STL Containers begin() / end() returns const_iterator ?

03 May, 2013 - 2 min read

map<string,Shopable*>::iterator it = mymap.begin();

The iterator appears to be constant, but items.begin() doesn't return a constant iterator. Or, that's what I think because the mouseover error is something like:

Compiler Error:

"No conversion from 'std::Tree_const_iterator<...> to std::Tree_iterator<...> exists'".


Use const_iterator as :

map<string,Shopable*>::const_iterator it = mymap.begin();

From the error, its clear that mymap.begin() returns const_iterator. That is because mymap isconst in the function where you've written this, something like following:

void f(const std::map<int,int> & m)
{    //^^^^^ note this

      std::map<int,int>::const_iterator it = m.begin(); //m is const in f()
                       //^^^^^ note this

void g(std::map<int,int> & m)
      std::map<int,int>::iterator it = m.begin(); //m is non-const in g()

That is, const container (whether its std::mapstd::vector etc) returns const_iterator and non-const container returns iterator.

Every container has overloaded functions of begin() and end(). So const container invokes the overloaded begin() which returns const_iterator and non-const container invokes the other overloaded begin() which returns iterator. And same for end() overloaded functions. Ex:


iterator begin();
const_iterator begin() const;

The problem is that mymap in the code above is a constant map, not a mutable map (maybe it is a member of a class and that code is inside constant member function?). Thus the call tomymap.begin() will pichup the overload that returns a const_iterator instead of the overload that returns an iterator.