Vector Iterators

There are 2 ways to access the beginning and end of a vector:

• .cbegin(), .begin() : these will give you an iterator that points to the first element. The difference is that accessing with .cbegin() will not allow you to modify the vector (the c is for const). You can increment an iterator loop over the contents of the vector.

• .cend(), .end() : these will return an iterator that points to one past the last element.

Tip

An iterator can be thought of as a special type of pointer. Iterators have restrictions on their use, depending on the container—this makes them more safe to use.

If we think about an iterator like:

auto it = container.cbegin();

Then we can access the next element in container by incrementing the iterator, it++.

If we want to see the value in container that the iterator is pointing to, then we need to dereference it—just as with pointers, this is done with the * operator:

std::cout << "cbegin is " << *it << std::endl;

Note

C++ doesn’t support printing the iterator itself, e.g.,

std::cout << it << std::endl;

does not compile. If you want to see the memory address (for some reason), you need to dereference the iterator and then take the address of the object it is pointing to, e.g.,

std::cout << &(*it) << std::endl;

Looping with iterators

Here’s an example of looping over an entire vector using iterators:

Listing 84 iterator_loop.cpp

#include <iostream>
#include <vector>

int main() {

    std::vector<int> container{0, 1, 1, 2, 3, 5, 8, 13, 21};

    for (auto it = container.cbegin(); it < container.cend(); ++it) {
        std::cout << *it << std::endl;
    }

}

Notice that we advance the iterator to point to the next element in the vector using ++it. We could also do it += 4 to skip 4 elements (but it is up to us to ensure that we don’t go past the end).

Important

Remember that container.cend() points to one past the last element. This is why we use the < operator here, and not <=.

There’s really no advantage in this example in using iterators vs. the ranged-for loop we’ve been using until now.