Classes (I)
Published by Juan Soulie
Last update on Feb 25, 2009 at 12:25pm UTC
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:
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6 | class CRectangle {
int x, y;
public:
void set_values (int,int);
int area (void);
} rect;
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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:
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:
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2 | rect.set_values (3,4);
myarea = rect.area();
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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:
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22 | // 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 definition 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 or 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:
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24 | // 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:
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23 | // 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: " << rectb.area() << endl;
return 0;
}
| rect area: 12
rectb area: 30 |
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
width and
height 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:
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2 | CRectangle rect (3,4);
CRectangle rectb (5,6);
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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.
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30 | // 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;
}
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;
}
| rect area: 12
rectb area: 30 |
Overloading Constructors
Like any other function, a constructor can also be overloaded with more than one function that have the same name but different types or number of parameters. Remember that for overloaded functions the compiler will call the one whose parameters match the arguments used in the function call. In the case of constructors, which are automatically called when an object is created, the one executed is the one that matches the arguments passed on the object declaration:
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29 | // overloading class constructors
#include
using namespace std;
class CRectangle {
int width, height;
public:
CRectangle ();
CRectangle (int,int);
int area (void) {return (width*height);}
};
CRectangle::CRectangle () {
width = 5;
height = 5;
}
CRectangle::CRectangle (int a, int b) {
width = a;
height = b;
}
int main () {
CRectangle rect (3,4);
CRectangle rectb;
cout << "rect area: " << rect.area() << endl;
cout << "rectb area: " << rectb.area() << endl;
return 0;
}
| rect area: 12
rectb area: 25 |
In this case,
rectb was declared without any arguments, so it has been initialized with the constructor that has no parameters, which initializes both
width and
height with a value of 5.
Important: Notice how if we declare a new object and we want to use its default constructor (the one without parameters), we do not include parentheses
():
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2 | CRectangle rectb; // right
CRectangle rectb(); // wrong!
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Default constructor
If you do not declare any constructors in a class definition, the compiler assumes the class to have a default constructor with no arguments. Therefore, after declaring a class like this one:
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public:
int a,b,c;
void multiply (int n, int m) { a=n; b=m; c=a*b; };
};
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The compiler assumes that
CExample has a default constructor, so you can declare objects of this class by simply declaring them without any arguments:
But as soon as you declare your own constructor for a class, the compiler no longer provides an implicit default constructor. So you have to declare all objects of that class according to the constructor prototypes you defined for the class:
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public:
int a,b,c;
CExample (int n, int m) { a=n; b=m; };
void multiply () { c=a*b; };
};
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Here we have declared a constructor that takes two parameters of type int. Therefore the following object declaration would be correct:
But,
Would
not be correct, since we have declared the class to have an explicit constructor, thus replacing the default constructor.
But the compiler not only creates a default constructor for you if you do not specify your own. It provides three special member functions in total that are implicitly declared if you do not declare your own. These are the
copy constructor, the
copy assignment operator, and the default destructor.
The copy constructor and the copy assignment operator copy all the data contained in another object to the data members of the current object. For
CExample, the copy constructor implicitly declared by the compiler would be something similar to:
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a=rv.a; b=rv.b; c=rv.c;
}
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Therefore, the two following object declarations would be correct:
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CExample ex2 (ex); // copy constructor (data copied from ex)
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Pointers to classes
It is perfectly valid to create pointers that point to classes. We simply have to consider that once declared, a class becomes a valid type, so we can use the class name as the type for the pointer. For example:
is a pointer to an object of class
CRectangle.
As it happened with data structures, in order to refer directly to a member of an object pointed by a pointer we can use the arrow operator (
->) of indirection. Here is an example with some possible combinations:
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34 | // pointer to classes example
#include
using namespace std;
class CRectangle {
int width, height;
public:
void set_values (int, int);
int area (void) {return (width * height);}
};
void CRectangle::set_values (int a, int b) {
width = a;
height = b;
}
int main () {
CRectangle a, *b, *c;
CRectangle * d = new CRectangle[2];
b= new CRectangle;
c= &a;
a.set_values (1,2);
b->set_values (3,4);
d->set_values (5,6);
d[1].set_values (7,8);
cout << "a area: " << a.area() << endl;
cout << "*b area: " << b->area() << endl;
cout << "*c area: " << c->area() << endl;
cout << "d[0] area: " << d[0].area() << endl;
cout << "d[1] area: " << d[1].area() << endl;
delete[] d;
delete b;
return 0;
}
| a area: 2
*b area: 12
*c area: 2
d[0] area: 30
d[1] area: 56 |
Next you have a summary on how can you read some pointer and class operators (
*,
&,
.,
->,
[ ]) that appear in the previous example:
| expression | can be read as |
|---|
| *x | pointed by x |
| &x | address of x |
| x.y | member y of object x |
| x->y | member y of object pointed by x |
| (*x).y | member y of object pointed by x (equivalent to the previous one) |
| x[0] | first object pointed by x |
| x[1] | second object pointed by x |
| x[n] | (n+1)th object pointed by x |
Be sure that you understand the logic under all of these expressions before proceeding with the next sections. If you have doubts, read again this section and/or consult the previous sections about pointers and data structures.
Classes defined with struct and union
Classes can be defined not only with keyword
class, but also with keywords
struct and
union.
The concepts of class and data structure are so similar that both keywords (
struct and
class) can be used in C++ to declare classes (i.e.
structs can also have function members in C++, not only data members). 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. For all other purposes both keywords are equivalent.
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 function members. The default access in union classes is public.
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