Classes (I)
A class is an expanded concept of a data structure: instead of holding only data, it can hold
both data and functions.
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 generally declared using the keyword class, with the following format:
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, that can be
either data or function declarations, and optionally access specifiers.
All is very similar to the declaration on data structures, except that we can now include also
functions and members, but also this new thing called access specifier. An access specifier is
one of the following three keywords: private, public or protected. These specifiers
modify the access rights that the members following them acquire:
• 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 members of their same class and 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 one other class specifier
automatically has private access. For example:
class CRectangle {
int x, y;
public:
void set_values (int,int);
int area (void);
} rect;
Declares a class (i.e., a type) called CRectangle and an object (i.e., a variable) of this class
called rect. This class contains four members: two data members of type int (member x
and member y) with private access (because private is the default access level) and two
member functions with public access: set_values() and area(), of which for now we have
only included their declaration, not their definition.
Notice the difference between the class name and the object name: In the previous example,
CRectangle was the class name (i.e., the type), whereas rect was an object of type
CRectangle. 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 previous declarations of CRectangle and rect, we can refer within the body of the
program to any of the public members of the object rect as if they were normal functions or
normal variables, just by putting the object's name followed by a dot (.) and then the name
of the member. All very similar to what we did with plain data structures before. For example:
rect.set_values (3,4);
myarea = rect.area();
The only members of rect that we cannot access from the body of our program outside the
class are x and y, since they have private access and they can only be referred from within
other members of that same class.
Here is the complete example of class CRectangle:
// classes example
#include
using namespace std;
class CRectangle {
int x, y;
public:
void set_values (int,int);
int area () {return (x*y);}
};
void CRectangle::set_values (int a,
int b) {
x = a;
y = b;
}
int main () {
CRectangle rect;
rect.set_values (3,4);
cout << "area: " << rect.area();
return 0;
}
area: 12
The most important new thing in this code is the operator of scope (::, two colons) included
in the definition of set_values(). It is used to define a member of a class from outside the
class declaration itself.
You may notice that the definition of the member function area() has been included directly
within the definition of the CRectangle class given its extreme simplicity, whereas
set_values() has only its prototype declared within the class, but its definition is outside it.
In this outside declaration, we must use the operator of scope (::) to specify that we are
defining a function that is a member of the class CRectangle and not a regular global
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, in the function set_values() of the previous code,
we have been able to use the variables x and y, which are private members of class
CRectangle, which means they are only accessible from other members of their class.
The only difference between defining a class member function completely within its class and
to include only the prototype and later its definition, is that in the first case the function will
automatically be considered an inline member function by the compiler, while in the second it
will be a normal (not-inline) class member function, which in fact supposes no difference in
behavior.
Members x and y have private access (remember that if nothing else is said, all members of a
class defined with keyword class have private access). By declaring them private we deny
access to them from anywhere outside the class. 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 an utility in
protecting those two variables, 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).
One of the greater advantages of a class is that, as any other type, we can declare several
objects of it. For example, following with the previous example of class CRectangle, we could
have declared the object rectb in addition to the object rect:
// example: one class, two objects
#include
using namespace std;
class CRectangle {
int x, y;
public:
void set_values (int,int);
int area () {return (x*y);}
};
void CRectangle::set_values (int a,
int b) {
x = a;
y = b;
}
int main () {
CRectangle 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 concrete case, the class (type of the objects) to which we are talking about is
CRectangle, of which there are two instances or 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 CRectangle has its own variables x and
y, as they, in some way, have also their own function members set_value() and area()
that each uses its object's own variables to operate.
That is the basic concept of object-oriented programming: Data and functions are both
members of the object. We no longer use sets of global variables that we pass from one
function to another as parameters, but instead we handle objects that have their own data
and functions embedded as members. Notice that we have not had to give any parameters in
any of the calls to rect.area or rectb.area. Those member functions directly used the
data members of their respective objects rect and rectb.
Constructors and destructors
Objects generally need to initialize variables or assign dynamic memory during their process of
creation to become operative and to avoid returning unexpected values during their execution.
For example, what would happen if in the previous example we called the member function
area() before having called function set_values()? Probably we would have gotten an
undetermined result since the members x and y would have never been assigned a value.
In order to avoid that, a class can include a special function called constructor, which is
automatically called whenever a new object of this class is created. This constructor function
must have the same name as the class, and cannot have any return type; not even void.
We are going to implement CRectangle including a constructor:
// example: class constructor
#include
using namespace std;
class CRectangle {
int width, height;
public:
CRectangle (int,int);
int area () {return
(width*height);}
};
CRectangle::CRectangle (int a, int
b) {
width = a;
height = b;
}
int main () {
CRectangle rect (3,4);
CRectangle rectb (5,6);
cout << "rect area: " <<
rect.area() << endl;
cout << "rectb area: " <<
rect area: 12
rectb area: 30
rectb.area() << endl;
return 0;
}
As you can see, the result of this example is identical to the previous one. But now we have
removed the member function set_values(), and have included instead a constructor that
performs a similar action: it initializes the values of x and y with the parameters that are
passed to it.
Notice how these arguments are passed to the constructor at the moment at which the objects
of this class are created:
CRectangle rect (3,4);
CRectangle rectb (5,6);
Constructors cannot be called explicitly as if they were regular member functions. They are
only executed when a new object of that class is created.
You can also see how neither the constructor prototype declaration (within the class) nor the
latter constructor definition include a return value; not even void.
The destructor fulfills the opposite functionality. It is automatically called when an object is
destroyed, either because its scope of existence has finished (for example, if it was defined as
a local object within a function and the function ends) or because it is an object dynamically
assigned and it is released using the operator delete.
The destructor must have the same name as the class, but preceded with a tilde sign (~) and
it must also return no value.
The use of destructors is especially suitable when an object assigns dynamic memory during
its lifetime and at the moment of being destroyed we want to release the memory that the
object was allocated.
// example on constructors and
destructors
#include
using namespace std;
class CRectangle {
int *width, *height;
public:
CRectangle (int,int);
~CRectangle ();
int area () {return (*width *
*height);}
};
CRectangle::CRectangle (int a, int
b) {
width = new int;
height = new int;
*width = a;
*height = b;
}
rect area: 12
rectb area: 30
CRectangle::~CRectangle () {
delete width;
delete height;
}
int main () {
CRectangle rect (3,4), rectb
(5,6);
cout << "rect area: " <<
rect.area() << endl;
cout << "rectb area: " <<
rectb.area() << endl;
return 0;
}
