The Class Constructor

The Class Constructor:

A class constructor is a special member function of a class that is executed whenever we create new objects of that class.

A constructor will have exact same name as the class and it does not have any return type at all, not even void. Constructors can be very useful for setting initial values for certain member variables.

Following example explains the concept of constructor:

#include <iostream>

using namespace std;

class Line

{

public:

voidsetLength( double len );

doublegetLength( void );

Line(); // This is the constructor private:

double length;

};

// Member functions definitions including constructor Line::Line(void)

{

cout<< "Object is being created" <<endl;

}

void Line::setLength( double len )

{

length = len;

}

double Line::getLength( void )

{

return length;

}

//  Main function for the program int main( )

{

Line line;

//  set line length line.setLength(6.0);

cout<< "Length of line : " <<line.getLength() <<endl;

return 0;

}

When the above code is compiled and executed, it produces the following result: Object is being created

Length of line : 6

Parameterized Constructor:

A default constructor does not have any parameter, but if you need, a constructor can have parameters. This helps you to assign initial value to an object at the time of its creation as shown in the following example:

#include <iostream> using namespace std;

class Line

{

public:

voidsetLength( double len );

doublegetLength( void );

Line(double len); // This is the constructor private:

double length;

};

//  Member functions definitions including constructor

Line::Line( double len)

{

cout<< "Object is being created, length = " <<len<<endl; length = len;

}

void Line::setLength( double len )

{

length = len;

}

double Line::getLength( void )

{

return length;

}

//  Main function for the program

int main( )

{

Line line(10.0);

// get initially set length.

cout<< "Length of line : " <<line.getLength() <<endl;

// set line length again

line.setLength(6.0);

cout<< "Length of line : " <<line.getLength() <<endl;

return 0;

}

When the above code is compiled and executed, it produces the following result: Object is being created, length = 10

Length of line : 10 Length of line : 6

Using Initialization Lists to Initialize Fields:

In case of parameterized constructor, you can use following syntax to initialize the fields: Line::Line( double len): length(len)

{

cout<< "Object is being created, length = " <<len<<endl;

}

Above syntax is equal to the following syntax:

Line::Line( double len)

{

cout<< "Object is being created, length = " <<len<<endl;

length = len;

}

If for a class C, you have multiple fields X, Y, Z, etc., to be initialized, then use can use same syntax and separate the fields by comma as follows:

C::C( double a, double b, double c): X(a), Y(b), Z(c)

{

....

}