following works, though:

//#include <boost/lambda/bind.hpp>
//#include <boost/lambda/lambda.hpp>
//#include <boost/range/adaptor/map.hpp>
//#include <boost/range/algorithm/remove_copy_if.hpp>

//namespace gcl {
//struct SectionConnectionT {
//    bool NotConnectedTo(RType r) const;
//    // ...
//};
//}

boost::remove_copy_if(it_pair | boost::adaptors::map_values,
    std::back_inserter(sectionConnections),
    boost::lambda::bind(&gcl::SectionConnectionT::NotConnectedTo, boost::lambda::_1, r));