Different efficiency of iterator and const_iterator (STL)

Question

In Qt there are similar classes to list an map. These classes provide a begin_const() method that returns a const_iterator. The documentation says that these const_iterators should be used whenever possible since they are faster.

The STL only gives you a const_iterator if the instance itself is const. Only one begin() method is implemented (overloaded for const).

Is there any difference when read-accessing elements with iterator and const_iterator? (I dont'w know why there's a difference for them in Qt)

Answer 1

The documentation says that these const_iterators should be used whenever possible since they are faster.

It sure does. From http://qt-project.org/doc/qt-4.8/containers.html#stl-style-iterators:

For each container class, there are two STL-style iterator types: one that provides read-only access and one that provides read-write access. Read-only iterators should be used wherever possible because they are faster than read-write iterators.

What a stupid thing to say.

Safer? Yes. Faster? Even if this were the case (it apparently isn't with gcc and clang), it is rarely a reason to prefer const iterators over non-const ones. This is premature optimization. The reason to prefer const iterators over non-const ones is safety. If you don't need the pointed-to contents to be modified, use a const iterator. Think of what some maintenance programmer will do to your code.

As far as begin versus cbegin is concerned, that's a C++11 addition. This allows the auto keyword to use a const iterator, even in a non-const setting.