O que são referências
I think a correction to my last post is in order.
When there is a constructor, the strange behavior mentioned in my last post doesn't occur. My guess is that php was treating reftest() as a constructor (maybe because it was the first function?) and running it upon instantiation.
<?php
class reftest
{
public $a = 1;
public $c = 1;
public function __construct()
{
return 0;
}
public function reftest()
{
$b =& $this->a;
$b++;
}
public function reftest2()
{
$d =& $this->c;
$d++;
}
}
$reference = new reftest();
$reference->reftest();
$reference->reftest2();
echo $reference->a; //Echoes 2.
echo $reference->c; //Echoes 2.
?>
O que as referências fazem
Referências PHP permitem fazer duas variáveis se referirem ao mesmo conteúdo. Ou seja:
<?php
$a =& $b;
?>
Nota: $a e $b são completamente iguais aqui, mas não porque $a está apontando para $b ou vice versa, mas sim que $a e $b apontam para o mesmo lugar.
Nota: Se o array com referências é copiado, seus valores não são referenciados. Isto é válido também para arrays passados por valor para funções.
Nota: Se você atribuir, passar ou retornar uma variável indefinida por referência, ela irá ser criada.
Exemplo #1 Usando referência com variáveis indefinidas
<?php
function foo(&$var) { }
foo($a); // $a é "criada" e setada par null
$b = array();
foo($b['b']);
var_dump(array_key_exists('b', $b)); // bool(true)
$c = new StdClass;
foo($c->d);
var_dump(property_exists($c, 'd')); // bool(true)
?>
A mesma sintaxe pode ser utilizada com funções, que retornem referências, e com o operador new (a partir do PHP 4.0.4):
<?php
$bar =& new fooclass();
$foo =& find_var ($bar);
?>
Nota: A não utilização do operador & causará a cópia do objeto. Se você utiliza $this em classes, ele operará na instância atual do objeto. A assimilação sem & irá copiar a instância (o objeto em si) e $this irá operar na cópia, podendo não ser esse procedimento sempre desejável. Normalmente você precisará trabalhar com uma instância única, seja por motivos de performance ou de consumo de memória.
Você pode utilizar o operador @ para esconder quaisquer erros em construtores na forma @new, mas isto não funciona quando utilizada a instrução &new. Esta é uma limitação da Zend Engine e irá gerar um erro de interpretação (parser error).
Aviso
Veja global $var; como atalho para $var
=& $GLOBALS['var'];. Assim atribuir outra referência
para $var somente modifica a variável de referência local.
Se você atribuir uma referência para uma variável declarada global dentro da função, a referência irá ser visível somente dentro da função. Você pode evitar isto usando o array $GLOBALS.
Exemplo #2 Referenciando variáveis globais de dentro de funções
<?php
$var1 = "Example variable";
$var2 = "";
function global_references($use_globals)
{
global $var1, $var2;
if (!$use_globals) {
$var2 =& $var1; // visível somente dentro da função
} else {
$GLOBALS["var2"] =& $var1; // visível também no contexto global
}
}
global_references(false);
echo "var2 is set to '$var2'\n"; // var2 is set to ''
global_references(true);
echo "var2 is set to '$var2'\n"; // var2 is set to 'Example variable'
?>
Nota: Se você atribuir um valor para uma variável com referência no comando foreach, a referência é modificada também.
Exemplo #3 Referências e o comando foreach
<?php
$ref = 0;
$row =& $ref;
foreach (array(1, 2, 3) as $row) {
// faz alguma coisa
}
echo $ref; // 3 - último elemento do array iterado
?>
A segunda coisa que referências permitem é passar variáveis por referência. Isto é feito marcando uma variável local de uma função e a variável do escopo chamador como referências ao mesmo conteúdo. Exemplo:
<?php
function foo (&$var)
{
$var++;
}
$a=5;
foo ($a);
?>
Em terceiro lugar, referências permitem também retorno por referência.
add a note
User Contributed NotesO que as referências fazem
Amaroq
16-Jan-2010 11:14
16-Jan-2010 11:14
Amaroq
15-Jan-2010 02:08
15-Jan-2010 02:08
When using references in a class, you can reference $this-> variables.
<?php
class reftest
{
public $a = 1;
public $c = 1;
public function reftest()
{
$b =& $this->a;
$b = 2;
}
public function reftest2()
{
$d =& $this->c;
$d++;
}
}
$reference = new reftest();
$reference->reftest();
$reference->reftest2();
echo $reference->a; //Echoes 2.
echo $reference->c; //Echoes 2.
