Why STL containers can insert using const iterator

Question

Why STL containers can insert using const_iterator? For example, there is

iterator std::list::insert (const_iterator position, const value_type& val);

I have thought that const_iterator's does not allow to modify container.

Answer 1

When an element is inserted into a list, the other elements are not modified. The iterator is only used to determine the insertion position, not to modify (= call non-const member function on) any of the existing elements.

Even erase can take a const_iterator. Again, this is because erasing an element does not modify it, either. This may sound strange, but it's trivial if you consider this:

void f()
{
    std::string const s = "foo";
    // the function ends and s is destroyed even though it was constant
}

Answer 2

They are called const_iterators because they don't allow you to change underlying value, not the container itself. You still need to have a non-const list to insert elements to, because insert is a non-const member of std::list, but the interface is made in the least restricting way.

Consider that you have a function that lets you find and iterator to some element. It doesn't need to change anything, so you design it to return const_iterator. And then you want to insert to list at this iterator, but you wouldn't be able to, if the interface for the list was more restricted then it is.