Cloning (programming): Difference between revisions

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{{refimprove|date=August 2015}}

In [[computer science]], '''cloning''' refers to the making of an exact copy of an [[object (programming)|object]], frequently under the [[programming paradigm|paradigm]] of [[instance-based programming]], or [[object-oriented programming]] (OOP).

==Shallow copies==

In most programming languages (exceptions include: [[Ruby (programming language)|Ruby]]), [[primitive type]]s such as <code>double</code>, <code>float</code>, <code>int</code>, <code>long</code>, etc. simply store their values somewhere in the computer's memory (often the [[call stack]]). By using simple assignment, you can copy the contents of the variable to another one:

Copying primitive types in Java or [[C++]]:

<sourcesyntaxhighlight lang="java">

int original = 42;

int copy = 0;

copy = original;

</syntaxhighlight>

</source>

Many OOP programming languages (including [[C SharpJava (programming language)|C#Java]], [[D (programming language)|D]], [[ECMAScript]], and [[JavaC Sharp (programming language)|JavaC#]]) make use of object references. Object references, which are similar to pointers in other languages, allow for objects to be passed around by [[Pointer (computer programming)|address]] so that the whole object need not be copied.

A Java example, when "copying" an object using simple assignment:

<sourcesyntaxhighlight lang="java">

Object original = new Object();

Object copy = null;

copy = original; // does not copy object but only its reference

</syntaxhighlight>

</source>

The object is not duplicated, the variables 'original' and 'copy' are actually referring to the same object.

The object is not duplicated, the variables 'original' and 'copy' are actually referring to the same object. In C++, the equivalent code

<syntaxhighlight lang="c">

Object* original = new Object();

Object* copy = NULL;

copy = original;

</syntaxhighlight>

makes it clear that it is a ''pointer'' to the object being copied, not the object itself.

==Cloning==

The process of actually making another exact replica of the object instead of just its reference is called cloning. In most languages, the language or libraries can facilitate some sort of cloning. In Java, the <code>Object</code> class contains the [[Clone (Java method)|<code>clone()</code> method]], which copies the object and returns a reference to that copied object. Since it is in the <code>Object</code> class, all classes defined in Java will have a clone method available to the programmer (although to function correctly it needs to be overridden at each level it is used).

Cloning an object in Java:

<sourcesyntaxhighlight lang="java">

Object originaloriginalObj = new Object();

Object copycopyObj = null;

copycopyObj = originaloriginalObj.clone(); // duplicates the object and assigns the new reference to 'copycopyObj'

</syntaxhighlight>→

</source>

C++ objects in general behave like primitive types, so to copy a C++ object one could use the '<code>=</code>' (assignment) operator. There is a default assignment operator provided for all classes, but its effect may be altered through the use of [[operator overloading]]. There are dangers when using this technique (see [[Object slicing|slicing]]). A method of avoiding slicing can be implementing a similar solution to the Java <code>clone()</code> method for the classes, and using pointers. (Note that thereThere is no built-in <code>clone()</code> method.)

A C++ example of object cloning:

<sourcesyntaxhighlight lang="cpp">

Object originaloriginalObj;

Object copycopyObj(originaloriginalObj); // creates a copy of originaloriginalObj named copycopyObj

</syntaxhighlight>

</source>

A C++ example of object cloning using pointers (avoidsto avoid slicing): see<ref>See Q&A at [http://en.allexperts.com/q/C-1040/constructor-virtual.htm en.allexperts.com] {{Webarchive|url=https://web.archive.org/web/20090718130949/http://en.allexperts.com/q/C-1040/constructor-virtual.htm |date=2009-07-18 }}</ref>):

<sourcesyntaxhighlight lang="cpp">

Object * originaloriginalObj = new Object;

Object * copycopyObj = NULLnullptr;

copycopyObj = new Object(* originaloriginalObj); // creates a copy of originaloriginalObj and assigns its address to copycopyObj

</syntaxhighlight>

</source>

==References==

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