ArrayAccess::offsetExists

(PHP 5, PHP 7, PHP 8)

ArrayAccess::offsetExists — 检查一个偏移位置是否存在

说明

public ArrayAccess::offsetExists(mixed $offset): bool

检查一个偏移位置是否存在。

对一个实现了 ArrayAccess 接口的对象使用 isset() 或 empty() 时，此方法将执行。

注意:
当使用 empty() 并且仅当 ArrayAccess::offsetExists() 返回 true 时，ArrayAccess::offsetGet() 将被调用以检查是为否空。

参数

offset: 需要检查的偏移位置。

返回值

成功时返回 true，或者在失败时返回 false。

注意:
如果一个非布尔型返回值被返回，将被转换为 bool 。

示例

示例 #1 ArrayAccess::offsetExists() 示例

<?php
class obj implements ArrayAccess {
    public function offsetSet($offset, $value): void {
        var_dump(__METHOD__);
    }
    public function offsetExists($var): bool {
        var_dump(__METHOD__);
        if ($var == "foobar") {
            return true;
        }
        return false;
    }
    public function offsetUnset($var): void {
        var_dump(__METHOD__);
    }
    #[\ReturnTypeWillChange]
    public function offsetGet($var) {
        var_dump(__METHOD__);
        return "value";
    }
}

$obj = new obj;

echo "Runs obj::offsetExists()\n";
var_dump(isset($obj["foobar"]));

echo "\nRuns obj::offsetExists() and obj::offsetGet()\n";
var_dump(empty($obj["foobar"]));

echo "\nRuns obj::offsetExists(), *not* obj:offsetGet() as there is nothing to get\n";
var_dump(empty($obj["foobaz"]));
?>

以上示例的输出类似于：

Runs obj::offsetExists()
string(17) "obj::offsetExists"
bool(true)

Runs obj::offsetExists() and obj::offsetGet()
string(17) "obj::offsetExists"
string(14) "obj::offsetGet"
bool(false)

Runs obj::offsetExists(), *not* obj:offsetGet() as there is nothing to get
string(17) "obj::offsetExists"
bool(true)

发现了问题？

了解如何改进此页面 • 提交拉取请求 • 报告一个错误

＋添加备注

用户贡献的备注 1 note

down

pierstoval at gmail dot com ¶

15 days ago

Please note something:

The docs explain clearly that this method is called when "isset()" or "empty()" are called on the object's key.

This means that there is a huge difference in your custom implementation when you have an internal array on which you choose to call either "isset()" or "array_key_exists()".

Even though the method says "offsetExists", it is *not* supposed to be used only when the offset exists, because this is not at all the behavior of neither "isset" nor "empty" internally.

This means you can have issues like this (more explanations below):

<?php

class Value {
    public function __construct(
        public string $value,
    ) {
    }
}

class MyArray implements ArrayAccess {
    private array $internal = [];

    public function offsetExists(mixed $offset): bool
    {
        return array_key_exists($offset, $this->internal);
    }

    // ... rest of the implementation
    public function offsetGet(mixed $offset): mixed
    {
        return $this->offsetExists($offset) ? $this->internal[$offset] : null;
    }

    public function offsetSet(mixed $offset, mixed $value): void
    {
        if (is_null($offset)) {
            $this->internal[] = $value;
        } else {
            $this->internal[$offset] = $value;
        }
    }

    public function offsetUnset(mixed $offset): void
    {
        unset($this->internal[$offset]);
    }
}

$object = new MyArray();
$object['key'] = null;

// This is where the error occurs:
// PHP Fatal error:  Uncaught TypeError: Value::__construct(): Argument #1 ($value) must be of type string, null given
$otherValue = isset($object['key']) ? new Value($object['key']) : null;
?>

The thing here is that we have some code that cannot use the "??" operator because we need the output of the "isset" call to return true, and only then we want to use.

With a real array, this should be fairly common because we know how "isset" works.

However, since the "offsetExists" method has a lot of different implementations in PHP libaries, you should *not* trust the output in "isset" with objects implementing ArrayAccess.

A workaround is to create an intermediate variable and run "isset()" on it:

<?php

// Before
$otherValue = isset($arrayObject['key']) ? new Value($arrayObject['key']) : null;

// After
$rawValue = $arrayObject['key'] ?? null;
$otherValue = isset($rawValue) ? new Value($rawValue) : null;

?>