Classes (I)

Classes are an expanded concept of data structures: like data structures, they can contain data members, but they can also contain functions as members.

An object is an instantiation of a class. In terms of variables, a class would be the type, and an object would be the variable.

Classes are defined using either keyword class or keyword struct, with the following syntax:

class class_name {
  access_specifier_1:
    member1;
  access_specifier_2:
    member2;
  ...
} object_names;

Where class_name is a valid identifier for the class, object_names is an optional list of names for objects of this class. The body of the declaration can contain members, which can either be data or function declarations, and optionally access specifiers.

Classes have the same format as plain data structures, except that they can also include functions and have these new things called access specifiers. An access specifier is one of the following three keywords: private, public or protected. These specifiers modify the access rights for the members that follow them:

  • private members of a class are accessible only from within other members of the same class (or from their "friends").
  • protected members are accessible from other members of the same class (or from their "friends"), but also from members of their derived classes.
  • Finally, public members are accessible from anywhere where the object is visible.

By default, all members of a class declared with the class keyword have private access for all its members. Therefore, any member that is declared before any other access specifier has private access automatically. For example: 

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class Rectangle {
    int width, height;
  public:
    void set_values (int,int);
    int area (void);
} rect;


Declares a class (i.e., a type) called Rectangle and an object (i.e., a variable) of this class, called rect. This class contains four members: two data members of type int (member width and member height) with private access (because private is the default access level) and two member functions with public access: the functions set_values and area, of which for now we have only included their declaration, but not their definition.

Notice the difference between the class name and the object name: In the previous example, Rectangle was the class name (i.e., the type), whereas rect was an object of type Rectangle. It is the same relationship int and a have in the following declaration:

 
 
int a;


where int is the type name (the class) and a is the variable name (the object).

After the declarations of Rectangle and rect, any of the public members of object rect can be accessed as if they were normal functions or normal variables, by simply inserting a dot (.) between object name and member name. This follows the same syntax as accessing the members of plain data structures. For example: 

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rect.set_values (3,4);
myarea = rect.area();


The only members of rect that cannot be accessed from outside the class are width and height, since they have private access and they can only be referred to from within other members of that same class.

Here is the complete example of class Rectangle:
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// classes example
#include <iostream>
using namespace std;

class Rectangle {
    int width, height;
  public:
    void set_values (int,int);
    int area() {return width*height;}
};

void Rectangle::set_values (int x, int y) {
  width = x;
  height = y;
}

int main () {
  Rectangle rect;
  rect.set_values (3,4);
  cout << "area: " << rect.area();
  return 0;
}
 
area: 12


This example reintroduces the scope operator (::, two colons), seen in earlier chapters in relation to namespaces. Here it is used in the definition of function set_values to define a member of a class outside the class itself.

Notice that the definition of the member function area has been included directly within the definition of class Rectangle given its extreme simplicity. Conversely, set_values it is merely declared with its prototype within the class, but its definition is outside it. In this outside definition, the operator of scope (::) is used to specify that the function being defined is a member of the class Rectangle and not a regular non-member function.

The scope operator (::) specifies the class to which the member being declared belongs, granting exactly the same scope properties as if this function definition was directly included within the class definition. For example, the function set_values in the previous example has access to the variables width and height, which are private members of class Rectangle, and thus only accessible from other members of the class, such as this.

The only difference between defining a member function completely within the class definition or to just include its declaration in the function and define it later outside the class, is that in the first case the function is automatically considered an inline member function by the compiler, while in the second it is a normal (not-inline) class member function. This causes no differences in behavior, but only on possible compiler optimizations.

Members width and height have private access (remember that if nothing else is specified, all members of a class defined with keyword class have private access). By declaring them private, access from outside the class is not allowed. This makes sense, since we have already defined a member function to set values for those members within the object: the member function set_values. Therefore, the rest of the program does not need to have direct access to them. Perhaps in a so simple example as this, it is difficult to see how restricting access to these variables may be useful, but in greater projects it may be very important that values cannot be modified in an unexpected way (unexpected from the point of view of the object).

The most important property of a class is that it is a type, and as such, we can declare multiple objects of it. For example, following with the previous example of class Rectangle, we could have declared the object rectb in addition to object rect:

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// example: one class, two objects
#include <iostream>
using namespace std;

class Rectangle {
    int width, height;
  public:
    void set_values (int,int);
    int area () {return width*height;}
};

void Rectangle::set_values (int x, int y) {
  width = x;
  height = y;
}

int main () {
  Rectangle rect, rectb;
  rect.set_values (3,4);
  rectb.set_values (5,6);
  cout << "rect area: " << rect.area() << endl;
  cout << "rectb area: " << rectb.area() << endl;
  return 0;
}
 
rect area: 12
rectb area: 30


In this particular case, the class (type of the objects) is Rectangle, of which there are two instances (i.e., objects): rect and rectb. Each one of them has its own member variables and member functions.

