Section 7 Static Members of Classes [7]_PHP Tutorial

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 to be encapsulated by the class, but do not include the functions and data of the specific object. Static members include Static methods and static properties.

Static properties contain data to be encapsulated in 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, static properties of a class Very similar to the global variable of the function

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

Static methods implement the functions that the class needs to encapsulate, and specific Object-agnostic. Static methods are very similar to global functions. Static methods can fully access the properties of the class, and can also be accessed by instances of the object, regardless of the access qualifier.

In example 6.3, 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 call it when there is no valid object , then static methods should be used. PHP will not create this variable inside static methods, even if you call them from an object.

Example 6.7 is derived from 6.3 changing getCount to a static method. The Static keyword cannot prevent An instance uses the -> operator to call getCount, but PHP will not create this variable inside the method. If you use this-> to call, an error will occur.

//6.3 Example refers to Section 4 --Examples in constructors and destructors (see the previous article). By comparing the two examples, you can better grasp the difference between
//static methods and ordinary methods.

You 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, this method will always be static no matter how it is called.

Your class can also define constant properties, without using 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.

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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