Question

I have a class that contains some data, and I would like to add begin() and end() functions that provide iterators over the IDs of the data.

I’m using the Boost counting_iterator:

#include <iostream>
#include <vector>
#include <boost/foreach.hpp>
#include <boost/iterator/counting_iterator.hpp>

template<class T>   
class ContainerTpl {
 public:
  typedef std::size_t Id;
  typedef boost::counting_iterator<Id> const_iterator;
  ContainerTpl() {}
  const_iterator begin() {
    return boost::counting_iterator<Id>(0);
  }
  const_iterator end() {
    return boost::counting_iterator<Id>(container_.size());
  }
 private:
  std::vector<T> container_;
};

int main () {
  typedef ContainerTpl<double> Container;
  Container c;
  BOOST_FOREACH (Container::Id cid, c) {
    std::cerr << cid << std::endl;
  }
  return 0;
}

Please note that this is minimal example code; in reality the class contains more functionality, so that, for example, a typedef to vector would not be sufficient. I really need that class, with an iterator over IDs.

Unfortunately, the code above gives me pretty nasty compiler errors:

In file included from boost/foreach.hpp:71,
                 from a.cpp:3:
boost/mpl/eval_if.hpp: In instantiation of ‘boost::mpl::eval_if<mpl_::bool_<false>, boost::range_const_iterator<ContainerTpl<double> >, boost::range_mutable_iterator<ContainerTpl<double> > >’:
boost/foreach.hpp:355:   instantiated from ‘boost::foreach_detail_::foreach_iterator<ContainerTpl<double>, mpl_::bool_<false> >’
a.cpp:25:   instantiated from here
boost/mpl/eval_if.hpp:38: error: no type named ‘type’ in ‘struct boost::range_mutable_iterator<ContainerTpl<double> >’
a.cpp: In function ‘int main()’:
a.cpp:25: error: no matching function for call to ‘begin(const boost::foreach_detail_::auto_any_base&, boost::foreach_detail_::type2type<ContainerTpl<double>, mpl_::bool_<false> >*, boost::mpl::o\
r_<boost::mpl::and_<boost::mpl::not_<boost::is_array<ContainerTpl<double> > >, mpl_::bool_<false>, mpl_::bool_<true>, mpl_::bool_<true>, mpl_::bool_<true> >, boost::mpl::and_<boost::mpl::not_<boo\
st::foreach::is_noncopyable<ContainerTpl<double> > >, boost::foreach::is_lightweight_proxy<ContainerTpl<double> >, mpl_::bool_<true>, mpl_::bool_<true>, mpl_::bool_<true> >, mpl_::bool_<false>, m\
pl_::bool_<false>, mpl_::bool_<false> >*)’
a.cpp:25: error: no matching function for call to ‘end(const boost::foreach_detail_::auto_any_base&, boost::foreach_detail_::type2type<ContainerTpl<double>, mpl_::bool_<false> >*, boost::mpl::or_\
<boost::mpl::and_<boost::mpl::not_<boost::is_array<ContainerTpl<double> > >, mpl_::bool_<false>, mpl_::bool_<true>, mpl_::bool_<true>, mpl_::bool_<true> >, boost::mpl::and_<boost::mpl::not_<boost\
::foreach::is_noncopyable<ContainerTpl<double> > >, boost::foreach::is_lightweight_proxy<ContainerTpl<double> >, mpl_::bool_<true>, mpl_::bool_<true>, mpl_::bool_<true> >, mpl_::bool_<false>, mpl\
_::bool_<false>, mpl_::bool_<false> >*)’
a.cpp:25: error: no matching function for call to ‘deref(const boost::foreach_detail_::auto_any_base&, boost::foreach_detail_::type2type<ContainerTpl<double>, mpl_::bool_<false> >*)’

How can I make the code work?

UPDATE: Following the answer, I add the following code, which makes it work:

namespace boost
{
// specialize range_mutable_iterator and range_const_iterator in                                                                                                                                    
// namespace boost                                                                                                                                                                                  
template<class T>
struct range_mutable_iterator< ContainerTpl<T> > {
  typedef typename ContainerTpl<T>::const_iterator type;
};

template<class T>
struct range_const_iterator< ContainerTpl<T> > {
  typedef typename ContainerTpl<T>::const_iterator type;
};
} // end namespace         

Answer 1

There’s a page in the Boost docs about that:

http://boost-sandbox.sourceforge.net/libs/foreach/doc/html/foreach/extensibility.html

In short you need to define boost::range_mutable_iterator<> for your type so that the compiler can instantiate the template type that BOOST_FOREACH is trying to use.

edit for future googlers:

I’m not sure if that “sandbox” URL will always point at the latest version or is a transient location that will eventually break. This is the link for the current version, which might be more stable, though it will become outdated:

http://www.boost.org/doc/libs/1_50_0/doc/html/foreach/extensibility.html