?: operator (the 'Elvis operator') in PHP

问题

I saw this today in some PHP code:

$items = $items ?: $this->_handle->result(\'next\', $this->_result, $this);

I\'m not familiar with the ?: operator being used here. It looks like a ternary operator, but the expression to evaluate to if the predicate is true has been omitted. What does it mean?

回答1:

It evaluates to the left operand if the left operand is truthy, and the right operand otherwise.

In pseudocode,

foo = bar ?: baz;

roughly resolves to

foo = bar ? bar : baz;

if (bar) {
    foo = bar;
} else {
    foo = baz;
}

with the difference that bar will only be evaluated once.

You can also use this to do a "self-check" of foo as demonstrated in the code example you posted:

foo = foo ?: bar;

This will assign bar to foo if foo is null or falsey, else it will leave foo unchanged.

Some more examples:

<?php
    var_dump(5 ?: 0); // 5
    var_dump(false ?: 0); // 0
    var_dump(null ?: 'foo'); // 'foo'
    var_dump(true ?: 123); // true
    var_dump('rock' ?: 'roll'); // 'rock'
?>

By the way, it's called the Elvis operator.

回答2:

See the docs:

Since PHP 5.3, it is possible to leave out the middle part of the ternary operator. Expression expr1 ?: expr3 returns expr1 if expr1 evaluates to TRUE, and expr3 otherwise.

回答3:

Be careful with arrays. We must write a checking variable after ?, because:

  $params = ['param1' => 'value1',
             'param2' => 'value2',
             'param3' => 'value3',];

  $param1 = isset($params['param1'])?:null;
  $param2 = !empty($params['param2'])?:null;
  $param3 = $params['param3']?:null; // get E_NOTICE, if $params['param3'] eq false

  var_dump($param1,$param2,$param3);
  true // would like to expect `value1`
  true // would like to expect `value2`
  param3 // properly, but problem above

Updated

From RFC. In the future (in PHP 7) operator Null Coalesce Operator will do it, for example:

$param1 = $params['param1'] ?? null;
// Equivalent to:  $param1 = isset($params['param1']) ? $params['param1'] : null;

回答4:

Another important consideration: The Elvis Operator breaks the Zend Opcache tokenization process. I found this the hard way! While this may have been fixed in later versions, I can confirm this problem exists in PHP 5.5.38 (with in-built Zend Opcache v7.0.6-dev).

If you find that some of your files 'refuse' to be cached in Zend Opcache, this may be one of the reasons... Hope this helps!

回答5:

Yes, this is new in PHP 5.3. It returns either the value of the test expression if it is evaluated as TRUE, or the alternative value if it is evaluated as FALSE.

来源：https://stackoverflow.com/questions/1993409/operator-the-elvis-operator-in-php

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