13.1 — Welcome to object-oriented programming

Object-oriented programming (OOP) provides us with the ability to create objects that tie together both properties and behaviors into a self-contained, reusable package. This leads to code that looks more like this:

you.driveTo(work);

Note that the term “object” is overloaded a bit, and this causes some amount of confusion. In traditional programming, an object is a piece of memory to store values. And that’s it. In object-oriented programming, an “object” implies that it is both an object in the traditional programming sense, and that it combines both properties and behaviors. From this point forward, when we use the term “object”, we’ll be referring to “objects” in the object-oriented sense.

13.2 — Classes and class members

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

struct DateStruct
{
    int year {};
    int month {};
    int day {};
};

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:

DateStruct today { 2020, 10, 14 }; // use uniform initialization
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:

#include <iostream>

struct DateStruct
{
    int year {};
    int month {};
    int day {};
};

void print(const DateStruct& date)
{
    std::cout << date.year << '/' << date.month << '/' << date.day;
}

int main()
{
    DateStruct today { 2020, 10, 14 }; // use uniform initialization

    today.day = 16; // use member selection operator to select a member of the struct
    print(today);

    return 0;
}

This program prints:

2020/10/16

Classes

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

struct DateStruct
{
    int year {};
    int month {};
    int day {};
};

class DateClass
{
public:
    int m_year {};
    int m_month {};
    int m_day {};
};

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.

Warning

Just like with structs, one of the easiest mistakes to make in C++ is to forget the semicolon at the end of a class declaration. This will cause a compiler error on the next line of code. Modern compilers like Visual Studio 2010 will give you an indication that you may have forgotten a semicolon, but older or less sophisticated compilers may not, which can make the actual error hard to find.

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:

DateClass today { 2020, 10, 14 }; // declare a variable of class DateClass

A reminder

Initialize the member variables of a class at the point of declaration.

Member Functions

Best practice

Name your classes starting with a capital letter.

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

void x()
{
// You can't call y() from here unless the compiler has already seen a forward declaration for y()
}

void y()
{
}

With member functions, this limitation doesn’t apply:

class foo
{
public:
     void x() { y(); } // okay to call y() here, even though y() isn't defined until later in this class
     void y() { };
};

Member types

A note about structs in C++

Best practice

Use the struct keyword for data-only structures. Use the class keyword for objects that have both data and functions.

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!

13.3 — Public vs private access specifiers

Public and private members

The code outside of a struct or class is sometimes called the public: the public is only allowed to access the public members of a struct or class, which makes sense.

Access specifiers

Mixing access specifiers

A class can (and almost always does) use multiple access specifiers to set the access levels of each of its members. There is no limit to the number of access specifiers you can use in a class.

In general, member variables are usually made private, and member functions are usually made public. We’ll take a closer look at why in the next lesson.

Best practice

Make member variables private, and member functions public, unless you have a good reason not to.

Some programmers prefer to list private members first, because the public members typically use the private ones, so it makes sense to define the private ones first. However, a good counterargument is that users of the class don’t care about the private members, so the public ones should come first. Either way is fine.

Access controls work on a per-class basis

#include <iostream>

class DateClass // members are private by default
{
	int m_month {}; // private by default, can only be accessed by other members
	int m_day {}; // private by default, can only be accessed by other members
	int m_year {}; // private by default, can only be accessed by other members

public:
	void setDate(int month, int day, int year)
	{
		m_month = month;
		m_day = day;
		m_year = year;
	}

	void print()
	{
		std::cout << m_month << '/' << m_day << '/' << m_year;
	}

	// Note the addition of this function
	void copyFrom(const DateClass& d)
	{
		// Note that we can access the private members of d directly
		m_month = d.m_month;
		m_day = d.m_day;
		m_year = d.m_year;
	}
};

int main()
{
	DateClass date;
	date.setDate(10, 14, 2020); // okay, because setDate() is public

	DateClass copy {};
	copy.copyFrom(date); // okay, because copyFrom() is public
	copy.print();
	std::cout << '\n';

	return 0;
}

One nuance of C++ that is often missed or misunderstood is that access control works on a per-class basis, not a per-object basis. This means that when a function has access to the private members of a class, it can access the private members of any object of that class type that it can see.

In the above example, copyFrom() is a member of DateClass, which gives it access to the private members of DateClass. This means copyFrom() can not only directly access the private members of the implicit object it is operating on (copy), it also means it has direct access to the private members of DateClass parameter d! If parameter d were some other type, this would not be the case.

This can be particularly useful when we need to copy members from one object of a class to another object of the same class. We’ll also see this topic show up again when we talk about overloading operator<< to print members of a class in the next chapter.

Structs vs classes revisited

Now that we’ve talked about access specifiers, we can talk about the actual differences between a class and a struct in C++. A class defaults its members to private. A struct defaults its members to public.

That’s it!

(Okay, to be pedantic, there’s one more minor difference – structs inherit from other classes publicly and classes inherit privately. We’ll cover what this means in a future chapter, but this particular point is practically irrelevant since you should never rely on the defaults anyway).

