Type Conversion – Class to Class

Now that we have understood how to convert basic data types to class types and vice-versa, it is time to learn how to convert objects of one class type to another class type.

The conversion between objects of different classes can be done using either a one-argument constructor or a conversion function. The choice depends upon whether the conversion routine has to be declared in the source class or in the destination class. To illustrate, consider a program that contains two classes: A and B. Also consider the statement:

object_A = object_B;

In the above statement, object_B of type B is converted to type A and assigned to object_A. Therefore, object_B is the source and object_A is the destination.

For the conversion, the user can use either a conversion function or a constructor with one argument, depending on whether it is specified in the source class or the destination class. In other words, if class B handles the conversion, it will hold a conversion function. On the other hand, if class A carries out the conversion, it will do that through a constructor that takes an argument of type class B.

Note that only one of the two conversion routines should be specified; the compiler will yield an error if both are specified since it will not be able to figure out which routine to call.

Sample Program

.cf { font-family: Lucida Console; font-size: 9pt; color: black; background: white; }
.cl { margin: 0px; }
.cb1 { color: green; }
.cb2 { color: blue; }
.cb3 { color: maroon; }

 

/*

 * Program converts from one class type to

 * another using a conversion function

 * and a constructor

 * https://www.tech-faq.com

 */

#include <iostream>

using namespace std;

class Kilometers

{

private:

    double kilometers;

public:

    Kilometers(double kilometers): kilometers(kilometers) {}

    void display()

    {

        cout << kilometers << " kilometeres";

    }

    double getValue()

    {

        return kilometers;

    }

};

class Miles

{

private:

    double miles;

public:

    Miles(double miles) : miles(miles) {}

    void display()

    {

        cout << miles << " miles";

    }

    operator Kilometers()

    {

        return Kilometers(miles*1.609344);

    }

    Miles(Kilometers kilometers)

    {

        miles = kilometers.getValue()/1.609344;

    }

};

int main(void)

{

    /*

    * Converting using the conversion function

    */

    Miles m1 = 100;

    Kilometers k1 = m1;

    m1.display();

    cout << " = ";

    k1.display();

    cout << endl;

    /*

    * Converting using the constructor

    */

    Kilometers k2 = 100;

    Miles m2 = k2;    // same as:  Miles m2 = Miles(k2);

    k2.display();

    cout << " = ";

    m2.display();

    cout << endl;

}

 

Output

 

100 miles = 160.934 kilometeres

100 kilometeres = 62.1371 miles

 

In this Series

• Introduction to type conversion
• Conversion from basic type to class type
• Conversion from class type to basic type
• Conversion from one class type to another class type