?>
However, this doesn't appear to be completely trustworthy. In some cases, it can act strangely.
<?php
class reftest
{
public $a = 1;
public $c = 1;
public function reftest()
{
$b =& $this->a;
$b++;
}
public function reftest2()
{
$d =& $this->c;
$d++;
}
}
$reference = new reftest();
$reference->reftest();
$reference->reftest2();
echo $reference->a; //Echoes 3.
echo $reference->c; //Echoes 2.
?>
In this second code block, I've changed reftest() so that $b increments instead of just gets changed to 2. Somehow, it winds up equaling 3 instead of 2 as it should.
strata_ranger at hotmail dot com
26-Sep-2009 10:29
26-Sep-2009 10:29
An interesting if offbeat use for references: Creating an array with an arbitrary number of dimensions.
For example, a function that takes the result set from a database and produces a multidimensional array keyed according to one (or more) columns, which might be useful if you want your result set to be accessible in a hierarchial manner, or even if you just want your results keyed by the values of each row's primary/unique key fields.
<?php
function array_key_by($data, $keys, $dupl = false)
/*
* $data - Multidimensional array to be keyed
* $keys - List containing the index/key(s) to use.
* $dupl - How to handle rows containing the same values. TRUE stores it as an Array, FALSE overwrites the previous row.
*
* Returns a multidimensional array indexed by $keys, or NULL if error.
* The number of dimensions is equal to the number of $keys provided (+1 if $dupl=TRUE).
*/
{
// Sanity check
if (!is_array($data)) return null;
// Allow passing single key as a scalar
if (is_string($keys) or is_integer($keys)) $keys = Array($keys);
elseif (!is_array($keys)) return null;
// Our output array
$out = Array();
// Loop through each row of our input $data
foreach($data as $cx => $row) if (is_array($row))
{
// Loop through our $keys
foreach($keys as $key)
{
$value = $row[$key];
if (!isset($last)) // First $key only
{
if (!isset($out[$value])) $out[$value] = Array();
$last =& $out; // Bind $last to $out
}
else // Second and subsequent $key....
{
if (!isset($last[$value])) $last[$value] = Array();
}
// Bind $last to one dimension 'deeper'.
// First lap: was &$out, now &$out[...]
// Second lap: was &$out[...], now &$out[...][...]
// Third lap: was &$out[...][...], now &$out[...][...][...]
// (etc.)
$last =& $last[$value];
}
if (isset($last))
{
// At this point, copy the $row into our output array
if ($dupl) $last[$cx] = $row; // Keep previous
else $last = $row; // Overwrite previous
}
unset($last); // Break the reference
}
else return NULL;
// Done
return $out;
}
// A sample result set to test the function with
$data = Array(Array('name' => 'row 1', 'foo' => 'foo_a', 'bar' => 'bar_a', 'baz' => 'baz_a'),
Array('name' => 'row 2', 'foo' => 'foo_a', 'bar' => 'bar_a', 'baz' => 'baz_b'),
Array('name' => 'row 3', 'foo' => 'foo_a', 'bar' => 'bar_b', 'baz' => 'baz_c'),
Array('name' => 'row 4', 'foo' => 'foo_b', 'bar' => 'bar_c', 'baz' => 'baz_d')
);
// First, let's key it by one column (result: two-dimensional array)
print_r(array_key_by($data, 'baz'));
// Or, key it by two columns (result: 3-dimensional array)
print_r(array_key_by($data, Array('baz', 'bar')));
// We could also key it by three columns (result: 4-dimensional array)
print_r(array_key_by($data, Array('baz', 'bar', 'foo')));
?>
dnhuff at acm dot org
09-Jun-2008 06:33
09-Jun-2008 06:33
In reply to Drewseph using foo($a = 'set'); where $a is a reference formal parameter.
$a = 'set' is an expression. Expressions cannot be passed by reference, don't you just hate that, I do. If you turn on error reporting for E_NOTICE, you will be told about it.
Resolution: $a = 'set'; foo($a); this does what you want.
Drewseph
29-May-2008 11:15
29-May-2008 11:15
If you set a variable before passing it to a function that takes a variable as a reference, it is much harder (if not impossible) to edit the variable within the function.
Example:
<?php
function foo(&$bar) {
$bar = "hello\n";
}
foo($unset);
echo($unset);
foo($set = "set\n");
echo($set);
?>
Output:
hello
set
It baffles me, but there you have it.
Amaroq
01-Apr-2008 06:56
01-Apr-2008 06:56
The order in which you reference your variables matters.
