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;
Select2ndIt(CONT& cont) : cont(cont) {}
typedef typename CONT::mapped_type value_type;
struct iterator : public std::iterator
{
typename CONT::iterator it;
// 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; }
iterator& operator--() { --it; return *this; }
iterator& operator--(int) { it--; return *this; }
reference operator*() const { return it->second; }
pointer operator->() const { return &it->second; }
};
struct const_iterator : public std::iterator
{
typename CONT::const_iterator it;
// typedef typename CONT::const_iterator::difference_type difference_type;
typedef const value_type* pointer;
typedef const value_type& reference;
const_iterator(typename CONT::const_iterator it) : it(it) {}
bool operator==(const const_iterator& other) const { return it == other.it; }
bool operator!=(const const_iterator& other) const { return it != other.it; }
const_iterator& operator++() { ++it; return *this; }
const_iterator& operator++(int) { it++; return *this; }
const_iterator& operator--() { --it; return *this; }
const_iterator& operator--(int) { it--; return *this; }
reference operator*() const { return it->second; }
pointer operator->() const { return &it->second; }
};
iterator begin()
{
return iterator(cont.begin());
}
iterator end()
{
return iterator(cont.end());
}
const_iterator begin() const
{
return const_iterator(cont.begin());
}
const_iterator end() const
{
return const_iterator(cont.end());
}
};
template<typename CONT>
Select2ndIt<CONT> select2nd(CONT& cont) { return Select2ndIt<CONT>(cont); }
And usage:
std::map cont;
BOOST_FOREACH(unsigned& value, alg::select2nd(cont))
value = 0;
And MSVS 8.0 tells: error C2440: 'initializing' : cannot convert from
'const unsigned int' to 'unsigned int &'
Without wrappers I can easily iterate as a non-const value:
std::map cont;
typedef std::pair value_type;
BOOST_FOREACH(value_type& value, cont)
value.second = 0;
works great.
I guess my wrappers very close to STL iterators (they even inherit
from std::iterator), however some unwanted constness is appearing, so
I afraid it is some issue with Boost.Foreach library.
I hope for some help or advice about this problem.
Thanks. Evgeny.
