Use of PHP object-oriented static, const, and final keywords_PHP tutorial

The Static keyword is used to describe member properties and member methods in a class as static, and the application of the final keyword This keyword can only be used to define classes and methods. The final keyword cannot be used to define member properties, because final means constant. We use the define() function to define constants in PHP, so final cannot be used. Defining member attributes

The Static keyword describes the member attributes and member methods in the class as static; what are the benefits of static members? Earlier we declared the human being "Person". If we add an attribute of "the country to which the person belongs" to the "Person" class, hundreds or more instance objects can be instantiated using the "Person" class. Each object has the attribute of "country to which it belongs." If the project is developed for Chinese people, then each object will have the attribute of a country that is "China." The other attributes are different. If we make the "country" attribute a static member, so that there is only one country attribute in the memory, and hundreds or more objects share this attribute, the static member can restrict external access, because Static members belong to the class and do not belong to any object instance. They are allocated space when the class is loaded for the first time. They are inaccessible to other classes and are only shared with instances of the class. This member of the class can be accessed to a certain extent. form protection.

Let’s analyze it from the perspective of memory. The memory is logically divided into four segments. The objects are placed in the “heap memory”, the references of the objects are placed in the “stack memory”, and the static members are placed in the “stack memory”. When it comes to the "initialization static segment", it is placed when the class is loaded for the first time, so that it can be shared by every object in the heap memory, as shown below;

Static variables of a class are very similar to global variables and can be shared by all instances of the class. The same is true for static methods of the class, similar to global functions.

The code is as follows

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代码如下	复制代码
class Person     {     //下面是人的静态成员属性     public static $myCountry="中国";     // var $name;  //人的名子     //这是人的静态成员方法     public static function say()     {         echo "我是中国人 ";     } } //输出静态属性 echo Person::$myCountry; //访问静态方法 Person::say(); //重新给静态属性赋值 Person::$myCountry="美国"; echo Person::$myCountry;

class Person
{
//The following are the static member attributes of people
Public static $myCountry="China";
// var $name; //The person’s name //This is a person’s static member method
Public static function say()
{
echo "I am Chinese
";
}
} //Output static attributes
echo Person::$myCountry; //Access static methods
Person::say(); //Reassign the static properties
Person::$myCountry="United States";
echo Person::$myCountry;

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Use of static and const keywords
The Static keyword describes the member attributes and member methods in the class as static; what are the benefits of static members? Earlier we declared the human being "Person". If we add an attribute of "the country to which the person belongs" to the "Person" class, hundreds or more instance objects can be instantiated using the "Person" class. Each object has the attribute of "country to which it belongs." If the project is developed for Chinese people, then each object will have the attribute of a country that is "China." The other attributes are different. If we make the "country" attribute a static member, so that there is only one country attribute in the memory, and hundreds or more objects share this attribute, the static member can restrict external access, because Static members belong to the class and do not belong to any object instance. They are allocated space when the class is loaded for the first time. They are inaccessible to other classes and are only shared with instances of the class. This member of the class can be accessed to a certain extent. form protection.

Let’s analyze it from the perspective of memory. The memory is logically divided into four segments. The objects are placed in the “heap memory”, the references of the objects are placed in the “stack memory”, and the static members are placed in the “stack memory”. When it comes to the "initialization static segment", it is placed when the class is loaded for the first time, so that it can be shared by every object in the heap memory, as shown below;

Static variables of a class are very similar to global variables and can be shared by all instances of the class. The same is true for static methods of the class, similar to global functions.

The code is as follows

Copy code

代码如下

复制代码

class Person       {       //下面是人的静态成员属性       public static $myCountry="中国";       // var $name;  //人的名子         //这是人的静态成员方法       public static function say()       {           echo "我是中国人  ";       }   }     //输出静态属性   echo Person::$myCountry;     //访问静态方法   Person::say();     //重新给静态属性赋值   Person::$myCountry="美国";   echo Person::$myCountry;  class Person     {     //下面是人的静态成员属性     public static $myCountry="中国";     // var $name;  //人的名子     //这是人的静态成员方法     public static function say()     {         echo "我是中国人 ";     } } //输出静态属性 echo Person::$myCountry; //访问静态方法 Person::say(); //重新给静态属性赋值 Person::$myCountry="美国";

