Polymorphic copy: Difference between revisions

}

</lang>

=={{header|D}}==

{{works with|Tango}}

If we assume there are no data members, this will be quite short and simple:

<lang D>

import tango.io.Stdout;

class T {

char[] toString() { return "I'm the instance of T"; }

T duplicate() { return new T; }

}

class S : T {

char[] toString() { return "I'm the instance of S p: " ~ str; }

override

T duplicate() { return new S; }

}

void main ()

{

T orig = new S;

T copy = orig.duplicate;

Stdout (orig).newline;

Stdout (copy).newline; // should have 'X' at the beginning

}

</lang>

Hovever this doesn't happen often in reality.

If we want to copy data fields we should have something like copy ctor, that will

do the deep copy.

<lang D>

import tango.io.Stdout;

class T {

this(T t = null) { } // ctor that will be used for copying

char[] toString() { return "I'm the instance of T"; }

T duplicate() { return new T(this); }

int custom(int x) { return 0; }

}

class S : T {

char[] str;

this(S s = null) {

super(s);

if (s !is null)

str = s.str.dup; // do the deep-copy

else // all newly created will get that

str = "123".dup;

}

char[] toString() { return "I'm the instance of S p: " ~ str; }

override

T duplicate() { return new S(this); }

// additional proc, just to test deep-copy

override

int custom(int x) {

if (str !is null) str[0] = x;

return str is null;

}

}

void main ()

{

T orig = new S;

orig.custom('X');

T copy = orig.duplicate;

orig.custom('Y');

Stdout (orig).newline;

Stdout (copy).newline; // should have 'X' at the beginning