PHP 中对象属性?中表示什么意思 哪个版本开始支持的?

[xoxo419]

private static ?Singleton $instance = null;

https://github.com/domnikl/DesignPatternsPHP/blob/9f61e21dcf1bf3b49ef70ccc6d7849fdd82b0874/Creational/Singleton/Singleton.php#L7

[jfcherng]

php 7.1, 關鍵字 nullable

[Still4]

我这一般是这么用的

public function initTask($task_info, $task_num, $task_total): ?Thread

public function initTask($task_info, $task_num, $task_total): Thread

当返回结果可能为空的时候用前一个，不这么写返回空会报错

[xoxo419]

@jfcherng
@Still4

用 PHP7.2 执行报错误

```
There was 1 error:

1) DesignPatterns\Creational\Singleton\Tests\SingletonTest::testUniqueness
ParseError: syntax error, unexpected '?', expecting function (T_FUNCTION) or const (T_CONST)

/var/www/base/test/php/DesignPatterns/Creational/Singleton/Singleton.php:7
/var/www/base/test/php/DesignPatterns/Creational/Singleton/Tests/SingletonTest.php:12

```

用 php7.4 执行 OK

[jfcherng]

如果你問的不是那個 "?" 的話, typed property php 7.4
https://www.php.net/manual/en/migration74.new-features.php#migration74.new-features.core.typed-properties