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  • 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
     * http://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

     

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    One comment
    1. Vikas

      30 November, 2015 at 11:23 am

      what does this line means ” Kilometers(double kilometers): kilometers(kilometers) {}”?

      Reply
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