PHP三元运算符vs空合并运算符

快来打我* 2022-11-27 10:27 231阅读 0赞

本文翻译自：[PHP ternary operator vs null coalescing operator][]

*Can someone explain the differences between ternary operator shorthand ( `?:` ) and null coalescing operator ( `??` ) in PHP?* **有人可以解释PHP中的三元运算符速记（ `?:` ：）和空合并运算符（ `??` ）之间的区别吗？**

*When do they behave differently and when in the same way (if that even happens)?* **它们什么时候表现不同，什么时候以相同的方式表现（如果甚至发生）？**

$a ?: $b

*VS.* **VS。**

$a ?? $b

--------------------

### \#1楼 ###

参考：[https://stackoom.com/question/2l3A6/PHP三元运算符vs空合并运算符][https_stackoom.com_question_2l3A6_PHP_vs]

--------------------

### \#2楼 ###

*Scroll down on [this][] link and view the section, it gives you a comparative example as seen below:* **向下滚动[此][this]链接并查看该部分，它为您提供了一个比较示例，如下所示：**

<?php
 /** Fetches the value of $_GET['user'] and returns 'nobody' if it does not exist. **/
 $username = $_GET['user'] ?? 'nobody';
 /** This is equivalent to: **/
 $username = isset($_GET['user']) ? $_GET['user'] : 'nobody';
 
 /** Coalescing can be chained: this will return the first defined value out of $_GET['user'], $_POST['user'], and 'nobody'. **/
 $username = $_GET['user'] ?? $_POST['user'] ?? 'nobody';
 ?>

*However, it is not advised to chain the operators as it makes it harder to understand the code when reading it later on.* **但是，不建议将运算符链接在一起，因为这会使以后阅读代码时更难理解代码。**

> *The null coalescing operator (??) has been added as syntactic sugar for the common case of needing to use a ternary in conjunction with isset().* **对于需要将三元数与isset（）结合使用的常见情况，已将空合并运算符（??）添加为语法糖。** *It returns its first operand if it exists and is not NULL;* **如果它存在且不为NULL，则返回其第一个操作数；否则返回第一个操作数。** *otherwise it returns its second operand.* **否则返回第二个操作数。**

*Essentially, using the coalescing operator will make it auto check for null unlike the ternary operator.* **本质上，与三元运算符不同，使用合并运算符将使其自动检查是否为空。**

--------------------

### \#3楼 ###

*When your first argument is null, they're basically the same except that the null coalescing won't output an `E_NOTICE` when you have an undefined variable.* **当您的第一个参数为null时，它们基本相同，只是当您有未定义的变量时，空合并不会输出`E_NOTICE` 。** *The [PHP 7.0 migration docs][] has this to say:* **[PHP 7.0迁移文档的][PHP 7.0 migration docs]内容如下：**

*Here's some example code to demonstrate this:* **这是一些示例代码来演示这一点：**

<?php
 
 $a = null;
 
 print $a ?? 'b'; // b
 print "\n";
 
 print $a ?: 'b'; // b
 print "\n";
 
 print $c ?? 'a'; // a
 print "\n";
 
 print $c ?: 'a'; // Notice: Undefined variable: c in /in/apAIb on line 14
 print "\n";
 
 $b = array('a' => null);
 
 print $b['a'] ?? 'd'; // d
 print "\n";
 
 print $b['a'] ?: 'd'; // d
 print "\n";
 
 print $b['c'] ?? 'e'; // e
 print "\n";
 
 print $b['c'] ?: 'e'; // Notice: Undefined index: c in /in/apAIb on line 33
 print "\n";

*The lines that have the notice are the ones where I'm using the shorthand ternary operator as opposed to the null coalescing operator.* **注意的行是我使用速记三元运算符而不是空合并运算符的行。** *However, even with the notice, PHP will give the same response back.* **但是，即使有此通知，PHP也会给出相同的响应。**

*Execute the code: [https://3v4l.org/McavC][https_3v4l.org_McavC]* **执行代码： [https][https_3v4l.org_McavC] : [//3v4l.org/McavC][https_3v4l.org_McavC]**

*Of course, this is always assuming the first argument is `null` .* **当然，这总是假设第一个参数为`null` 。** *Once it's no longer null, then you end up with differences in that the `??`* **一旦它不再为null，则最终会导致区别`??`** *operator would always return the first argument while the `?:` shorthand would only if the first argument was truthy, and that relies on how [PHP would type-cast things to a boolean][] .* **仅当第一个参数为true时，运算符始终会返回`?:`简写形式的第一个参数，这取决于[PHP][PHP would type-cast things to a boolean]如何[将内容类型转换为boolean][PHP would type-cast things to a boolean] 。**

*So:* **所以：**

$a = false ?? 'f'; // false
    $b = false ?: 'g'; // 'g'

*would then have `$a` be equal to `false` and `$b` equal to `'g'` .* **则`$a`等于`false`而`$b`等于`'g'` 。**

--------------------

### \#4楼 ###

*Both of them behave differently when it comes to dynamic data handling.* **在动态数据处理方面，两者的行为都不同。**

*If the variable is empty ( '' ) the null coalescing will treat the variable as true but the shorthand ternary operator won't.* **如果变量为空（''），则空合并将变量视为true，但速记三元运算符则不会。** *And that's something to have in mind.* **这是要牢记的。**

$a = NULL;
    $c = '';
    
    print $a ?? '1b';
    print "\n";
    
    print $a ?: '2b';
    print "\n";
    
    print $c ?? '1d';
    print "\n";
    
    print $c ?: '2d';
    print "\n";
    
    print $e ?? '1f';
    print "\n";
    
    print $e ?: '2f';