Notice that the call to rect.area() does not give the same result as the call to rectb.area(). This is because each object of class Rectangle has its own variables width and height, as they -in some way- have also their own function members set_value and area that operate on the object's own member variables.

Classes allow programming using object-oriented paradigms: Data and functions are both members of the object, reducing the need to pass and carry handlers or other state variables as arguments to functions, because they are part of the object whose member is called. Notice that no arguments were passed on the calls to rect.area or rectb.area. Those member functions directly used the data members of their respective objects rect and rectb.

Constructors

What would happen in the previous example if we called the member function area before having called set_values? An undetermined result, since the members width and height had never been assigned a value.

In order to avoid that, a class can include a special function called its constructor, which is automatically called whenever a new object of this class is created, allowing the class to initialize member variables or allocate storage.

This constructor function is declared just like a regular member function, but with a name that matches the class name and without any return type; not even void.

The Rectangle class above can easily be improved by implementing a constructor:

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// example: class constructor
#include <iostream>
using namespace std;

class Rectangle {
    int width, height;
  public:
    Rectangle (int,int);
    int area () {return (width*height);}
};

Rectangle::Rectangle (int a, int b) {
  width = a;
  height = b;
}

int main () {
  Rectangle rect (3,4);
  Rectangle rectb (5,6);
  cout << "rect area: " << rect.area() << endl;
  cout << "rectb area: " << rectb.area() << endl;
  return 0;
}
 
rect area: 12
rectb area: 30


The results of this example are identical to those of the previous example. But now, class Rectangle has no member function set_values, and has instead a constructor that performs a similar action: it initializes the values of width and height with the arguments passed to it.

Notice how these arguments are passed to the constructor at the moment at which the objects of this class are created:

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Rectangle rect (3,4);
Rectangle rectb (5,6);


Constructors cannot be called explicitly as if they were regular member functions. They are only executed once, when a new object of that class is created.

Notice how neither the constructor prototype declaration (within the class) nor the latter constructor definition, have return values; not even void: Constructors never return values, they simply initialize the object.

Overloading constructors

Like any other function, a constructor can also be overloaded with different versions taking different parameters: with a different number of parameters and/or parameters of different types. The compiler will automatically call the one whose parameters match the arguments:

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// overloading class constructors
#include <iostream>
using namespace std;

class Rectangle {
    int width, height;
  public:
    Rectangle ();
    Rectangle (int,int);
    int area (void) {return (width*height);}
};

Rectangle::Rectangle () {
  width = 5;
  height = 5;
}

Rectangle::Rectangle (int a, int b) {
  width = a;
  height = b;
}

int main () {
  Rectangle rect (3,4);
  Rectangle rectb;
  cout << "rect area: " << rect.area() << endl;
  cout << "rectb area: " << rectb.area() << endl;
  return 0;
}
 
rect area: 12
rectb area: 25


In the above example, two objects of class Rectangle are constructed: rect and rectb. rect is constructed with two arguments, like in the example before.

But this example also introduces a special kind constructor: the default constructor. The default constructor is the constructor that takes no parameters, and it is special because it is called when an object is declared but is not initialized with any arguments. In the example above, the default constructor is called for rectb. Note how rectb is not even constructed with an empty set of parentheses - in fact, empty parentheses cannot be used to call the default constructor:

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Rectangle rectb;   // ok, default constructor called
Rectangle rectc(); // oops, default constructor NOT called


This is because the empty set of parentheses would make of rectc a function declaration instead of an object declaration: It would be a function that takes no arguments and returns a value of type Rectangle.

Uniform initialization

The way of calling constructors by enclosing their arguments in parentheses, as shown above, is known as functional form. But constructors can also be called with other syntaxes:

First, constructors with a single parameter can be called using the variable initialization syntax (an equal sign followed by the argument):

class_name object_name = initialization_value;

More recently, C++ introduced the possibility of constructors to be called using uniform initialization, which essentially is the same as the functional form, but using braces ({}) instead of parentheses (()):

class_name object_name { value, value, value, ... }

Optionally, this last syntax can include an equal sign before the braces.

Here is an example with four ways to construct objects of a class whose constructor takes a single parameter:

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// classes and uniform initialization
#include <iostream>
using namespace std;

class Circle {
    double radius;
  public:
    Circle(double r) { radius = r; }
    double circum() {return 2*radius*3.14159265;}
};

int main () {
  Circle foo (10.0);   // functional form
  Circle bar = 20.0;   // assignment init.
  Circle baz {30.0};   // uniform init.
  Circle qux = {40.0}; // POD-like

  cout << "foo's circumference: " << foo.circum() << '\n';
  return 0;
}
 
foo's circumference: 62.8319


An advantage of uniform initialization over functional form is that, unlike parentheses, braces cannot be confused with function declarations, and thus can be used to explicitly call default constructors:

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Rectangle rectb;   // default constructor called
Rectangle rectc(); // function declaration (default constructor NOT called)
Rectangle rectd{}; // default constructor called


The choice of syntax to call constructors is largely a matter of style. Most existing code currently uses functional form, and some newer style guides suggest to choose uniform initialization over the others, even though it also has its potential pitfalls for its preference of initializer_list as its type.

