Community
Learn
Tools Library
AI Tools
Leisure
search
English
Home > php教程 > php手册 > Keywords in Part 4 of the Object-Oriented Series for Front-end Learning PHP

Keywords in Part 4 of the Object-Oriented Series for Front-end Learning PHP

WBOY
Release： 2016-11-16 10:24:02
Original
1186 people have browsed it
×
目录
[1]public [2]protected [3]private[4]final[5]static[6]const[7]this[8]self[9]parent

前面的话

　　php实现面向对象的一个显著特征是大量使用关键字，本文将详细介绍关键字

 

public

　　public表示公有，它具有最大的访问权限，被定义为公有的类成员可以在任何地方被访问

　　如果属性用 var 定义，则被视为公有，如果方法没有设置关键字，则该方法默认为公有

<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> demo{
    </span><span style="color: #0000ff;">public</span> <span style="color: #800080;">$public</span> = 1<span style="color: #000000;">;
    </span><span style="color: #0000ff;">function</span> test(<span style="color: #800080;">$var</span><span style="color: #000000;">){
        </span><span style="color: #0000ff;">echo</span> "{<span style="color: #800080;">$var</span>}000"<span style="color: #000000;">;
    }
}
</span><span style="color: #800080;">$d1</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> demo;
</span><span style="color: #800080;">$d1</span>->test(<span style="color: #800080;">$d1</span>-><span style="color: #0000ff;">public</span>);<span style="color: #008000;">//</span><span style="color: #008000;">1000</span>
?>
Copy after login

 

protected

　　protected表示受保护的，被定义为受保护的类成员则可以被其自身以及其子类和父类访问

<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> demo{
    </span><span style="color: #0000ff;">protected</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> fn(){
        </span><span style="color: #0000ff;">echo</span> '111'<span style="color: #000000;">;
    }
}
</span><span style="color: #0000ff;">class</span> demo1 <span style="color: #0000ff;">extends</span><span style="color: #000000;"> demo{
    </span><span style="color: #0000ff;">function</span><span style="color: #000000;"> test(){
        parent</span>::<span style="color: #000000;">fn();
    }    
}
</span><span style="color: #800080;">$d1</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> demo1;
</span><span style="color: #800080;">$d1</span>->test();<span style="color: #008000;">//</span><span style="color: #008000;">111</span>
?>
Copy after login

 

private

　　private表示私有的，被定义为私有的类成员则只能被其定义所在的类访问

<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> demo{
    </span><span style="color: #0000ff;">private</span> <span style="color: #800080;">$private</span> = 1<span style="color: #000000;">;
    </span><span style="color: #0000ff;">function</span><span style="color: #000000;"> test(){
        </span><span style="color: #0000ff;">echo</span>(<span style="color: #800080;">$this</span>-><span style="color: #0000ff;">private</span><span style="color: #000000;">);
    }
}
</span><span style="color: #800080;">$d1</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> demo;
</span><span style="color: #800080;">$d1</span>->test();<span style="color: #008000;">//</span><span style="color: #008000;">1</span>
?>
Copy after login

 

final

　　PHP5新增了final关键字，它只能用来修饰类和方法，不能使用final这个关键字来修饰成员属性，因为final是常量的意思，我们在PHP里定义常量使用的是define()函数和const关键字，所以不能使用final来定义成员属性

　　如果父类中的方法被声明为final，则子类无法覆盖该方法。如果一个类被声明为 final，则不能被继承

<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> BaseClass {
   </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> test() {
       </span><span style="color: #0000ff;">echo</span> "BaseClass::test() called\n"<span style="color: #000000;">;
   }
   </span><span style="color: #0000ff;">final</span> <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> moreTesting() {
       </span><span style="color: #0000ff;">echo</span> "BaseClass::moreTesting() called\n"<span style="color: #000000;">;
   }
}
</span><span style="color: #0000ff;">class</span> ChildClass <span style="color: #0000ff;">extends</span><span style="color: #000000;"> BaseClass {
   </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> moreTesting() {
       </span><span style="color: #0000ff;">echo</span> "ChildClass::moreTesting() called\n"<span style="color: #000000;">;
   }
}
</span><span style="color: #008000;">//</span><span style="color: #008000;"> Results in Fatal error: Cannot override final method BaseClass::moreTesting()</span>
?>
Copy after login