<?php
$a1 = "One";
$a2 = "Two";
$b1 = "Three";
$b2 = "Four";
$b1 =& $a1;
$a2 =& $b2;
echo $a1; //Echoes "One"
echo $b1; //Echoes "One"
echo $a2; //Echoes "Four"
echo $b2; //Echoes "Four"
?>
charles at org oo dot com
19-Oct-2007 10:59
19-Oct-2007 10:59
points to post below me.
When you're doing the references with loops, you need to unset($var).
for example
<?php
foreach($var as &$value)
{
...
}
unset($value);
?>
Hlavac
09-Oct-2007 09:25
09-Oct-2007 09:25
Watch out for this:
foreach ($somearray as &$i) {
// update some $i...
}
...
foreach ($somearray as $i) {
// last element of $somearray is mysteriously overwritten!
}
Problem is $i contians reference to last element of $somearray after the first foreach, and the second foreach happily assigns to it!
dovbysh at gmail dot com
06-Jul-2007 07:50
06-Jul-2007 07:50
Solution to post "php at hood dot id dot au 04-Mar-2007 10:56":
<?php
$a1 = array('a'=>'a');
$a2 = array('a'=>'b');
foreach ($a1 as $k=>&$v)
$v = 'x';
echo $a1['a']; // will echo x
unset($GLOBALS['v']);
foreach ($a2 as $k=>$v)
{}
echo $a1['a']; // will echo x
?>
amp at gmx dot info
08-Jun-2007 05:59
08-Jun-2007 05:59
Something that might not be obvious on the first look:
If you want to cycle through an array with references, you must not use a simple value assigning foreach control structure. You have to use an extended key-value assigning foreach or a for control structure.
A simple value assigning foreach control structure produces a copy of an object or value. The following code
$v1=0;
$arrV=array(&$v1,&$v1);
foreach ($arrV as $v)
{
$v1++;
echo $v."\n";
}
yields
0
1
which means $v in foreach is not a reference to $v1 but a copy of the object the actual element in the array was referencing to.
The codes
$v1=0;
$arrV=array(&$v1,&$v1);
foreach ($arrV as $k=>$v)
{
$v1++;
echo $arrV[$k]."\n";
}
and
$v1=0;
$arrV=array(&$v1,&$v1);
$c=count($arrV);
for ($i=0; $i<$c;$i++)
{
$v1++;
echo $arrV[$i]."\n";
}
both yield
1
2
and therefor cycle through the original objects (both $v1), which is, in terms of our aim, what we have been looking for.
(tested with php 4.1.3)
firespade at gmail dot com
03-Apr-2007 02:11
03-Apr-2007 02:11
Here's a good little example of referencing. It was the best way for me to understand, hopefully it can help others.
$b = 2;
$a =& $b;
$c = $a;
echo $c;
// Then... $c = 2
php at hood dot id dot au
05-Mar-2007 06:56
05-Mar-2007 06:56
I discovered something today using references in a foreach
<?php
$a1 = array('a'=>'a');
$a2 = array('a'=>'b');
foreach ($a1 as $k=>&$v)
$v = 'x';
echo $a1['a']; // will echo x
foreach ($a2 as $k=>$v)
{}
echo $a1['a']; // will echo b (!)
?>
After reading the manual this looks like it is meant to happen. But it confused me for a few days!
(The solution I used was to turn the second foreach into a reference too)
ladoo at gmx dot at
17-Apr-2005 09:05
17-Apr-2005 09:05
I ran into something when using an expanded version of the example of pbaltz at NO_SPAM dot cs dot NO_SPAM dot wisc dot edu below.
This could be somewhat confusing although it is perfectly clear if you have read the manual carfully. It makes the fact that references always point to the content of a variable perfectly clear (at least to me).
<?php
$a = 1;
$c = 2;
$b =& $a; // $b points to 1
$a =& $c; // $a points now to 2, but $b still to 1;
echo $a, " ", $b;
// Output: 2 1
?>
php.devel at homelinkcs dot com
15-Nov-2004 11:16
15-Nov-2004 11:16
In reply to lars at riisgaardribe dot dk,
When a variable is copied, a reference is used internally until the copy is modified. Therefore you shouldn't use references at all in your situation as it doesn't save any memory usage and increases the chance of logic bugs, as you discoved.
joachim at lous dot org
10-Apr-2003 10:46
10-Apr-2003 10:46
So to make a by-reference setter function, you need to specify reference semantics _both_ in the parameter list _and_ the assignment, like this:
class foo{
var $bar;
function setBar(&$newBar){
$this->bar =& newBar;
}
}
Forget any of the two '&'s, and $foo->bar will end up being a copy after the call to setBar.