class Person {  //The following are the static member attributes of people Public static $myCountry="China"; // var $name; //The person’s name //This is a static member method of people Public static function say() {  echo "I am Chinese "; }   } //Output static attributes echo Person::$myCountry; //Access static method Person::say(); //Reassign static properties Person::$myCountry="United States"; echo Person::$myCountry; class Person { //The following are the static member attributes of people Public static $myCountry="China"; // var $name; //The person’s name //This is a static member method of human Public static function say() { echo "I am Chinese "; } } //Output static attributes echo Person::$myCountry; //Access static method Person::say(); //Reassign the static attributes Person::$myCountry="United States";

echo Person::$myCountry; Because static members are created when the class is first loaded, there is no need for an object outside the class and you can access the static members using the class name; as mentioned above , Static members are shared by each instance object of this class, so can we access static members in the class using objects? From the picture above, we can see that static members do not exist inside each object, but each object can be shared, so if we use objects to access members, there will be no such attribute definition and cannot be accessed using objects. Static members. In other object-oriented languages, such as Java, you can use objects to access static members. If you can use objects to access static members in PHP, we should try not to use them because we use static members. When working on a project, the purpose is to use the class name to access.

Static methods in a class can only access the static attributes of the class. Static methods in the class cannot access non-static members of the class. The reason is very simple. We want to access the methods of this class. For other members, we need to use the $this reference, and the $this reference pointer represents the object that calls this method. We said that static methods are not called with objects, but are accessed using class names, so there is no object at all. exists, there is no reference to $this. Without the reference to $this, non-static members in the class cannot be accessed. And because static members in the class can be accessed without objects, the static methods in the class can only To access the static properties of a class, since $this does not exist, we use a special class "self" to access other static members in a static method; self is similar to $this, except that self represents the location of this static method. kind. So in a static method, you can use the "class name" of the class where the method is located, or you can use "self" to access other static members. If there are no special circumstances, we usually use the latter, that is, "self:: member attribute " way.

The code is as follows

Copy code

代码如下	复制代码
class Person {     //下面是人的静态成员属性     public static $myCountry="中国";     //这是人的静态成员方法, 通过self访问其它静态成员     public static function say()     {         echo "我是".self::$myCountry."";     } } //访问静态方法 Person::say();

class Person
{
//The following are the static member attributes of people
Public static $myCountry="China";

//This is a human static member method, access other static members through self
Public static function say()
{
echo "I am".self::$myCountry."";
}
}
//Access static methods
Person::say();

代码如下	复制代码
class MyClass {     //定义一个常量constant     const constant = 'constant value';     function showConstant() {         echo  self::constant . "n";  //使用self访问，不要加”$”     } } echo MyClass::constant . "n";  //使用类名来访问,也不加”$” $class = new MyClass(); $class->showConstant(); //echo $class::constant;  是不允许的

Can static members be accessed in non-static methods? Of course it is possible, but you cannot use "$this" for reference and you must use the class name or "self:: member attribute form". const is a keyword for defining constants. To define constants in PHP, you use the "define()" function, but to define constants in a class, you use the "const" keyword, which is similar to #define in C. If in If its value is changed in the program, an error will occur. The access method of member properties modified with "const" is similar to the access method of members modified with "static". The "class name" is also used, and "self" is used in the method. Keywords. But you don't need to use the "$" symbol, and you can't use objects to access it.

The code is as follows	Copy code
class MyClass { //Define a constant const constant = 'constant value'; function showConstant() {               echo self::constant . "n"; //Use self to access, do not add "$" } } echo MyClass::constant . "n"; //Use the class name to access without adding "$" $class = new MyClass(); $class->showConstant(); //echo $class::constant; is not allowed

Application of final keyword
This keyword can only be used to define classes and methods. The final keyword cannot be used to define member properties, because final means constant. We use the define() function to define constants in PHP, so it cannot Use final to define member properties.

Classes marked with the final key cannot be inherited;

The code is as follows

代码如下	复制代码
final class Person { } class Student extends Person { }  //会出现下面错误: Fatal error: Class Student may not inherit from final class (Person) 使用final关键标记的方法不能被子类覆盖，是最终版本； class Person {  final function say() {} } class Student extends Person {  function say() {} } //会出现下面错误: Fatal error: Cannot override final method Person::say()

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final class Person {

}

class Student extends Person { } //The following error will appear: Fatal error: Class Student may not inherit from final class (Person) Methods marked with final key cannot be overridden by subclasses and are the final version; class Person { final function say() {}

} class Student extends Person

{

function say() {} } //The following error will appear: Fatal error: Cannot override final method Person::say() http://www.bkjia.com/PHPjc/629203.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/629203.htmlTechArticleStatic keyword is used to describe member properties and member methods in a class as static. The key to the application of final keyword is Words can only be used to define classes and methods. The keyword final cannot be used...