13.4 — Access functions and encapsulation

Encapsulation

Note: The word encapsulation is also sometimes used to refer to the packaging of data and functions that work on that data together. We prefer to just call that object-oriented programming.

Benefit: encapsulated classes are easier to use and reduce the complexity of your programs

Benefit: encapsulated classes help protect your data and prevent misuse

Access functions

Best practice

Getters should return by value or const reference.

13.5 — Constructors

A constructor is a special kind of class member function that is automatically called when an object of that class is created. Constructors are typically used to initialize member variables of the class to appropriate user-provided values, or to do any setup steps necessary for the class to be used (e.g. open a file or database).

After a constructor executes, the object should be in a well-defined, usable state.

Unlike normal member functions, constructors have specific rules for how they must be named:

1. Constructors must have the same name as the class (with the same capitalization)
2. Constructors have no return type (not even void)

Default constructors and default initialization

Value-initialization

Best practice

Favor value-initialization over default-initialization for class objects.

Note that value initialization uses empty braces, not empty parenthesis:

Fraction f1 {}; // value initialization of Fraction f1
Fraction f2();  // forward declaration of function f2

Direct- and list-initialization using constructors with parameters

Best practice

Favor brace initialization to initialize class objects.

Copy initialization using equals with classes

Much like with fundamental variables, it’s also possible to initialize classes using copy initialization:

Fraction six = Fraction{ 6 }; // Copy initialize a Fraction, will call Fraction(6, 1)
Fraction seven = 7; // Copy initialize a Fraction.  The compiler will try to find a way to convert 7 to a Fraction, which will invoke the Fraction(7, 1) constructor.

However, we recommend you avoid this form of initialization with classes, as it may be less efficient. Although direct-initialization, list-initialization, and copy-initialization all work identically with fundamental types, copy-initialization does not work the same with classes (though the end-result is often the same). We’ll explore the differences in more detail in a future chapter.

Reducing your constructors

In the above two-constructor declaration of the Fraction class, the default constructor is actually somewhat redundant. We could simplify this class as follows:

#include <cassert>

class Fraction
{
private:
    int m_numerator {};
    int m_denominator {};

public:
    // Default constructor
    Fraction(int numerator=0, int denominator=1)
    {
        assert(denominator != 0);

        m_numerator = numerator;
        m_denominator = denominator;
    }

    int getNumerator() { return m_numerator; }
    int getDenominator() { return m_denominator; }
    double getValue() { return static_cast<double>(m_numerator) / m_denominator; }
};

Although this constructor is still a default constructor, it has now been defined in a way that it can accept one or two user-provided values as well.

Fraction zero; // will call Fraction(0, 1)
Fraction zero{}; // will call Fraction(0, 1)
Fraction six{ 6 }; // will call Fraction(6, 1)
Fraction fiveThirds{ 5, 3 }; // will call Fraction(5, 3)

When implementing your constructors, consider how you might keep the number of constructors down through smart defaulting of values.

A reminder about default parameters

If we want to be able to construct a Something with only a double, we’ll need to add a second (non-default) constructor:

class Something
{
public:
	// Default constructor
	Something(int n = 0, double d = 1.2) // allows us to construct a Something(int, double), Something(int), or Something()
	{
	}

	Something(double d)
	{
	}
};

int main()
{
	Something s1 { 1, 2.4 }; // calls Something(int, double)
	Something s2 { 1 }; // calls Something(int, double)
	Something s3 {}; // calls Something(int, double)

	Something s4 { 2.4 }; // calls Something(double)

	return 0;
}

An implicitly generated default constructor

Best practice

If you have constructors in your class and need a default constructor that does nothing (e.g. because all your members are initialized using non-static member initialization), use = default.

Classes containing class members

Constructor notes

Best practice

Always initialize all member variables in your objects.

Quiz time

Question #2

What happens if you don’t declare a default constructor?

Hide Solution

If you haven’t defined any other constructors, the compiler will create an empty public default constructor for you. This means your objects will be instantiable with no parameters. If you have defined other constructors (default or otherwise), the compiler will not create a default constructor for you. Assuming you haven’t provided a default constructor yourself, your objects will not be instantiable without arguments.

13.6 — Constructor member initializer lists

Member initializer lists

To solve this problem, C++ provides a method for initializing class member variables (rather than assigning values to them after they are created) via a member initializer list (often called a “member initialization list”). Do not confuse these with the similarly named initializer list that we can use to assign values to arrays.

Now let’s write the same code using an initialization list:

#include <iostream>

class Something
{
private:
    int m_value1 {};
    double m_value2 {};
    char m_value3 {};

public:
    Something() : m_value1{ 1 }, m_value2{ 2.2 }, m_value3{ 'c' } // Initialize our member variables
    {
    // No need for assignment here
    }

    void print()
    {
         std::cout << "Something(" << m_value1 << ", " << m_value2 << ", " << m_value3 << ")\n";
    }
};

int main()
{
    Something something{};
    something.print();
    return 0;
}

The member initializer list is inserted after the constructor parameters. It begins with a colon (