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php命名空间的设计思想和缺点

May 17, 2018 pm 12:01 PM
php Namespaces Thought shortcoming design

相比C#等语言,你可以在php函数里面随意定义变量并赋值,而不用担心覆盖了全局变量,或者类变量;你也可以随意的定义类变量,而不用担心会和函数名冲突,因为变量前面都有个$。

php的命名空间和全局变量、局部变量的设计思想一样,都是为了尽可能的减少命名冲突。php的命名空间可以和文件系统类似,有相对命名空间和绝对命名空间。

定义命名空间

必须是第一个语句,前面不能有任何语句,注释可以。 
必须是绝对命名空间。

namespace bookrpg\config;
//或者
namespace bookrpg\config
{
}
namespace \bookrpg\config;  //错误,不能有前导\12345678910
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导入命名空间

必须是绝对命名空间。

use bookrpg\config\ClassName;
use \bookrpg\config\ClassName; //可以有前导\,但不建议加
use bookrpg\config\ClassName as NewClassName;
use Exception;
use \Exception; //可以有前导\,但不建议加123456
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使用命名空间

使用绝对命名空间

$cls = new \bookrpg\config\ClassName();
class MyException extends \Exception
{
}123456
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使用相对命名空间

namespace bookrpg
{
    //等同于new \bookrpg\config\ClassName();
    $cls = new config\ClassName(); 
    //错误,php会认为是\bookrpg\Exception,而不是全局类Exception
    $ex = new Exception(); 
}123456789
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怪异的地方

C++、Java、C#都有相似的命名空间概念,而php的命名空间不跟随大流,有自己的一套独立的思想,对于先入为主的人来来说就显得怪异和令人迷惑。

命名空间中居然有反斜杠? 
点号(.)已经被用作字符串连接符了,又要区分绝对和相对命名空间,new .a.b.c()太不美观,所以选择反斜杠吧。

一旦使用了命名空间,使用全局类也要加上“\”或者use一下,累不累呀? 
没办法,为了严格类比文件系统的概念,也为了随便定义类名,而不用担心类名冲突。

既然是严格类比文件系统,为什么定义和导入命名空间的时候,前面不加\呢? 
只能说,php的设计者不是完美主义者。

但php的命名空间确实缺乏逻辑完美性:

前导\的使用不统一,同样是绝对命名空间,定义的时候不允许用,导入的时候却允许用。

即使是全局类,在非全局命名空间下,也必须使用前导\,而全局函数和变量又不受此限制。

使用建议

定义和导入命名空间的时候,统一不加前导\

所有的类都使用use导入,包括全局类,只命名空间类,有冲突的就as一下

禁止直接使用相对命名空间

理解了php命名空间的设计思想,并避免其设计不完美的地方,这样看来php的命名空间也是很容易接受的。

上面是我整理给大家的php命名空间的设计思想和缺点,希望今后会对大家有帮助。

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