12.2 — Classes and class members

While C++ provides a number of fundamental data types (e.g. char, int, long, float, double, etc…) that are often sufficient for solving relatively simple problems, it can be difficult to solve complex problems using just these types. One of C++’s more useful features is the ability to define your own data types that better correspond to the problem being solved. You have already seen how enumerated types and structs can be used to create your own custom data types.

Here is an example of a struct used to hold a date:

Enumerated types and data-only structs (structs that only contain variables) represent the traditional non-object-oriented programming world, as they can only hold data. We can create and initialize this struct as follows:

Now, if we want to print the date to the screen (something we probably want to do a lot), it makes sense to write a function to do this. Here’s a full program:

This program prints:

2020/10/16

Classes

In the world of object-oriented programming, we often want our types to not only hold data, but provide functions that work with the data as well. In C++, this is typically done via the class keyword. The class keyword defines a new user-defined type called a class.

In C++, classes and structs are essentially the same. In fact, the following struct and class are effectively identical:

Note that the only significant difference is the public: keyword in the class. We will discuss the function of this keyword in the next lesson.

Just like a struct declaration, a class declaration does not allocate any memory. It only defines what the class looks like.

Class (and struct) definitions are like a blueprint -- they describe what the resulting object will look like, but they do not actually create the object. To actually create an object of the class, a variable of that class type must be defined:

Member Functions

In addition to holding data, classes (and structs) can also contain functions! Functions defined inside of a class are called member functions (or sometimes methods). Member functions can be defined inside or outside of the class definition. We’ll define them inside the class for now (for simplicity), and show how to define them outside the class later.

Here is our Date class with a member function to print the date:

Just like members of a struct, members (variables and functions) of a class are accessed using the member selector operator (.):

This prints:

2020/10/16

Note how similar this program is to the struct version we wrote above.

However, there are a few differences. In the DateStruct version of print() from the example above, we needed to pass the struct itself to the print() function as the first parameter. Otherwise, print() wouldn’t know what DateStruct we wanted to use. We then had to reference this parameter inside the function explicitly.

Member functions work slightly differently: All member function calls must be associated with an object of the class. When we call “today.print()”, we’re telling the compiler to call the print() member function, associated with the today object.

Now let’s take a look at the definition of the print member function again:

What do m_year, m_month, and m_day actually refer to? They refer to the associated object (as determined by the caller).

So when we call “today.print()”, the compiler interprets m_day as today.m_day, m_month as today.m_month, and m_year as today.m_year. If we called “tomorrow.print()”, m_day would refer to tomorrow.m_day instead.

In this way, the associated object is essentially implicitly passed to the member function. For this reason, it is often called the implicit object.

We’ll talk more about how the implicit object passing works in detail in a later lesson in this chapter.

The key point is that with non-member functions, we have to pass data to the function to work with. With member functions, we can assume we always have an implicit object of the class to work with!

Using the “m_” prefix for member variables helps distinguish member variables from function parameters or local variables inside member functions. This is useful for several reasons. First, when we see an assignment to a variable with the “m_” prefix, we know that we are changing the state of the class instance. Second, unlike function parameters or local variables, which are declared within the function, member variables are declared in the class definition. Consequently, if we want to know how a variable with the “m_” prefix is declared, we know that we should look in the class definition instead of within the function.

By convention, class names should begin with an upper-case letter.

Here’s another example of a class:

This produces the output:

Name: Alex  Id: 1  Wage: $25
Name: Joe  Id: 2  Wage: $22.25

With normal non-member functions, a function can’t call a function that’s defined “below” it (without a forward declaration):

With member functions, this limitation doesn’t apply:

Member types

In addition to member variables and member functions, classes can have member types or nested types (including type aliases). In the following example, we’re creating a calculator where we can swiftly change the type of number it’s using if we ever need to.

Output

7
123
7
123

In such a context, the class name effectively acts like a namespace for the nested type. From inside the class, we only need reference number_type. From outside the class, we can access the type via Calculator::number_type.

When we decide that an int no longer fulfills our needs and we want to use a double, we only need to update the type alias, rather than having to replace every occurrence of int with double.

Type alias members make code easier to maintain and can reduce typing. Template classes, which we’ll cover later, often make use of type alias members. You’ve already seen this as std::vector::size_type, where size_type is an alias for an unsigned integer.

Up to now, we used a “_t” suffix for type aliases. For member type aliases, a “_type” or no suffix at all is more common.

Nested types cannot be forward declared. Generally, nested types should only be used when the nested type is used exclusively within that class. Note that since classes are types, it’s possible to nest classes inside other classes -- this is uncommon and is typically only done by advanced programmers.

A note about structs in C++

In C, structs can only hold data, and do not have associated member functions. In C++, after designing classes (using the class keyword), Bjarne Stroustrup spent some amount of time considering whether structs (which were inherited from C) should be granted the ability to have member functions. Upon consideration, he determined that they should, in part to have a unified ruleset for both. So although we wrote the above programs using the class keyword, we could have used the struct keyword instead.

Many developers (including myself) feel this was the incorrect decision to be made, as it can lead to dangerous assumptions. For example, it’s fair to assume a class will clean up after itself (e.g. a class that allocates memory will deallocate it before being destroyed), but it’s not safe to assume a struct will. Consequently, we recommend using the struct keyword for data-only structures, and the class keyword for defining objects that require both data and functions to be bundled together.

You have already been using classes without knowing it

It turns out that the C++ standard library is full of classes that have been created for your benefit. std::string, std::vector, and std::array are all class types! So when you create an object of any of these types, you’re instantiating a class object. And when you call a function using these objects, you’re calling a member function.

Conclusion

The class keyword lets us create a custom type in C++ that can contain both member variables and member functions. Classes form the basis for Object-oriented programming, and we’ll spend the rest of this chapter and many of the future chapters exploring all they have to offer!

Quiz time

Pair(1, 1)
Pair(2, 2)

12.3 -- Public vs private access specifiers

Index

12.1 -- Welcome to object-oriented programming