PHP面向对象——构造函数、析构函数

     我们在创建和销毁对象时需要执行一些任务。例如，在创建对象时给属性赋值，在对象销毁时关闭数据连接等，这时就需要构造函数和析构函数。

在PHP中构造函数和析构函数是固定的，如下：

<font face="微软雅黑"><span style="color: #008000">//</span><span style="color: #008000"> 构造函数</span>
<span style="color: #0000ff">function</span> __construct([argument1,argument2,...,</font><font face="微软雅黑"><span style="color: #000000">argumentN]){
    </span><span style="color: #008000">/*</span><span style="color: #008000"> Class initialization code </span><span style="color: #008000">*/</span></font><span style="color: #000000">
<font face="微软雅黑">}

</font></span><font face="微软雅黑"><span style="color: #008000">//</span><span style="color: #008000"> 析构函数</span>
<span style="color: #0000ff">function</span></font><font face="微软雅黑"><span style="color: #000000"> __destruct(){
    </span><span style="color: #008000">//</span><span style="color: #008000"> ...</span>
}</font>

构造函数

• 构造函数可以接受参数，能够在创建对象时赋值给对象属性
• 构造函数可以调用类方法或其他函数
• 构造函数可以调用其他类的构造函数

    构造函数使用例子：

<span style="color: #000000">php
</span><span style="color: #0000ff">class</span><span style="color: #000000"> Person{
    </span><span style="color: #0000ff">private</span> <span style="color: #800080">$name</span><span style="color: #000000">;
    </span><span style="color: #0000ff">private</span> <span style="color: #800080">$age</span><span style="color: #000000">;
    </span><span style="color: #0000ff">private</span> <span style="color: #800080">$gender</span><span style="color: #000000">;

    </span><span style="color: #0000ff">public</span> <span style="color: #0000ff">function</span> __construct(<span style="color: #800080">$name</span>,<span style="color: #800080">$age</span>,<span style="color: #800080">$gender</span><span style="color: #000000">){
        </span><span style="color: #800080">$this</span>->setName(<span style="color: #800080">$name</span><span style="color: #000000">);
        </span><span style="color: #800080">$this</span>->setAge(<span style="color: #800080">$age</span><span style="color: #000000">);
        </span><span style="color: #800080">$this</span>->setGender(<span style="color: #800080">$gender</span><span style="color: #000000">);
    }

    </span><span style="color: #0000ff">public</span> <span style="color: #0000ff">function</span> setName(<span style="color: #800080">$name</span><span style="color: #000000">){
        </span><span style="color: #800080">$this</span>->name = <span style="color: #800080">$name</span><span style="color: #000000">;
    }

    </span><span style="color: #008000">//</span><span style="color: #008000"> ... getter setter 方法</span>
<span style="color: #000000">
}

</span><span style="color: #800080">$peron</span> = <span style="color: #0000ff">new</span> Person("lee",18,'男'<span style="color: #000000">);

</span>?>

    调用父类构造函数、调用无关类的构造函数：

<span style="color: #0000ff">function</span><span style="color: #000000"> __construct(){
    parent</span>::__construct(); <span style="color: #008000">//</span><span style="color: #008000"> 调用父类的构造函数必须显示的使用parent调用父类构造函数</span>
    classname::__construct(); <span style="color: #008000">//</span><span style="color: #008000"> 调用其他类的构造函数，classname是类名
    //其他操作</span>
}

继承和构造函数

    PHP中的子类的构造函数不会主动调用父类的构造函数，要显示的使用parent::__construct()调用：

<span style="color: #000000">php
</span><span style="color: #0000ff">class</span><span style="color: #000000"> Animal{
    </span><span style="color: #0000ff">private</span> <span style="color: #800080">$name</span><span style="color: #000000">;
    </span><span style="color: #0000ff">function</span> __construct(<span style="color: #800080">$name</span><span style="color: #000000">){
        </span><span style="color: #800080">$this</span>->setName(<span style="color: #800080">$name</span><span style="color: #000000">)
        </span><span style="color: #0000ff">echo</span> "动物类被创建！"<span style="color: #000000">;
    }
    </span><span style="color: #008000">//</span><span style="color: #008000"> ... 其他方法</span>
<span style="color: #000000">
}