 

static

　　static关键字表示静态的意思，用于修饰类的成员属性和成员方法（即为静态属性和静态方法）

　　类中的静态属性和静态方法不用实例化(new)就可以直接使用类名访问

　　[注意]静态属性不能通过一个类已实例化的对象来访问，但静态方法可以

　　由于静态方法不需要通过对象即可调用，所以伪变量 $this 在静态方法中不可用，静态属性不可以由对象通过 -> 操作符来访问

　　用静态方式调用一个非静态方法会导致一个 E_STRICT 级别的错误

　　就像其它所有的 PHP 静态变量一样，静态属性只能被初始化为文字或常量，不能使用表达式。所以可以把静态属性初始化为整数或数组，但不能初始化为另一个变量或函数返回值，也不能指向一个对象

<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> Foo
{
    </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #800080;">$my_static</span> = 'foo'<span style="color: #000000;">;
    </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> staticValue() {
        </span><span style="color: #0000ff;">return</span> self::<span style="color: #800080;">$my_static</span><span style="color: #000000;">;
    }
}
</span><span style="color: #0000ff;">class</span> Bar <span style="color: #0000ff;">extends</span><span style="color: #000000;"> Foo
{
    </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> fooStatic() {
        </span><span style="color: #0000ff;">return</span> parent::<span style="color: #800080;">$my_static</span><span style="color: #000000;">;
    }
}
</span><span style="color: #0000ff;">print</span> Foo::<span style="color: #800080;">$my_static</span> . "\n";<span style="color: #008000;">//</span><span style="color: #008000;">'foo'</span>
<span style="color: #800080;">$foo</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> Foo();
</span><span style="color: #0000ff;">print</span> <span style="color: #800080;">$foo</span>->staticValue() . "\n";<span style="color: #008000;">//</span><span style="color: #008000;">'foo'</span>
<span style="color: #0000ff;">print</span> <span style="color: #800080;">$foo</span>::<span style="color: #800080;">$my_static</span> . "\n";<span style="color: #008000;">//</span><span style="color: #008000;">'foo'</span>
<span style="color: #0000ff;">print</span> <span style="color: #800080;">$foo</span>->my_static . "\n"; <span style="color: #008000;">//</span><span style="color: #008000;">报错 </span>
?>
Copy after login

 

const

　　可以把在类中始终保持不变的值定义为常量。在定义和使用常量的时候不需要使用$符号，而是使用const

　　常量的值必须是一个定值，不能是变量，类属性，数学运算的结果或函数调用

<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> MyClass
{
    </span><span style="color: #0000ff;">const</span> <span style="color: #008080;">constant</span> = 'constant value'<span style="color: #000000;">;
    </span><span style="color: #0000ff;">function</span><span style="color: #000000;"> showConstant() {
        </span><span style="color: #0000ff;">echo</span>  self::<span style="color: #008080;">constant</span> . "\n"<span style="color: #000000;">;
    }
}
</span><span style="color: #0000ff;">echo</span> MyClass::<span style="color: #008080;">constant</span> . "\n";<span style="color: #008000;">//</span><span style="color: #008000;">'constant value'</span>
<span style="color: #800080;">$classname</span> = "MyClass"<span style="color: #000000;">;
</span><span style="color: #0000ff;">echo</span> <span style="color: #800080;">$classname</span>::<span style="color: #008080;">constant</span> . "\n"; <span style="color: #008000;">//</span><span style="color: #008000;">'constant value'</span>
<span style="color: #800080;">$class</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> MyClass();
</span><span style="color: #800080;">$class</span>->showConstant();<span style="color: #008000;">//</span><span style="color: #008000;">'constant value'</span>
<span style="color: #0000ff;">echo</span> <span style="color: #800080;">$class</span>::<span style="color: #008080;">constant</span>."\n";<span style="color: #008000;">//</span><span style="color: #008000;">'constant value'</span>
?>
Copy after login

 

this

　　当一个方法在类定义内部被调用时，有一个可用的伪变量this，特殊对象的引用this就是在对象内部的成员方法中，代表本对象的一个引用，但只能在对象的成员方法中使用，不管是在对象内部使用$this访问自己对象内部成员。还是在对象外部通过对象的引用名称访问对象中的成员，都需要使用特殊的运算符“->”来完成访问

