Constructors and Destructors:
A Class is defined as constructor by declaring a constructor in the form of function inside
the class. In other word a function in the name of class is a constructor with few exceptions from the regular function. Constructor is a special member function. The constructor must have identical name as the class. Constructor is the extension of class with additional features and certain limitations. The class cannot be defined without parameters or arguments, that is class can be defined in the form of function and thus class can be declared with argument and without argument. The class without argument is called default constructor with argument is known as parameterized constructors. The construction is used to initialize the class; as seen earlier a class cannot be initialized; this limitation is over come by having a constructor inside the class. The constructor can be overload.
Rules of Constructor:
- The constructors should have the same name as the class.
- The constructors will get the form of function prototype.
- The constructors are used to initialize the class and its objects.
- The constructors should he declared in the public section of the class.
- The constructors are invoked automatically as soon as the objects are created.
- The constructors cannot be inherited.
- The constructors can have a default argument.
- The constructors may or may not have argument.
- The constructors also can be defined as inline function.
- The constructors cannot have its class like argument.
- The constructors can have its class as argument but through reference only. This is known as copy constructor.
- The constructor can be constant, thus constructor arguments never be modified.
- The constructors do not return type even void like function.
- Constructor can be empty or without statement inside it. It is a do nothing constructor it must be produced when overloading operator is defined in the class.
- Constructor also can have other than initialization statement.
A constructor is defined as follows which will interpret the rule of constructor.
Constructor without argument or default constructor:
class integer
{int m,n;
public:
integer(void);
};
integer::integer(void)
{m=0; n=0;
}
Constructor with argument or parameterized constructor:
class integer
{int m,n;
public:
integer(int x, int y);
};
integer::integer(int x, int y)
{m=x; n=y;
}
Calling a constructor:
1. Explicit calling. 2. Implicit calling.
Explicit calling, it is declared by combining the declaration and passing the argument to the constructor.
integer int1=integer(10,20);
The above statement can re written in the following form.
integer int1;
int1(10,20);
The above method is implicit calling it can also be made shorter in the following way. integer int1(10,20);
Inline constructor:
class integer
{
int m,n;
public:
integer(int x, int y)
{m=x; n=y;}
};
Copy constructor:
class A
{int m,n;
public:
A(A&); //This is invalid A(A) it is the rule of constructor;
};
Multiple Constructors:
class integer
{int m,n;
public:
integer ( )
{m=0; n=0;}
integer(int x, int y)
{m=x; n=y;}
integer(integer &i) //Copy constructor
{m=i.m; n=i.n;}
};
The above constructor can be called as follows. Calling constructor without parameter integer I1;
Calling constructor without parameter integer I2(20,40); Calling constructor without parameter integer I3(I2);
Depending on the argument passed the constructor will be evoked. A class any number of constructors in its class, but the names must be identical. This is works exactly like function overloading and thus it is known as constructor overloading.
Constructor with default argument:
class integer
{int m,n; public: integer( )
{m=20;}
integer(int x=20);
};
integer::integer(int x)
{m=x;
}
When there is only one argument in a constructor and that too is declared as default argument. Then there will conflict in calling the constructor with default argument and default constructor. Default constructor is a constructor without argument. This conflict is overcome as follows.