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382 lines
10 KiB
XML
382 lines
10 KiB
XML
<?xml version="1.0" encoding="utf-8"?>
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<!-- $Revision$ -->
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<!-- EN-Revision: 801e7a15e8c9ee2d16966487dc9058d992450b46 Maintainer: HaoHappy Status: ready -->
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<!-- CREDITS: Luffy, mowangjuanzi -->
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<sect1 xml:id="language.oop5.properties" xmlns="http://docbook.org/ns/docbook">
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<title>属性</title>
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<para>
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类的变量成员叫做<emphasis>属性</emphasis>，或者叫<emphasis>字段</emphasis>，在本文档统一称为<emphasis>属性</emphasis>。
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属性开头至少使用一个修饰符（比如 <xref linkend="language.oop5.visibility"/>、<xref linkend="language.oop5.static"/>或者自 PHP 8.1.0 起支持的 <link linkend="language.oop5.properties.readonly-properties">readonly</link>），
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除了 <code>readonly</code> 属性之外都是可选的，然后自 PHP 7.4
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起可以跟一个类型声明，然后跟一个普通的变量声明来组成。属性中的变量可以初始化，但是初始化的值必须是 <link linkend="language.constants">常量</link>值。
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</para>
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<note>
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<para>
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另一种过时的声明类属性的方法是使用 <literal>var</literal> 关键字，而不是使用修饰符。
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</para>
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</note>
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<note>
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<simpara>
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没有声明 <xref linkend="language.oop5.visibility"/> 修饰符的属性将默认声明为 <literal>public</literal>。
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</simpara>
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</note>
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<para>
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在类的成员方法里面，可以用 <literal>-></literal>（对象运算符）：<varname>$this->property</varname>（其中
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<literal>property</literal> 是该属性名）这种方式来访问非静态属性。静态属性则是用
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<literal>::</literal>（双冒号）：<varname>self::$property</varname>
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来访问。更多静态属性与非静态属性的区别参见 <xref linkend="language.oop5.static" />。
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</para>
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<para>
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当一个方法在类定义内部被调用时，有一个可用的伪变量
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<varname>$this</varname>。<varname>$this</varname>
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是一个到主叫对象的引用（通常是该方法所从属的对象，但如果是从第二个对象<link
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linkend="language.oop5.static">静态</link>调用时也可能是另一个对象）。
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</para>
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<para>
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<example>
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<title>属性声明</title>
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<programlisting role="php" annotations="non-interactive">
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<![CDATA[
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<?php
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class SimpleClass
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{
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public $var1 = 'hello ' . 'world';
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public $var2 = <<<EOD
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hello world
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EOD;
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public $var3 = 1+2;
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// 错误的属性声明
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public $var4 = self::myStaticMethod();
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public $var5 = $myVar;
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// 正确的属性声明
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public $var6 = myConstant;
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public $var7 = array(true, false);
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public $var8 = <<<'EOD'
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hello world
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EOD;
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// 没有访问控制修饰符：
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static $var9;
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readonly int $var10;
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}
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?>
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]]>
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</programlisting>
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</example>
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</para>
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<note>
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<para>
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更多关于类／对象的处理函数，请查看<link linkend="ref.classobj">类／对象函数</link>。
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</para>
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</note>
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<sect2 xml:id="language.oop5.properties.typed-properties">
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<title>类型声明</title>
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<para>
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从 PHP 7.4.0 开始，属性定义可以包含<xref linkend="language.types.declarations" />，但
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<type>callable</type> 除外。
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<example>
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<title>类型声明的示例</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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class User
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{
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public int $id;
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public ?string $name;
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public function __construct(int $id, ?string $name)
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{
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$this->id = $id;
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$this->name = $name;
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}
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}
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$user = new User(1234, null);
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var_dump($user->id);
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var_dump($user->name);
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?>
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]]>
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</programlisting>
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&example.outputs;
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<screen>
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<![CDATA[
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int(1234)
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NULL
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]]>
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</screen>
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</example>
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</para>
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<para>
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类型属性必须在访问前初始化，否则会抛出 <classname>Error</classname> 。
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<example>
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<title>访问属性</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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class Shape
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{
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public int $numberOfSides;
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public string $name;
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public function setNumberOfSides(int $numberOfSides): void
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{
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$this->numberOfSides = $numberOfSides;
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}
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public function setName(string $name): void
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{
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$this->name = $name;
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}
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public function getNumberOfSides(): int
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{
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return $this->numberOfSides;
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}
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public function getName(): string
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{
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return $this->name;
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}
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}
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$triangle = new Shape();
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$triangle->setName("triangle");
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$triangle->setNumberofSides(3);
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var_dump($triangle->getName());
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var_dump($triangle->getNumberOfSides());
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$circle = new Shape();
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$circle->setName("circle");
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var_dump($circle->getName());
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var_dump($circle->getNumberOfSides());
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?>
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]]>
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</programlisting>
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&example.outputs;
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<screen>
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<![CDATA[
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string(8) "triangle"
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int(3)
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string(6) "circle"
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Fatal error: Uncaught Error: Typed property Shape::$numberOfSides must not be accessed before initialization
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]]>
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</screen>
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</example>
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</para>
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</sect2>
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<sect2 xml:id="language.oop5.properties.readonly-properties">
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<title>只读属性</title>
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<para>
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自 PHP 8.1.0 起，可以使用 <code>readonly</code> 修饰符声明属性，防止初始化后修改属性。
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在 PHP 8.4.0 之前，<code>readonly</code> 属性是隐式的私有设置，只能从同一类写入。
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从 PHP 8.4.0 开始，<code>readonly</code> 属性是隐式的 <link linkend="language.oop5.visibility-members-aviz"><literal>protected(set)</literal></link>，
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因此可以从子类设置。如果需要，可以显式覆盖。
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<example>
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<title>只读属性示例</title>
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<programlisting role="php" annotations="non-interactive">
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<![CDATA[
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<?php
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class Test {
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public readonly string $prop;
