Section 4--Constructor and Destructor--ClassesandObjectsinPHP54_PHP Tutorial

/* +-------------------------------------------------- --------------------------------+ | = This article is read by Haohappy> | = Classes and Objects 1 Chapter's notes | = Translation + personal experience | = To avoid unnecessary trouble, please do not reprint, thank you | = Criticisms and corrections are welcome, and I hope to make progress together with all PHP enthusiasts! +------- -------------------------------------------------- -----------------------+ */ Section 4 - Constructors and destructors If you declare a function in a class, 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 any other function, the constructor may have parameters or default values. You can define A class creates an object and puts 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. Inheritance is a powerful feature of classes. One class (subclass/derived class) can inherit the functions 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 inherit a class using the extends keyword. You may wonder how constructors are inherited When they are inherited along with other methods, they will not be executed when the object is created. If you need this functionality, 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 constructor of the parent class. Some object-oriented languages ​​name the constructor after the class. This was also the case in previous versions of PHP, and this method is still valid today. That is: if you name a class Animal and create a method named Animal, 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 __construct is regarded as a constructor. This allows classes written in previous PHP versions to still be used. But new scripts (PHP5) should use __construct. This new method of declaring constructors in PHP can make the constructor 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 the constructor in PHP the same access method as other class methods The access method will affect the ability to instantiate the object 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 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 execute arbitrary code to clear memory after using an object. When PHP decides that your script is no longer related to the object, destructor The constructor will be called. 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 an object, you can give a pointer to the object. Assign any other value to the variable. 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 is incremented starting from the constructor and decremented 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. 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 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.

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