</span><span style="color: #0000ff">class</span> Birds <span style="color: #0000ff">extends</span><span style="color: #000000"> Animal{
    </span><span style="color: #0000ff">private</span> <span style="color: #800080">$name</span><span style="color: #000000">;
    </span><span style="color: #0000ff">private</span> <span style="color: #800080">$leg</span><span style="color: #000000">;
    </span><span style="color: #0000ff">function</span> __construct(<span style="color: #800080">$name</span>,<span style="color: #800080">$leg</span><span style="color: #000000">){
        parent</span>::__construct(<span style="color: #800080">$name</span>); <span style="color: #008000">//</span><span style="color: #008000"> 显示调用</span>
        <span style="color: #800080">$this</span>->setLeg(<span style="color: #800080">$leg</span><span style="color: #000000">);
        </span><span style="color: #0000ff">echo</span> "鸟类被创建！"<span style="color: #000000">;
    }
    </span><span style="color: #008000">//</span><span style="color: #008000"> ... 其他方法</span>
<span style="color: #000000">}
</span>?>

    如果涉及多层继承，当调用parent::__construct()时，会沿着父类向上搜索，直到找到最合适的构造函数，例如：

<span style="color: #008000">//</span><span style="color: #008000"> 接上例</span>

<span style="color: #0000ff">class</span> Parrot <span style="color: #0000ff">extends</span><span style="color: #000000"> Birds{
    </span><span style="color: #0000ff">private</span> <span style="color: #800080">$name</span><span style="color: #000000">;
    </span><span style="color: #0000ff">private</span> <span style="color: #800080">$leg</span><span style="color: #000000">;
    </span><span style="color: #0000ff">private</span> <span style="color: #800080">$wing</span><span style="color: #000000">;
    </span><span style="color: #0000ff">function</span> __construct(<span style="color: #800080">$name</span><span style="color: #000000">){
        parent</span>::__construct(<span style="color: #800080">$name</span>); <span style="color: #008000">//</span><span style="color: #008000"> 此时没有找到父类（Birds类）合适的构造函数，只能向上搜索，搜索到Animal类时，才找到合适的构造函数</span>
        <span style="color: #0000ff">echo</span> "鹦鹉类被创建！"<span style="color: #000000">;
        </span><span style="color: #800080">$this</span>-><span style="color: #000000">smackTalk();
        </span><span style="color: #008000">/*</span><span style="color: #008000">
        输出结果：
        "动物类被创建！"
        "鹦鹉说话！"
        </span><span style="color: #008000">*/</span><span style="color: #000000">
    }
    </span><span style="color: #0000ff">function</span><span style="color: #000000"> smackTalk(){
        </span><span style="color: #0000ff">echo</span> "鹦鹉说话！"<span style="color: #000000">;    
    }

}</span>

    如果想要依次调用几个父类的构造函数，可以使用类名直接调用构造函数，例如：

<span style="color: #0000ff">function</span> __construct(<span style="color: #800080">$name</span>,<span style="color: #800080">$leg</span><span style="color: #000000">){
       Animal</span>::__construct(<span style="color: #800080">$name</span>); <span style="color: #008000">//</span><span style="color: #008000"> 调用Animal构造函数</span>
        Birds::__construct(<span style="color: #800080">$name</span>,<span style="color: #800080">$leg</span>); <span style="color: #008000">//</span><span style="color: #008000"> 调用Birds构造函数</span>
}

   PHP是不支持函数的重载，所有构造函数同样不能有多个。

析构函数

• 析构函数是在销毁对象时，自动调用，不能显示的调用。
• 析构函数不能带参数。

 

在以下几种情况下可能会调用析构函数（但不一定）：

• PHP页面加载完毕之后；
• unset()类；
• 变量引用指向别的对象或值时；

PHP的内存回收机制和JAVA的很类似，对没有任何引用的对象进行销毁回收，采用引用计数器的技术。

例子：

<span style="color: #000000">php
</span><span style="color: #0000ff">class</span><span style="color: #000000"> test{
    </span><span style="color: #0000ff">function</span><span style="color: #000000"> __destruct(){
        </span><span style="color: #0000ff">echo</span> "当对象销毁时会调用！！！"<span style="color: #000000">;
    }

}
</span><span style="color: #800080">$a</span> = <span style="color: #800080">$b</span> = <span style="color: #800080">$c</span> = <span style="color: #0000ff">new</span><span style="color: #000000"> test();

</span><span style="color: #800080">$a</span> = <span style="color: #0000ff">null</span><span style="color: #000000">;
</span><span style="color: #0000ff">unset</span>(<span style="color: #800080">$b</span><span style="color: #000000">);

</span><span style="color: #0000ff">echo</span> "<hr>"<span style="color: #000000">;

</span>?>

此例子，如下图，有三个变量引用$a,$b,$c指向test对象，test对象就有3个引用计数，当$a = null时，$a对test对象的引用丢失，计数-1，变为2，当$b被unset()时，$b对test对象的引用也丢失了，计数再-1，变为1，最后页面加载完毕，$c指向test对象的引用自动被释放，此时计数再-1，变为0，test对象已没有变量引用，就会被销毁，此时就会调用析构函数。