What is the difference between const_iterator and non-const iterator in the C++ STL?

const_iterators don't allow you to change the values that they point to, regular iterators do.

As with all things in C++, always prefer const, unless there's a good reason to use regular iterators (i.e. you want to use the fact that they're not const to change the pointed-to value).

They should pretty much be self-explanatory. If iterator points to an element of type T, then const_iterator points to an element of type 'const T'.

It's basically equivalent to the pointer types:

T* // A non-const iterator to a non-const element. Corresponds to std::vector<T>::iterator
T* const // A const iterator to a non-const element. Corresponds to const std::vector<T>::iterator
const T* // A non-const iterator to a const element. Corresponds to std::vector<T>::const_iterator

A const iterator always points to the same element, so the iterator itself is const. But the element it points to does not have to be const, so the element it points to can be changed. A const_iterator is an iterator that points to a const element, so while the iterator itself can be updated (incremented or decremented, for example), the element it points to can not be changed.

Use const_iterator whenever you can, use iterator when you have no other choice.

(as others have said) const_iterator doesn't allow you modify the elements to which it points, this is useful inside of const class methods. It also allows you to express your intent.

ok Let me explain it with very simple example first without using constant iterator consider we have collection of random integers collection "randomData"

    for(vector<int>::iterator i = randomData.begin() ; i != randomData.end() ; ++i)*i = 0;
for(vector<int>::const_iterator i = randomData.begin() ; i!= randomData.end() ; ++i)cout << *i;

As can be seen for writing/editing data inside collection normal iterator is used but for reading purpose constant iterator has been used . If you try using constant iterator in first for loop you will get error . As a thumb rule use constant iterator to read data inside collection .