Does std::map::iterator return a copy of value or a value itself?

Question

I\'m trying to create a map inside a map:

typedef map inner_map;
typedef map outer_map;

Will I be ab

Answer 1

The value_type a map is a pair and therefore it has members first and second. As with all iterators, a map iterator is a pseudo-pointer, i.e. it points to data within a collection and not copies of that data.

It is almost certain internally to contain pointers rather than references due to the fact that iterators can be re-assigned (that is what you use them for) and you cannot reassign references to refer to other objects.

Even if you have a const_iterator and the type underneath is POD, it must have a pointer to it, in case someone does this:

map< int, int > m;
m.insert( make_pair( 1, 2 );
map::const_iterator citer = m.begin();
map::iterator iter = m.begin();
iter->second = 3;
std::cout << citer->second << '\n'; // should always print 3

The behaviour should be defined and should output 3, which would not happen if the const_iterator decided to "optimise" after all it's const and only int...