Thread: What is the purpose of iterators?

In your first example, it would be enough to write:

Code:

vector<int> v(100);

Or to be more explicitly:

Code:

vector<int> v(100, 0);

But now, suppose later you want to "reset" the elements to say, 123. You can then #include <algorithm> and write:

Code:

std::fill(v.begin(), v.end(), 123);

This is even "easier to read, much easier to type, just as optimised and just as safe". It also brings us to one answer to your question: you can use iterators to write function templates that implement generic algorithms.

Originally Posted by yaya

For the erase() function, you can simply do this:

In which case you are using an iterator.

Originally Posted by yaya

Yeah, I understand that there are better ways to do this, but I just wanted a bare-bones example.

My point is that iterators allow for the better ways to do this, and not only "to go through all of the elements of the vector", but to go through a range of elements via suitable iterators. This range could be a sub-range from a vector or some other container, or sometimes container objects are not even involved.

Originally Posted by yaya

I am assuming you want to go through all of the elements of the vector to do some sort of processing.

If you prefer to use array indexing to solve your particular problem, go ahead. It is not wrong.

All containers do not support indexing. Take a set, for example. Or a map. You can't use indexes there.
So how do you iterate through such containers? You do it through iterators.
Iterator is a concept that defines a way to iterate over a range. So it makes sense to have it apply to all containers, not just those where index operators do not work.
Some containers also support indices. They, too, have their uses.

Some of the vector functions require an iterator or you can't use them, insert, erase for example. So without them you cant use the full functionality of the type.

While they do require iterators, they could just as well have accepted indices. For a vector, there is no advantage in using iterators.
But for other containers, such as linked lists, there is a need. Sure, it could theoretically support indices, but then most operators would be O(N) instead of O(1).

If all containers have a very different interface, this would make it impossible to write generic code (templates) to work with many container types.

std::string supports both indices and iterators, though, for member functions (not that the abundance of overloads doesn't get confusing at times).

For example, suppose we want to fill an empty vector with a for loop. Why would we do it like this:

Yeah, iterators make for loops verbose (disregarding the fact that here there are alternative and better ways to fill containers with a value).

The new standard will give you tools to make it look better:

Code:

//automatic variable type deduction
for (auto it = vec.begin(); it != vec.end(); ++it) {
     *it = 0;
}

//range-based for loops
for (int& i: vec) {
    i = 0;
}