Section 4 Constructor and Destructor [4]_PHP Tutorial

If you declare a function in a class, named __construct, the 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 Likewise, 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 class called __destruct function, PHP will call this function before the object is destroyed. It is called a destructor.

Inheritance is a powerful feature of classes. A class (subclass/derived class) can inherit another class (parent class/base class) functions. 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. Just like 3.1. As shown in 2, you can use the extends keyword to inherit from a class. You may be wondering how constructors are inherited. When they are inherited along with other methods, they are not executed when creating the object.
If you need this functionality, you need to use the method mentioned in Chapter 2 ::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. This was also the case in previous versions of PHP, and this method still works. That is: if you name a class Animal and create a method in it named Animal, 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 constructors 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 in PHP an access method like other class methods. The access method will affect the ability to instantiate objects from a certain scope. This allows the implementation of some fixed design patterns, such as the Singleton pattern. Destructors are the opposite of constructors. PHP calls them to destroy an object from memory. By default, PHP only releases the memory occupied by the object's properties and destroys the resources associated with the object. Destructors allow you to use an object after Execute arbitrary code to clear memory. The destructor will be called when PHP determines that your script is no longer associated with the object. Within a function's namespace, this happens when the function returns. For global variables, this happens at the end of the script. If 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, the number of objects instantiated from the class is counted. The Counter class starts incrementing in the constructor and decrements in the destructor. Once you define a class, you can use new to create an instance of the class. The definition of the class is the blueprint, and the instances are the components placed on the assembly line. New takes the name of the class and returns an instance of the class. If the constructor requires parameters, you should enter the parameters after new. class Counter
{
private static $count = 0; function __construct()
{
self::$count++;
} function __destruct()
{
self::$count--;
} function getCount()
{
return self::$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.

http://www.bkjia.com/PHPjc/314091.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/314091.htmlTechArticleIf you declare a function in a class named __construct, this function will be regarded as a constructor and is executed when an object instance is created. To be clear, __ is two underscores...