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10.8 — Container classes

In real life, we use containers all the time. Your breakfast cereal comes in a box, the pages in your book come inside a cover and binding, and you might store any number of items in containers in your garage. Without containers, it would be extremely inconvenient to work with many of these objects. Imagine trying to read a book that didn’t have any sort of binding, or eat cereal that didn’t come in a box without using a bowl. It would be a mess. The value the container provides is largely in its ability to help organize and store items that are put inside it.

Similarly, a container class is a class designed to hold and organize multiple instances of another type (either another class, or a fundamental type). There are many different kinds of container classes, each of which has various advantages, disadvantages, and restrictions in their use. By far the most commonly used container in programming is the array, which you have already seen many examples of. Although C++ has built-in array functionality, programmers will often use an array container class (std::array or std::vector) instead because of the additional benefits they provide. Unlike built-in arrays, array container classes generally provide dynamic resizing (when elements are added or removed), remember their size when they are passed to functions, and do bounds-checking. This not only makes array container classes more convenient than normal arrays, but safer too.

Container classes typically implement a fairly standardized minimal set of functionality. Most well-defined containers will include functions that:

  • Create an empty container (via a constructor)
  • Insert a new object into the container
  • Remove an object from the container
  • Report the number of objects currently in the container
  • Empty the container of all objects
  • Provide access to the stored objects

Sometimes certain container classes will omit some of this functionality. For example, arrays container classes often omit the insert and remove functions because they are slow and the class designer does not want to encourage their use.

Container classes implement a member-of relationship. For example, elements of an array are members-of (belong to) the array. Note that we’re using “member-of” in the conventional sense, not the C++ class member sense.

Types of containers

Container classes generally come in two different varieties. Value containers are composition that store copies of the objects that they are holding (and thus are responsible for creating and destroying those copies), for example std::string. Reference containers or views are aggregations that store pointers or references to other objects (and thus are not responsible for creation or destruction of those objects), for example std::string_view.

Unlike in real life, where containers can hold whatever types of objects you put in them, in C++, containers typically only hold one type of data. For example, if you have an array of integers, it will only hold integers. Unlike some other languages, many C++ containers do not allow you to arbitrarily mix types. If you need containers to hold integers and doubles, you will generally have to write two separate containers to do this. Despite the restrictions on their use, containers are immensely useful, and they make programming easier, safer, and faster.

An array container class

In this example, we are going to write an integer array class from scratch that implements most of the common functionality that containers should have. This array class is going to be a value container, which will own the elements it’s organizing. As the name suggests, the container will hold an array of integers, similar to std::vector<int>.

First, let’s create the IntArray.h file:

Our IntArray is going to need to keep track of two values: the data itself, and the size of the array. Because we want our array to be able to change in size, we’ll have to do some dynamic allocation, which means we’ll have to use a pointer to store the data.

Now we need to add some constructors that will allow us to create IntArrays. We are going to add two constructors: one that constructs an empty array, and one that will allow us to construct an array of a predetermined size.

We’ll also need some functionality to help us clean up IntArrays. We’ll write a function called erase(), which will erase the array and set the length to 0.

Now let’s overload the [] operator so we can access the elements of the array. We perform bounds checking on the index to make sure it’s valid, which we do using the assert() function. By convention, element access operators don’t perform bounds checking, but at() functions do. We add an assert() anyway, because it only runs in debug mode, so it has no impact when we compile the program in release mode. We’ll also add an access function to return the length of the array. Here’s everything so far:

At this point, we already have an IntArray class that we can use. We can allocate IntArrays of a given size, and we can use the [] operator to retrieve or change the value of the elements.

Output

0 0 123 0 0

However, there are still a few thing we can’t do with our IntArray. We still can’t change its size, still can’t insert or delete elements, and we still can’t sort it.

First, let’s write begin() and end() member functions. These will allow us to use IntArray with many of the standard functions as well as range-based for-loops. begin() and end() are straightforward, because we can use the begin and end of the array the IntArray storing internally.

Second, let’s write some code that will allow us to resize an array. We are going to write a resize() function, that will keep any existing elements in the array when its size increases and add new elements with value 0. If the array shrinks, all elements that are past the new end of the array are discarded.

std::move() is a sneak peak at move semantics, which we mentioned when introducing std::unique_ptr. It transfers ownership of the new array from data to m_data. By using std::move(), data is reset to a nullptr and won’t try to destroy the resource when it dies.

Whew! That was a little tricky! But it was worth it, we can now use range-based for-loops and standard algorithms

Output

0 0 123 0 0
0 123 312

Many array container classes would stop here. However, just in case you want to see how insert and delete functionality would be implemented we’ll go ahead and write those too. Both of these algorithms are very similar to resize().

Here is our IntArray container class in its entirety.

IntArray.h:

Now, let’s test it just to prove it works:

This produces the result:

40 1 2 3 5 20 6 7 8 30

Although writing container classes can be pretty complex, the good news is that most of the time you can use standard containers instead, and if you do need to write custom containers, you only have to write them once. Once the container class is working, you can use and reuse it as often as you like without any additional programming effort required.

It is also worth explicitly mentioning that even though our sample IntArray container class holds a built-in data type (int), we could have just as easily used a user-defined type (e.g. a Point class).

One more thing: If a class in the standard library meets your needs, use that instead of creating your own. For example, instead of using IntArray, you’re better off using std::vector<int>. It’s battle tested, efficient, and plays nicely with the other classes in the standard library. But sometimes you need a specialized container class that doesn’t exist in the standard library, so it’s good to know how to create your own when you need to. We’ll talk more about containers in the standard library once we’ve covered a few more fundamental topics.

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