Member initialization in constructors

When a constructor is used to initialize other members, these other members can be initialized directly, without resorting to statements in its body. This is done by inserting, before the constructor's body, a colon (:) and a list of initializations for class members. For example, consider a class with the following declaration:

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class Rectangle {
    int width,height;
  public:
    Rectangle(int,int);
    int area() {return width*height;}
};


The constructor for this class could be defined, as usual, as:

 
 
Rectangle::Rectangle (int x, int y) { width=x; height=y; }


But it could also be defined using member initialization as:

 
 
Rectangle::Rectangle (int x, int y) : width(x) { height=y; }


Or even:

 
 
Rectangle::Rectangle (int x, int y) : width(x), height(y) { }


Note how in this last case, the constructor does nothing else than initialize its members, hence it has an empty function body.

For members of fundamental types, it makes no difference which of the ways above the constructor is defined, because they are not initialized by default, but for member objects (those whose type is a class), if they are not initialized after the colon, they are default-constructed.

Default-constructing all members of a class may or may always not be convenient: in some cases, this is a waste (when the member is then reinitialized otherwise in the constructor), but in some other cases, default-construction is not even possible (when the class does not have a default constructor). In these cases, members shall be initialized in the member initialization list. For example:

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// member initialization
#include <iostream>
using namespace std;

class Circle {
    double radius;
  public:
    Circle(double r) : radius(r) { }
    double area() {return radius*radius*3.14159265;}
};

class Cylinder {
    Circle base;
    double height;
  public:
    Cylinder(double r, double h) : base (r), height(h) {}
    double volume() {return base.area() * height;}
};

int main () {
  Cylinder foo (10,20);

  cout << "foo's volume: " << foo.volume() << '\n';
  return 0;
}
 
foo's volume: 6283.19


In this example, class Cylinder has a member object whose type is another class (base's type is Circle). Because objects of class Circle can only be constructed with a parameter, Cylinder's constructor needs to call base's constructor, and the only way to do this is in the member initializer list.

These initializations can also use uniform initializer syntax, using braces {} instead of parentheses ():

 
 
Cylinder::Cylinder (double r, double h) : base{r}, height{h} { }


Pointers to classes

Objects can also be pointed to by pointers: Once declared, a class becomes a valid type, so it can be used as the type pointed to by a pointer. For example: 

 
 
Rectangle * prect;


is a pointer to an object of class Rectangle.

Similarly as with plain data structures, the members of an object can be accessed directly from a pointer by using the arrow operator (->). Here is an example with some possible combinations:

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// pointer to classes example
#include <iostream>
using namespace std;

class Rectangle {
  int width, height;
public:
  Rectangle(int x, int y) : width(x), height(y) {}
  int area(void) { return width * height; }
};


int main() {
  Rectangle obj (3, 4);
  Rectangle * foo, * bar, * baz;
  foo = &obj;
  bar = new Rectangle (5, 6);
  baz = new Rectangle[2] { {2,5}, {3,6} };
  cout << "obj's area: " << obj.area() << '\n';
  cout << "*foo's area: " << foo->area() << '\n';
  cout << "*bar's area: " << bar->area() << '\n';
  cout << "baz[0]'s area:" << baz[0].area() << '\n';
  cout << "baz[1]'s area:" << baz[1].area() << '\n';       
  delete bar;
  delete[] baz;
  return 0;
}


This example makes use of several operators to operate on objects and pointers (operators *, &, ., ->, []). They can be interpreted as:

expressioncan be read as
*xpointed to by x
&xaddress of x
x.ymember y of object x
x->ymember y of object pointed to by x
(*x).ymember y of object pointed to by x (equivalent to the previous one)
x[0]first object pointed to by x
x[1]second object pointed to by x
x[n](n+1)th object pointed to by x

Most of these expressions have been introduced in earlier chapters. Most notably, the chapter about arrays introduced the offset operator ([]) and the chapter about plain data structures introduced the arrow operator (->).

Classes defined with struct and union

Classes can be defined not only with keyword class, but also with keywords struct and union.

The keyword struct, generally used to declare plain data structures, can also be used to declare classes that have member functions, with the same syntax as with keyword class. The only difference between both is that members of classes declared with the keyword struct have public access by default, while members of classes declared with the keyword class have private access by default. For all other purposes both keywords are equivalent in this context.

Conversely, the concept of unions is different from that of classes declared with struct and class, since unions only store one data member at a time, but nevertheless they are also classes and can thus also hold member functions. The default access in union classes is public.
Index
Index