Constructor and Destructor_PHP Tutorial-PHP Tutorial-php.cn

Constructor and Destructor_PHP Tutorial

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Release： 2016-07-20 10:59:58

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If you declare a function in a class and name it __construct, this function will be treated as a constructor and executed when creating an object instance. To be clear, __ is two underscores. Like 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 is called a destructor.

Continuation is a powerful feature of classes. One class (subclass/derived class) can continue 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 be used in the derived class Add other properties and methods to it. You can also override methods and properties of the base class. As shown in 3.1.2, you can use the extends keyword to extend a class.
You may be wondering how constructors are continued. When they are continued along with other methods, they are not executed when the object is created.
If you need this function, 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 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, no matter what 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 range. 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 when the script ends. 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 to increase in value from the constructor and decreases in value 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 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.