Question

I have a boost::bimap<int, boost::shared_ptr<A>> container and want to return an iterator to the left view ensuring constness of the contents. Returning container.left.begin() returns an iterator that dereferences to std::pair<int const, boost::shared_ptr<A> const>.

Obviously, this is not what I want, since it’s possible to change A by dereferencing the shared_ptr. I want an iterator that dereferences to std::pair<int const, boost::shared_ptr<A const>> (I don’t really care if the shared_ptr is const or not). I know I should probably be doing this with the use of boost::transform_iterator, but I can’t figure out what the “casting” function should look like.

Can anybody help me out with this? Or is there another, easier method to obtain what I want?

Edit: Here’s what I have so far, suffice to say, it’s giving me a good 2 screens worth of errors.

typedef boost::bimap<unsigned int, boost::shared_ptr<A> > container_type;
typedef container_type::left_const_iterator base_const_iterator;
typedef boost::transform_iterator<makeIterConst<A>, base_const_iterator> const_iterator;

template <typename T>
struct makeIterConst : std::unary_function<std::pair<unsigned int const, boost::shared_ptr<T> const>, std::pair<unsigned int const, boost::shared_ptr<T const> const> >
{
  std::pair<unsigned int const, boost::shared_ptr<T const> const> operator() (std::pair<int const, boost::shared_ptr<T> const> const & orig) const
  {
    std::pair<int const, boost::shared_ptr<T const> const> newPair(orig.first, boost::const_pointer_cast<T const>(orig.second));
    return newPair;
  }
};

Here’s the “core” error:

note: candidate function not viable:
no known conversion from ‘const
boost::bimaps::relation::structured_pair,
boost::bimaps::tags::tagged,
boost::bimaps::relation::member_at::right>,
mpl_::na,
boost::bimaps::relation::normal_layout>’
to ‘const std::pair >’ for
1st argument

Answer 1

The problem is that your value_type is not actually a std::pair (and is not implicitly convertible to one), and thus cannot be passed to makeIterConst::operator().

Make it take const base_const_iterator::value_type & instead.

#include <iostream>
#include <string>
#include <boost/shared_ptr.hpp>
#include <boost/make_shared.hpp>
#include <boost/bimap.hpp>
#include <boost/iterator/transform_iterator.hpp>

struct A
{
    std::string data;
    A(const std::string& s) : data(s) {}
};

typedef boost::bimap<unsigned int, boost::shared_ptr<A> > container_type;
typedef container_type::left_map::const_iterator base_const_iterator;

template <typename T>
struct makeIterConst : std::unary_function<base_const_iterator::value_type const &,
                                           std::pair<unsigned int const, boost::shared_ptr<T const> const> >
{
     std::pair<unsigned int const, boost::shared_ptr<T const> const> operator()
             (base_const_iterator::value_type const & orig) const
     {
         std::pair<int const, boost::shared_ptr<T const> const> newPair(orig.first, boost::const_pointer_cast<T const>(orig.second));
         return newPair;
     }
};

typedef boost::transform_iterator<makeIterConst<A>,
                                 base_const_iterator> const_iterator;

int main()
{
    container_type m;
    boost::shared_ptr<A> p = boost::make_shared<A>("foo");
    m.insert( container_type::value_type(1, p));

//  using regular iterator
    for( base_const_iterator left_iter = m.left.begin();
                         left_iter != m.left.end();
                         ++left_iter )
    {
        std::cout << left_iter->first << " --> " << left_iter->second->data << '\n';
        left_iter->second->data =  "bar"; // compiles
    }

    // using constified iterator
    for( const_iterator left_iter = boost::make_transform_iterator(m.left.begin(), makeIterConst<A>() );
              left_iter != boost::make_transform_iterator(m.left.end(), makeIterConst<A>() );
            ++left_iter )
    {
        std::cout << left_iter->first << " --> " << left_iter->second->data << '\n';
//   the following will give a compilation error, as expected
//      left_iter->second->data =  "changed_foo";
    }
}

test: https://ideone.com/fHIUe