Section 4--Constructor and Destructor_PHP Tutorial-PHP Tutorial-php.cn

Section 4--Constructor and Destructor_PHP Tutorial

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Release： 2016-07-21 16:00:57

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+------------------------------------------------- ----------------------------------+
| = This article is read by Haohappy<>
| = Notes from the Chapter Classes and Objects
| = Translation + personal experience
| = To avoid possible unnecessary trouble, please do not reprint, thank you
| = Criticisms are welcome Correction, hope to make progress together with all PHP enthusiasts!
| = PHP5 Research Center: http://blog.csdn.net/haohappy2004
+-------------- -------------------------------------------------- ----------------+
*/

Section 4 - Constructors and destructors

If you declare it in a class A function, named __construct, will be treated as a constructor and will be executed when creating an object instance. To be clear, __ is two underscores. Just like any other function, the constructor may have parameters Or the default value. You can define a class to create an object and put all its properties in one statement.

You can also define a function called __destruct, and PHP will Call this function before destruction. It is called a destructor.

Inheritance is a powerful feature of classes. A 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 the methods and properties of the base class. As shown in 3.1.2, you can Use the extends keyword to inherit a class.

You may wonder 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 feature, you need to use the :: operator mentioned in Chapter 2. It allows you to point to a namespace. parent points to the parent class namespace, and you can use parent::__construct to call the parent class's constructor.

Some object-oriented languages name the constructor after the class. This was also true for the first few versions of PHP, and this method still works. That is: if you name a class Animal and create a If the name is also an Animal method, then this method is a constructor. If a class has both a __construt constructor and a function with the same name as the class, PHP will regard __construct as a constructor. This makes it possible to use previous PHP versions to write Classes can 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 class it is in. What is the name. In this way, when you change the name of the class, you do not need to change the name of the constructor.

You may give the constructor an access method in PHP like other class methods. The access method will be Affects the ability to instantiate objects from a certain scope. This allows the implementation of some fixed design patterns, such as the Singleton pattern.

Destructors, as opposed to constructors. PHP calls them to remove an object from memory Destruction. By default, PHP only releases the memory occupied by the object's properties and destroys the resources associated with the object. The destructor allows you to execute arbitrary code to clear the memory after using an object.

When PHP determines your script The destructor will be called when the object is no longer associated. Within a function's namespace, this happens when the function returns. For global variables, this happens at the end of the script. If you want to explicitly destroy a 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. Counter The value of a class starts from the constructor and decreases from the destructor.

Once you define a class, you can use new to create an instance of this class. The definition of the class is the design diagram, and the instance is the placeholder Components on the assembly line. New takes the name of the class and returns an instance of that class. If the constructor requires parameters, you should enter the parameters after new.

Copy code The code is as follows:

 class Counter 
 { 
 private static $count = 0; 

 function __construct() 
 { Self::$count++; 
                                                                                           getCount() 
 { 
 return self::$count; 
 } } 
 } 

 //Create the first instance 
 $c = new Counter(); 

 //Output 1 
 print($c->getCount() . "
n"); 

 //Create a second instance 
 $c2 = new Counter(); 

 //Output 2 
 print($c->getCount() . "
n"); 

 //Destroy the instance 
 $c2 = NULL; 

 //Output 1 
 print($c->getCount() . "
n"); 
?> 




 When you create a new instance, memory is prepared to store all properties. Each instance has its own unique set of properties. But methods are shared by all instances of the class.