　　[注意]this在静态方法中不可用

<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> A
{
    </span><span style="color: #0000ff;">function</span><span style="color: #000000;"> foo()
    {
        </span><span style="color: #0000ff;">if</span> (<span style="color: #0000ff;">isset</span>(<span style="color: #800080;">$this</span><span style="color: #000000;">)) {
            </span><span style="color: #0000ff;">echo</span> '$this is defined ('<span style="color: #000000;">;
            </span><span style="color: #0000ff;">echo</span> <span style="color: #008080;">get_class</span>(<span style="color: #800080;">$this</span><span style="color: #000000;">);
            </span><span style="color: #0000ff;">echo</span> ")\n"<span style="color: #000000;">;
        } </span><span style="color: #0000ff;">else</span><span style="color: #000000;"> {
            </span><span style="color: #0000ff;">echo</span> "\$this is not defined.\n"<span style="color: #000000;">;
        }
    }
}
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> B
{
    </span><span style="color: #0000ff;">function</span><span style="color: #000000;"> bar()
    {
        </span><span style="color: #008000;">//</span><span style="color: #008000;"> Note: the next line will issue a warning if E_STRICT is enabled.</span>
        A::<span style="color: #000000;">foo();
    }
}

</span><span style="color: #800080;">$a</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> A();
</span><span style="color: #800080;">$a</span>->foo();<span style="color: #008000;">//</span><span style="color: #008000;">$this is defined (A) </span>
A::foo();<span style="color: #008000;">//</span><span style="color: #008000;">$this is not defined. </span>
<span style="color: #800080;">$b</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> B();
</span><span style="color: #800080;">$b</span>->bar();<span style="color: #008000;">//</span><span style="color: #008000;">$this is defined (B) </span>
B::bar();<span style="color: #008000;">//</span><span style="color: #008000;">$this is not defined.</span>
?>
Copy after login

 

self

　　在类的方法中，不能用this来引用静态变量或静态方法，而需要用self来引用

<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> MyClass
{
    </span><span style="color: #0000ff;">const</span> <span style="color: #008080;">constant</span> = 'constant value'<span style="color: #000000;">;
    </span><span style="color: #0000ff;">static</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> showConstant() {
        </span><span style="color: #0000ff;">echo</span>  self::<span style="color: #008080;">constant</span> . "\n"<span style="color: #000000;">;
    }
}
</span><span style="color: #800080;">$var</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> MyClass;
</span><span style="color: #0000ff;">echo</span> <span style="color: #800080;">$var</span>->showConstant();<span style="color: #008000;">//</span><span style="color: #008000;">constant value</span>
?>
Copy after login

 

parent

　　parent用于调用父类中定义的成员方法或常量

<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> MyClass{
    </span><span style="color: #0000ff;">function</span><span style="color: #000000;"> fn(){
        </span><span style="color: #0000ff;">echo</span>('111'<span style="color: #000000;">);
    }
    </span><span style="color: #0000ff;">const</span> A = 'a'<span style="color: #000000;">;
}
</span><span style="color: #0000ff;">class</span> Class1 <span style="color: #0000ff;">extends</span><span style="color: #000000;"> MyClass{
    </span><span style="color: #0000ff;">function</span><span style="color: #000000;"> test(){
        </span><span style="color: #0000ff;">echo</span> parent::fn().parent::<span style="color: #000000;">A;
    }
}
</span><span style="color: #800080;">$var</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> Class1;
</span><span style="color: #800080;">$var</span>->test();<span style="color: #008000;">//</span><span style="color: #008000;">111a</span>
?>
Copy after login
Related labels：
前端学PHP之面向对象系列第四篇——关键字
source：php.cn
Previous article：OpenCart How to Install vQmod Tutorial Next article：How to prevent SQL injection in php
Statement of this Website
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn
Latest Articles by Author
Latest Issues
function_exists() cannot determine the custom function Function test () {return true;} if (function_exists ('test')) {echo "test is function...
From 2024-04-29 11:01:01
0
3
2389
How to display the mobile version of Google Chrome Hello teacher, how can I change Google Chrome into a mobile version?
From 2024-04-23 00:22:19
0
11
2518
The child window operates the parent window, but the output does not respond. The first two sentences are executable, but the last sentence cannot be implemented.
From 2024-04-19 15:37:47
0
1
2138
There is no output in the parent window document.onclick = function(){ window.opener.document.write('I am the output of the child ...
From 2024-04-18 23:52:34
0
1
2014
Where is the courseware about CSS mind mapping? Courseware
From 2024-04-16 10:10:18
0
0
2092
Related Topics
More>
Popular Recommendations
Popular Tutorials
More>
Related Tutorials
Popular Recommendations
Latest courses
Latest Downloads
More>
Web Effects
Website Source Code
Website Materials
Front End Template