Static members such as HP5.0 object model in-depth exploration_PHP tutorial

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Static members of a class are different from general class members: static members have nothing to do with the instance of the object, only the class itself. They are used to implement the functions and data that the class wants to encapsulate, but do not include the functions and data of specific objects. Static members include static methods and static properties.

Static properties contain data that is encapsulated within a class and can be shared by all instances of the class. In fact, in addition to belonging to a fixed class and restricting access methods, the static properties of a class are very similar to the global variables of a function.

We use a static property Counter::$count in the following example. It belongs to the Counter class and not to any instance of Counter. You cannot use this to refer to it, but you can use self or other valid naming expressions. In the example, the getCount method returns self::$count, not Counter::$count.

Static methods implement functions that need to be encapsulated by the class and have nothing to do with specific objects. Static methods are very similar to global functions. Static methods can fully access the properties of the class or be accessed by instances of the object, regardless of the access qualifier. .

In the previous example, getCount is an ordinary method, called with ->. PHP creates a this variable, although the method is not used. However, getCount does not belong to any object. In some cases, we even want to If you call it when there is a valid object, then you should use static methods. PHP will not create this variable inside static methods, even if you call them from an object.

Example 6.7 comes from 6.3 changing getCount to a static method. The Static keyword does not prevent an instance from calling getCount using the -> operator, but PHP will not create this variable inside the method. If you use this-> to call it, Something will go wrong.

//6.3 examples refer to the examples in Section 4 - Constructors and Destructors (see the previous article). By comparing the two examples, you can have a good grasp of the difference between //static methods and ordinary methods.

You can write a method to show whether it is called statically or non-statically by determining whether this is created. Of course, if you use the static keyword, the method will always be static no matter how it is called.

Your class can also define constant properties. You don't need to use public static, just use the const keyword. Constant properties are always static. They are properties of the class, not the properties of the object that instantiates the class.
The following is the quoted content:

Listing 6.7 Static members

class Counter
{
private static $count = 0;
const VERSION = 2.0;

function __construct()
{
​self::$count ;
}

function __destruct()
{
​self::$count--;
}

static function getCount()
{
​return self::$count;
}
};

//Create an instance, then __construct() will execute
$c = new Counter();

//Output 1
print(Counter::getCount() . "n");

//Output the version attribute of the class
print("Version used: " . Counter::VERSION . "n");
?>

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