Cloning (programming): Difference between revisions

Revision as of 08:14, 5 July 2019

Revision as of 17:32, 11 May 2020

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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 [[Java (programming language)|Java]], [[D (programming language)|D]], [[ECMAScript]], and [[C Sharp (programming language)|C#]]) 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. In C++, the equivalent code

<sourcesyntaxhighlight lang="c">

Object* original = new Object();

Object* copy = NULL;

copy = original;

</syntaxhighlight>

</source>

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

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Cloning an object in Java:

<sourcesyntaxhighlight lang="java">

Object originalObj = new Object();

Object copyObj = null;

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

</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 there is no built-in <code>clone()</code> method)

A C++ example of object cloning:

<sourcesyntaxhighlight lang="cpp">

Object originalObj;

Object copyObj(originalObj); // creates a copy of originalObj named copyObj

</syntaxhighlight>

</source>

A C++ example of object cloning using pointers (to 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* originalObj = new Object;

Object* copyObj = nullptr;

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

</syntaxhighlight>

</source>

==References==