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public function __construct(string $prop) {
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// 初始化正常。
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$this->prop = $prop;
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}
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}
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$test = new Test("foobar");
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// 读取正常。
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var_dump($test->prop); // string(6) "foobar"
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// 再赋值异常。分配的值是否相同并不重要。
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$test->prop = "foobar";
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// Error: Cannot modify readonly property Test::$prop
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?>
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]]>
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</programlisting>
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</example>
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<note>
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<para>
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readonly 修饰符只能应用于<link linkend="language.oop5.properties.typed-properties">类型化属性</link>。可以使用
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<xref linkend="language.types.mixed" /> 类型创建没有类型约束的只读属性。
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</para>
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</note>
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<note>
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<para>
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不支持对静态属性只读。
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</para>
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</note>
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</para>
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<para>
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只读属性只能初始化一次，并且只能从声明它的作用域内初始化。对属性的任何赋值和修改都会导致 <classname>Error</classname> 异常。
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<example>
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<title>初始化只读属性异常</title>
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<programlisting role="php" annotations="non-interactive">
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<![CDATA[
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<?php
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class Test1 {
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public readonly string $prop;
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}
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$test1 = new Test1;
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// 私有作用域之外异常初始化。
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$test1->prop = "foobar";
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// Error: Cannot initialize readonly property Test1::$prop from global scope
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?>
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]]>
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</programlisting>
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</example>
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</para>
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<note>
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<para>
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禁止在只读属性上指定默认值，因为具有默认值的只读属性等同于常量，因此不是特别有用。
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<informalexample>
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<programlisting role="php" annotations="non-interactive">
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<![CDATA[
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<?php
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class Test {
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// Fatal error: Readonly property Test::$prop cannot have default value
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public readonly int $prop = 42;
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}
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?>
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]]>
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</programlisting>
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</informalexample>
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</para>
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</note>
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<note>
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<para>
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只读属性一旦初始化就不能 <function>unset</function>。但可以在初始化之前从声明属性的作用域中取消只读属性。
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</para>
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</note>
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<para>
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修改不一定是简单的赋值，以下所有行为也会导致 <classname>Error</classname> 异常：
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<informalexample>
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<programlisting role="php" annotations="non-interactive">
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<![CDATA[
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<?php
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class Test {
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public function __construct(
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public readonly int $i = 0,
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public readonly array $ary = [],
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) {}
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}
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$test = new Test;
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$test->i += 1;
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$test->i++;
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++$test->i;
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$test->ary[] = 1;
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$test->ary[0][] = 1;
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unset($test->ary[0]);
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$ref =& $test->i;
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$test->i =& $ref;
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byRef($test->i);
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foreach ($test as &$prop);
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?>
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]]>
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</programlisting>
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</informalexample>
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</para>
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<para>
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然而，只读属性并不会妨碍内部可变性。存储在只读属性中的对象（或资源）仍然可以在内部修改：
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<informalexample>
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<programlisting role="php" annotations="non-interactive">
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<![CDATA[
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<?php
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class Test {
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public function __construct(public readonly object $obj) {}
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}
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$test = new Test(new stdClass);
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// 内部可变正常。
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$test->obj->foo = 1;
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// 赋值异常。
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$test->obj = new stdClass;
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?>
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]]>
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</programlisting>
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</informalexample>
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</para>
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<para>
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自 PHP 8.3.0 起，使用 <link linkend="object.clone">__clone()</link>
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方法克隆对象时可以重新初始化 readonly 属性。
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<example>
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<title>readonly 属性和克隆</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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class Test1 {
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public readonly ?string $prop;
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public function __clone() {
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$this->prop = null;
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}
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public function setProp(string $prop): void {
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$this->prop = $prop;
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}
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}
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$test1 = new Test1;
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$test1->setProp('foobar');
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$test2 = clone $test1;
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var_dump($test2->prop); // NULL
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?>
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]]>
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</programlisting>
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</example>
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</para>
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</sect2>
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<sect2 xml:id="language.oop5.properties.dynamic-properties">
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<title>动态属性</title>
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<para>
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如果尝试在 &object; 上赋值不存在的属性，PHP
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将会自动创建相应的属性。动态创建的属性将<emphasis>仅能</emphasis>在此类实例上使用。
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</para>
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<warning>
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<simpara>
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自 PHP 8.2.0 起弃用动态属性。建议更改为属性声明。要处理任意属性名称，类应该实现魔术方法
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<link linkend="object.get">__get()</link> 和 <link
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linkend="object.set">__set()</link>。最后可以使用 <code>#[\AllowDynamicProperties]</code>
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注解标记此类。
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</simpara>
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</warning>
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</sect2>
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</sect1>
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<!-- Keep this comment at the end of the file
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Local variables:
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mode: sgml
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sgml-omittag:t
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sgml-shorttag:t
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sgml-minimize-attributes:nil
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sgml-always-quote-attributes:t
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sgml-indent-step:1
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sgml-indent-data:t
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indent-tabs-mode:nil
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sgml-parent-document:nil
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sgml-default-dtd-file:"~/.phpdoc/manual.ced"
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sgml-exposed-tags:nil
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sgml-local-catalogs:nil
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sgml-local-ecat-files:nil
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End:
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vim600: syn=xml fen fdm=syntax fdl=2 si
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vim: et tw=78 syn=sgml
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vi: ts=1 sw=1
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-->
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