Advanced Programming Techniques in D/

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toString and opEquals

toString and opEquals

Let’s learn about toString and opEquals functions for classes.

We'll cover the following...

toString

Same with structs, toString enables using class objects as strings. The following code shows how toString() is implicitly called when displaying class objects:

D
import std.stdio;



class Clock {

    int hour;

    int minute;

    int second;



    this(int hour, int minute, int second) {

        this.hour = hour; 

        this.minute = minute; 

        this.second = second;

    }

}



void main() {

    auto clock = new Clock(20, 30, 0);  

    writeln(clock); // Calls clock.toString()

}

The inherited toString() is usually not useful; it produces just the name of the type:

deneme.Clock

Here deneme is the name of the module. The output above indicates that Clock has been defined in the deneme module.

As we have seen in the previous chapter, this function is almost always overridden to produce a more meaningful string representation of an object:

D
import std.string;

import std.stdio;



class Clock {

    int hour;

    int minute;

    int second;





    this(int hour, int minute, int second) {

        this.hour = hour; 

        this.minute = minute; 

        this.second = second;

    }



    override string toString() const {

        return format("%02s:%02s:%02s", hour, minute, second); 

    }

    // ...

}



class AlarmClock : Clock {

    int alarmHour;

    int alarmMinute;



    this(int hour, int minute, int second, 

         int alarmHour, int alarmMinute) {

        super(hour, minute, second);

        this.alarmHour = alarmHour;

        this.alarmMinute = alarmMinute;

    }



    override string toString() const {

        return format("%s ♫%02s:%02s", super.toString(), 

                                       alarmHour, alarmMinute);

    }

    // ...

}

void main() {

    auto bedSideClock = new AlarmClock(20, 30, 0, 7, 0); 

    writeln(bedSideClock);

}

opEquals

As we have seen in the operator overloading chapter, this member function is about the behavior of the == operator (and the != operator indirectly). The return value of the operator must be true if the objects are considered to be equal and false otherwise.

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