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PHP memory management constructors and destructors_PHP tutorial
- WBOYOriginal
- 2016-07-13 10:33:51871browse
Friends who are learning the PHP language for the first time may not know much about the PHP destructor. In this article, we will explain the relevant knowledge in this aspect to you in detail. Hope it helps everyone.
Most classes have a special method called a constructor. When an object is created, it automatically calls the constructor, which is usually used to perform some useful initialization tasks. The declaration of a constructor is the same as the declaration of other operations, except that its name must be __construct(). This is a change in PHP5, in previous versions the name of the constructor had to be the same as the class name. For backward compatibility, if a class does not have a method named __construct(), PHP will search for a method with the same name as the class.
<?php
class person{
var $name;
var $age;
function __construct(){ // 构造函数
$this->name="lisi";
$this->age=28;
}
function say(){
echo "my name is ".$this->name."<br />";
echo "my age is ".$this->age."<p>";
}
}
$per=new person();
$per->say();
$per->name="zhangsan";
$per->age=26;
$per->say();
?>
The opposite of the constructor is the destructor. The destructor is a newly added content of PHP5. There is no destructor in PHP4. Destructors allow some operations or functions to be performed before a class is destroyed, which usually occurs automatically when all references to the class are reset or go out of scope. Similar to the name of the constructor, the name of a class's destructor must be __destruct(). The destructor cannot take any parameters.
<?php
class person {
function __destruct()
{
echo "bye bye !";
}
}
$a=new person();
?>
If you declare a function in a class named __construct, this function will be treated as a constructor and executed when creating an object instance. To be clear, __ is two underscores. Just like any other function, a constructor may have parameters or default values. You can define a class to create an object and put all its properties in a statement.
You can also define a function called __destruct, which PHP will call before the object is destroyed. It's called the PHP destructor.
Inheritance is a powerful feature of classes. One class (subclass/derived class) can inherit the functionality of another class (parent class/base class). The derived class will contain all the properties and methods of the base class, and can add other properties and methods in the derived class. You can also override base class methods and properties. As shown in 3.1.2, you can extend a class using the extends keyword.
You may be wondering how constructors are inherited. When they are inherited along with other methods, they are not executed when the object is created. If you need this functionality, you need to use the :: operator. It allows you to point to a namespace. parent points to the parent class namespace. You can use parent::__construct to call the parent class's constructor.
Some object-oriented languages name the constructor after the class. The same was true for previous versions of PHP, and this method still works today. That is: if you name a class Animal and create a method named Animal in it, then this method is the constructor. If a class has both a __construt constructor and a function with the same name as the class, PHP will treat __construct as a constructor. This allows classes written in previous PHP versions to still be used. But new scripts (PHP5) should use __construct.
PHP’s new way of declaring a constructor allows the constructor to have a unique name, regardless of the name of the class it is in. This way you don't need to change the name of the constructor when you change the name of the class.
You may give constructors the same access as other class methods in PHP. Access methods will affect the ability to instantiate objects from a certain range. This allows the implementation of some fixed design patterns, such as the Singleton pattern.
PHP destructor, the opposite of constructor. PHP calls them to destroy an object from memory. By default, PHP only releases the memory occupied by object properties and destroys object-related resources. Destructors allow you to execute arbitrary code to clear memory after using an object.
When PHP decides that your script is no longer associated with the object, the PHP destructor will be called. Within a function's namespace, this occurs when the function returns. For global variables, this happens at the end of the script. If you want to explicitly destroy an object, you can assign any other value to the variable pointing to the object. Usually assign the variable to NULL or call unset.
In the following example, count the number of objects instantiated from the class. The Counter class is incremented from the constructor and decremented in the PHP destructor.
Once you define a class, you can use new to create an instance of the class. The class definition is the design drawing, and the instances are the components placed on the assembly line. New takes the name of a class and returns an instance of that class. If the constructor requires parameters, you should enter the parameters after new.
<?php
class Counter {
private static $count = 0;
function __construct() { self::$count++; }
function __destruct() { self::$count--; }
function getCount() { return self::$count; } }
//建立第一个实例，调用构造函数，$count = 1
$c = new Counter();
//输出1
print($c->getCount() . "<br />");
//建立第二个实例，调用构造函数，$count = 2
$c2 = new Counter();
//输出2
print($c->getCount() . "<br />");
//销毁实例，调用析构函数，$count = 1
$c2 = NULL;
//输出1
print($c->getCount() . "<br />");
?>
Program execution result:
1 2 1
When you create a new PHP destructor instance, memory will be prepared to store all properties. Each instance has its own unique set of properties. But methods are shared by all instances of the class.