Short-circuit evaluation via the AND operator in PHP

问题

I'm trying to improve my coding ninja h4x skills, and I'm currently looking at different frameworks, and I have found sample code that's pretty hard to google.

I am looking at the FUEL framework used in a project.

The sample I don't understand is

$data and $this->template->set_global($data);

What is the and keyword doing in this line of code? It is used many places in the framework and it's the first that I have found that uses it.

回答1:

This is a type of "short circuit evaluation". The and/&& implies that both sides of the comparison must evaluate to TRUE.

The item on the left of the and/&& is evaluated to TRUE/FALSE and if TRUE, the item on the right is executed and evaluated. If the left item is FALSE, execution halts and the right side isn't evaluated.

$data = FALSE;
// $this->template->set_global($data) doesn't get evaluated!
$data and $this->template->set_global($data);

$data = TRUE;
// $this->template->set_global($data) gets evaluated
$data and $this->template->set_global($data);

Note these don't have to be actual boolean TRUE/FALSE, but can also be truthy/falsy values according to PHP's evaluation rules. See the PHP boolean docs for more info on evaluation rules.

回答2:

When you use logical operators, operands (the value on the left and the value on the right) are evaluated as boolean, so basically that code will do this, in a shorter way:

$o1 = (Bool)$data; // cast to bool
if($o1)
    $o2 = (Bool)$this->template->set_global($data); // cast to bool

Edit:

Some additional information:

$a = 33;
isset($a) && print($a) || print("not set");
echo "<br>";
isset($a) AND print($a) OR print("not set");
echo "<br>";

Try to comment/decomment $a = 33;. This is the difference between && and AND, and between || and OR (print returns true that is casted to "1" when converted to string).

回答3:

It is a valid statement and works like this:

If $data is valid (is not '', 0 or NULL) then run $this->template->set_global($data)

It's a quick way of saying:

if ($data)
{
    $this->template->set_global($data);
}

Btw you can also use && instead of and

回答4:

PHP supports both && and and for the logical AND operation, and they generally work identically, except and has a slightly lower operator precedence than &&: http://php.net/manual/en/language.operators.precedence.php

回答5:

It's a boolean operator which means it takes two operands and returns a boolean value-- true or false. If both operands evaluate to true (anything but and empty string, zero or null in PHP) it will return true, otherwise the result will be false.

Here's PHP's official docs on the and operator: http://www.php.net/manual/en/language.operators.logical.php

<?php
$a = true and false; # FALSE
$b = true and 5; # TRUE
$c = '' and 0; # FALSE
$d = null and true; # FALSE
?>

来源：https://stackoverflow.com/questions/7665101/short-circuit-evaluation-via-the-and-operator-in-php

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php

and-operator