*And the output:* **并输出：**

1b
    2b
    
    2d
    1f
    
    Notice: Undefined variable: e in /in/ZBAa1 on line 21
    2f

*Link: [https://3v4l.org/ZBAa1][https_3v4l.org_ZBAa1]* **链接： [https][https_3v4l.org_ZBAa1] ： [//3v4l.org/ZBAa1][https_3v4l.org_ZBAa1]**

--------------------

### \#5楼 ###

*If you use the shortcut ternary operator like this, it will cause a notice if `$_GET['username']` is not set:* **如果您这样使用快捷方式三元运算符，那么如果未设置`$_GET['username']` ，则会引起通知：**

$val = $_GET['username'] ?: 'default';

*So instead you have to do something like this:* **因此，您必须执行以下操作：**

$val = isset($_GET['username']) ? $_GET['username'] : 'default';

*The **null coalescing operator** is equivalent to the above statement, and will return 'default' if `$_GET['username']` is not set or is `null` :* **空合并运算符等效于上述语句，并且如果未设置`$_GET['username']`或为`null` ，则将返回'default'：**

$val = $_GET['username'] ?? 'default';

*Note that **it does not check truthiness** .* **请注意， 它不会检查真实性 。** *It checks only if it is set and not null.* **它仅检查是否已设置并且不为null。**

*You can also do this, and the first **defined** (set and not `null` ) value will be returned:* **您也可以执行此操作，并且将返回第一个定义的 （set而不是`null` ）值：**

$val = $input1 ?? $input2 ?? $input3 ?? 'default';

*Now that is a proper coalescing operator.* **现在，这是一个合适的合并运算符。**

--------------------

### \#6楼 ###

*It seems there are pros and cons to using either `??`* **似乎使用`??`都有利弊** *or `?:` .* **或`?:` 。** *The pro to using `?:` is that it evaluates false and null and "" the same.* **使用`?:`的优点是它对false和null以及“”的求值相同。** *The con is that it reports an E\_NOTICE if the preceding argument is null.* **缺点是，如果前面的参数为null，它将报告E\_NOTICE。** *With `??`* **与`??`** *the pro is that there is no E\_NOTICE, but the con is that it does not evaluate false and null the same.* **优点是没有E\_NOTICE，但是缺点是它不求假，并且null不变。** *In my experience, I have seen people begin using null and false interchangeably but then they eventually resort to modifying their code to be consistent with using either null or false, but not both.* **以我的经验，我看到人们开始交替使用null和false，但后来他们最终诉诸修改代码以与使用null或false一致，但不能同时使用两者。** *An alternative is to create a more elaborate ternary condition: `(isset($something) or !$something) ? $something : $something_else`* **另一种方法是创建一个更复杂的三元条件： `(isset($something) or !$something) ? $something : $something_else`** *`(isset($something) or !$something) ? $something : $something_else` .* **`(isset($something) or !$something) ? $something : $something_else` 。**

*The following is an example of the difference of using the `??`* **以下是使用`??`的区别的示例** *operator using both null and false:* **运算符同时使用null和false：**

$false = null;
 $var = $false ?? "true";
 echo $var . "--- ";//returns: true---
 
 $false = false;
 $var = $false ?? "true";
 echo $var . "--- "; //returns: ---

*By elaborating on the ternary operator however, we can make a false or empty string "" behave as if it were a null without throwing an e\_notice:* **但是，通过详细说明三元运算符，我们可以使错误或空字符串“”表现为好像是空值，而不会引发e\_notice：**

$false = null;
 $var = (isset($false) or !$false) ? $false : "true";
 echo $var . "--- ";//returns: ---
 
 $false = false;
 $var = (isset($false) or !$false) ? $false : "true";
 echo $var . "--- ";//returns: ---
 
 $false = "";
 $var = (isset($false) or !$false) ? $false : "true";
 echo $var . "--- ";//returns: ---
 
 $false = true;
 $var = (isset($false) or !$false) ? $false : "true";
 echo $var . "--- ";//returns: 1---

*Personally, I think it would be really nice if a future rev of PHP included another new operator: `:?`* **我个人认为，如果以后的PHP版本中包含另一个新的运算符，那将非常好`:?`** *that replaced the above syntax.* **替换了上面的语法。** *ie: `// $var = $false :? "true";`* **即： `// $var = $false :? "true";`** `// $var = $false :? "true";` *That syntax would evaluate null, false, and "" equally and not throw an E\_NOTICE...* **该语法将相等地评估null，false和“”，并且不会抛出E\_NOTICE ...**

[PHP ternary operator vs null coalescing operator]: https://oldbug.net/q/2l3A6/PHP-ternary-operator-vs-null-coalescing-operator
[https_stackoom.com_question_2l3A6_PHP_vs]: https://stackoom.com/question/2l3A6/PHP%E4%B8%89%E5%85%83%E8%BF%90%E7%AE%97%E7%AC%A6vs%E7%A9%BA%E5%90%88%E5%B9%B6%E8%BF%90%E7%AE%97%E7%AC%A6
[this]: http://php.net/manual/en/migration70.new-features.php
[PHP 7.0 migration docs]: http://php.net/manual/en/migration70.new-features.php#migration70.new-features.null-coalesce-op
[https_3v4l.org_McavC]: https://3v4l.org/McavC
[PHP would type-cast things to a boolean]: http://php.net/manual/en/language.types.boolean.php#language.types.boolean.casting
[https_3v4l.org_ZBAa1]: https://3v4l.org/ZBAa1