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6.17 — Introduction to iterators

Iterating through an array (or other structure) of data is quite a common thing to do in programming. And so far, we’ve covered many different ways to do so: with loops and an index (for-loops and while loops), with pointers and pointer arithmetic, and with range-based for-loops:

Looping using indexes is more typing than needed if we only use the index to access elements. It also only works if the container (e.g. the array) provides direct access to elements (which arrays do, but some other types of containers, such as lists, do not).

Looping with pointers and pointer arithmetic is verbose, and can be confusing to readers who don’t know the rules of pointer arithmetic. Pointer arithmetic also only works if elements are consecutive in memory (which is true for arrays, but not true for other types of containers, such as lists, trees, and maps).

For advanced readers

Pointers (without pointer arithmetic) can also be used to iterate through some non-sequential structures. In a linked list, each element is connected to the prior element by a pointer. We can iterate through the list by following the chain of pointers.

Range-based for-loops are a little more interesting, as the mechanism for iterating through our container is hidden -- and yet, they still work for all kinds of different structures (arrays, lists, trees, maps, etc…). How do these work? They use iterators.

Iterators

An iterator is an object designed to iterate through a container without having to care about the internal structure of the data.

The simplest kind of iterator is a pointer, which (using pointer arithmetic) works for consecutive data. Let’s revisit a simple array traversal using a pointer and pointer arithmetic:

Output:

0 1 2 3 4 5 6

In the above, we defined two variables: begin (which points to the beginning of our container), and end (which marks an end point). For arrays, the end marker is typically the place in memory where the last element would be if the container contained one more element.

The pointer then iterates between begin and end, and the current element can be accessed by dereferencing the pointer.

Warning

You might be tempted to calculate the end marker using the address-of operator and array syntax like so:

But this causes undefined behavior, because array[std::size(array)] accesses an element that is off the end of the array.

Instead, use:

Standard library iterators

Iterating is such a common operation that all standard library containers offer direct support for iteration. Instead of calculating our own begin and end points, we can simply ask the container for the begin and end points via functions conveniently named begin() and end():

This prints:

1 2 3

The iterator header also contains two generic functions (std::begin and std::end) that can be used:

This also prints:

1 2 3

Don’t worry about the types of the iterators for now, we’ll re-visit iterators in a later chapter. The important thing is that the iterator takes care of the details of iterating through the container. All we need are four things: the begin point, the end point, operator++ to move the iterator to the next element (or the end), and operator* to get the value of the current element.

Back to range-based for loops

All types that have begin and end member functions or can be used with std::begin and std::end are usable in range-based for-loops.

Behind the scenes, the range-based for-loop calls begin() and end() of the type to iterate over. std::array has begin and end member functions, so we can use it in a range-based loop. C-style fixed arrays can be used with std::begin and std::end functions, so we can loop through them with a range-based loop as well. Dynamic arrays don’t work though, because there is no std::end function for them (because the type information doesn’t contain the array’s length).

You’ll learn how to add functions to your types later, so that they can be used with range-based for-loops too.

Range-based for-loops aren’t the only thing that makes use of iterators. They’re also used in std::sort and other algorithms. Now that you know what they are, you’ll notice they’re used quite a bit in the standard library.

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