what is the difference between const_iterator and iterator? [duplicate]

Solution 1:

There is no performance difference.

A const_iterator is an iterator that points to const value (like a const T* pointer); dereferencing it returns a reference to a constant value (const T&) and prevents modification of the referenced value: it enforces const-correctness.

When you have a const reference to the container, you can only get a const_iterator.

Edited: I mentionned “The const_iterator returns constant pointers” which is not accurate, thanks to Brandon for pointing it out.

Edit: For COW objects, getting a non-const iterator (or dereferencing it) will probably trigger the copy. (Some obsolete and now disallowed implementations of std::string use COW.)

Solution 2:

Performance wise there is no difference. The only purpose of having const_iterator over iterator is to manage the accessesibility of the container on which the respective iterator runs. You can understand it more clearly with an example:

std::vector<int> integers{ 3, 4, 56, 6, 778 };

If we were to read & write the members of a container we will use iterator:

for( std::vector<int>::iterator it = integers.begin() ; it != integers.end() ; ++it )
       {*it = 4;  std::cout << *it << std::endl; }

If we were to only read the members of the container integers you might wanna use const_iterator which doesn't allow to write or modify members of container.

for( std::vector<int>::const_iterator it = integers.begin() ; it != integers.end() ; ++it )
       { cout << *it << endl; }

NOTE: if you try to modify the content using *it in second case you will get an error because its read-only.