Sorry for the delayed response. I was traveling last week, and didn't see this. johny5.coder@gmail.com wrote:
Hello to everybody.
Im using the great library Boost.Foreach and very happy with it, however currently I met some strange issue I want to show you.
Im using custom wrappers to iterate over mapped values in std::map<> container. Here is the wrapper:
template<typename CONT> struct Select2ndIt { CONT& cont;
OK, Select2nsIt is a container proxy.
Select2ndIt(CONT& cont) : cont(cont) {}
typedef typename CONT::mapped_type value_type;
struct iterator : public std::iterator <typename CONT::iterator::iterator_category, value_type, typename CONT::iterator::distance_type> { typename CONT::iterator it;
Actually, this is not quite correct. See below.
// typedef typename CONT::iterator::difference_type difference_type; typedef value_type* pointer; typedef value_type& reference;
iterator(typename CONT::iterator it) : it(it) {}
bool operator==(const iterator& other) const { return it == other.it; } bool operator!=(const iterator& other) const { return it != other.it; } iterator& operator++() { ++it; return *this; } iterator& operator++(int) { it++; return *this; }
Oops, that's wrong. Try this. iterator operator++(int) { iterator that(*this); ++it; return that; }
iterator& operator--() { --it; return *this; } iterator& operator--(int) { it--; return *this; }
Ditto for operator--(int)
reference operator*() const { return it->second; } pointer operator->() const { return &it->second; } };
So far so good. (Although you might simplify this code somewhat (and make it more correct) by using an iterator adaptor from the Boost.Iterators library.)
struct const_iterator : public std::iterator <typename CONT::const_iterator::iterator_category, value_type, typename CONT::const_iterator::distance_type> {
<snip> Here's the problem. Although it would seem to be correct to define separate iterator and const_iterator types in your container proxy, it's actually wrong. The const-ness of the iterators should match the const-ness of the underlying container, not the const-ness of the proxy. Consider the different meanings of the following: std::map<int,unsigned> m1; BOOST_FOREACH(unsigned& value, select2nd(m1)) // OK std::map<int,unsigned> const m2; // NOTE CONST HERE BOOST_FOREACH(unsigned const & value, select2nd(m2)) // OK Whether the values in the map are const or not actually has nothing to do with whether the Select2ndIt proxy object is const. It's just a proxy. So you can fix the issue simply by replacing the const_iterator definition with: typedef iterator const_iterator; However, you'll also need to change the definition of your iterator struct. If CONT is const-qualified, iterator should adapt CONT::const_iterator instead of CONT::iterator. HTH, -- Eric Niebler Boost Consulting www.boost-consulting.com