Question

I have a graph class that employs two dimension vertices using a map with another map nested inside, as such(V is my template):

map< V , map<  V , int> > vertices;

Defining an iterator as such seems to be ok:

typename map< V, map< V , int>::iterator i;

However when I try to use that iterator, like this i = vertices.begin() , I get a long error that looks like this. (main.cpp 81 is where I call the method that does the iteration). Where could I look at to find what’s wrong?

main.cpp:81:   instantiated from here
graph.h:326: error: no match for ‘operator=’ in ‘i = ((const GraphNameSpace::Graph<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >*)this)->GraphNameSpace::Graph<std::basic_string<char, std::char_traits<char>,      std::allocator<char> > >::vertices.std::map<_Key, _Tp, _Compare, _Alloc>::begin [with _Key = std::basic_string<char, std::char_traits<char>, std::allocator<char> >, _Tp = std::map<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int> > >, _Compare = std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, _Alloc = std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::map<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int> > > > >]()’
/usr/lib/gcc/x86_64-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_tree.h:152: note: candidates are: std::_Rb_tree_iterator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::map<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int> > > > >& std::_Rb_tree_iterator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::map<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int> > > > >::operator=(const std::_Rb_tree_iterator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::map<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int> > > > >&)
make: *** [main.o] Error 1

UPDATE:
At the risk of providing too much information, here is my graph class on pastebin. Right at the very bottom in the dump function is where I’m attempting to do the iteration.

Answer 1

The member function dump(), inside which you write i = vertices.begin(), is a const member function, which effectively makes vertices a const object, for it is a member of the class. So you need const_iterator:

typename map<V, map<V, int> >::const_iterator i;
                             //^^^^^ note this

It should work now. 🙂