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Adding documentation for references inside the constructor
git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@43099 c90b9560-bf6c-de11-be94-00142212c4b1
This commit is contained in:
André Langhorst
parent
95247980c8
commit
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1 changed files with 171 additions and 1 deletions
172
language/oop.xml
172
language/oop.xml
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@ -30,7 +30,7 @@ class Cart {
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}
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}
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}
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?>
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?>
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</programlisting>
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</informalexample>
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</para>
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@ -195,6 +195,176 @@ $different_cart = new Constructor_Cart ("20", 17);
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</simpara>
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</caution>
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</sect1>
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<sect1 id="keyword.newref">
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<title>references inside the constructor</title>
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<para>
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Creating references within the constructor can lead to confusing results. This tutorial like section helps you
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to avoid problems.
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<informalexample>
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<programlisting role="php">
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class foo {
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function foo($name) {
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// create a reference inside the global array $globalref
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global $globalref;
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$globalref[] = &$this;
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// set name to passed value
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$this->setName($name);
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// and put it out
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$this->echoName();
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}
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function echoName() {
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echo "<br>",$this->Name;
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}
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function setName($name) {
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$this->Name = $name;
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}
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}
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</programlisting>
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</informalexample>
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<para>
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Let us check out if there is a difference between <varname>$bar1</varname> which has been created using the copy <literal>=</literal> operator
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and <varname>$bar2</varname> which has been created using the reference <literal>=&</literal> operator...
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</para>
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<informalexample>
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<programlisting role="php">
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$bar1 = new foo('set in constructor');
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$bar1->echoName();
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$globalref[0]->echoName();
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/* output:
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set in constructor
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set in constructor
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set in constructor */
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$bar2 =& new foo('set in constructor');
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$bar2->echoName();
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$globalref[1]->echoName();
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/* output:
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set in constructor
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set in constructor
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set in constructor */
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</programlisting>
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</informalexample>
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<para>
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Apparently there is no difference, but in fact there is a very significant one:
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<varname>$bar1</varname> and <varname>$globalref[0]</varname> are _NOT_ referenced, they are NOT the same variable.
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This is because "new" does not return a reference by default, instead it returns a copy.
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<note>
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<simpara>
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There is no performance loss (since php 4 and up use reference counting) returning copies instead of references.
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On the contrary it is most often better to simply work with copies instead of references, because creating
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references takes some time where creating copies virtually takes no time (unless none of them is a large array or object
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and one of them gets changed and the other(s) one(s) subsequently, then it would be wise to use references to change them
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all concurrently).
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</simpara>
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</note>
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To prove what is written above let us watch the code below.
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</para>
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<informalexample>
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<programlisting role="php">
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// now we will change the name. what do you expect?
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// you could expect that both $bar and $globalref[0] change their names...
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$bar1->setName('set from outside');
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// as mentioned before this is not the case.
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$bar1->echoName();
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$globalref[0]->echoName();
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/* output:
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set on object creation
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set from outside */
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// let us see what is different with $bar2 and $globalref[1]
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$bar2->setName('set from outside');
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// luckily they are not only equyl, they are thesame variable
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// thus $bar2->Name and $globalref[1]->Name are the same too
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$bar2->echoName();
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$globalref[1]->echoName();
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/* output:
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set from outside
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set from outside */
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</programlisting>
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</informalexample>
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<para>
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Another final example, try to understand it.
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<informalexample>
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<programlisting role="php">
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class a {
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function a($i) {
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$this->value = $i;
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// try to figure out why we do not need a reference here
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$this->b = new b($this);
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}
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function createRef() {
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$this->c = new b($this);
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}
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function echoValue() {
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echo "<br>","class ",get_class($this),': ',$this->value;
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}
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}
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class b {
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function b(&$a) {
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$this->a = &$a;
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}
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function echoValue() {
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echo "<br>","class ",get_class($this),': ',$this->a->value;
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}
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}
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// try to undestand why using a simple copy here would yield
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// in an undesired result in the *-marked line
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$a =& new a(10);
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$a->createRef();
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$a->echoValue();
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$a->b->echoValue();
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$a->c->echoValue();
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$a->value = 11;
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$a->echoValue();
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$a->b->echoValue(); // *
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$a->c->echoValue();
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/*
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output:
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class a: 10
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class b: 10
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class b: 10
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class a: 11
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class b: 11
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class b: 11
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*/
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</programlisting>
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</informalexample>
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</para>
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</sect1>
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</chapter>
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