使用。或[]访问对象属性 - 有什么区别？ [英] Using . or [ ] to access Object properties - what's the difference?

问题描述

之间的code（i）及（ii）中写的？

What is the difference between the code (i) and (ii) written below ?

（一）

var obj:Object = new Object();
obj.attribute = value ;

（ⅱ）

var obj:Object = new Object();
obj["key"] = value;

是否有任何运行时间的影响，如果我这样写：

Are there any run-time implications if I write this :

var obj:Object = new Object();
obj.somekey = value1 ;
obj["someKey"] = value2 ;

请解释一下。

推荐答案

所不同的是，在查找机制：如果您使用点语法，编译器将在编译时知道您访问该对象的属性。如果您使用括号语法，该财产的实际查找是在运行，而且将有更多的类型检查 - 毕竟，你可以动态地组合密钥字符串，其值可以改变，或者你甚至可以调用一个函数，而不是一个变量，等等。

The difference is in the lookup mechanism: If you use the dot syntax, the compiler will know at compile time that you are accessing a property of that object. If you use the bracket syntax, the actual lookup of the property is done at runtime, and there will have to be more type checking - after all, you could compose the key string dynamically, the value could change, or you could even be calling a function instead of a variable, etc.

其结果是在性能上显著差异：支架语法如下只要点语法来执行的三倍左右

The result is a significant difference in performance: Bracket syntax takes about three times as long to execute as dot syntax.

这里有一个小速度测试来说明我的观点：

Here's a little speed test to illustrate my point:

var start : int = getTimer();

var obj:Object = { something : "something" };

for (var i : int = 0; i < 100000000; i++) {
    var n:String = obj.something;
}

trace ("Time with dot syntax: "+(getTimer() - start));

start = getTimer();

for (i = 0; i < 100000000; i++) {
    var o:String = obj["something"];
}

trace ("Time with bracket syntax: "+(getTimer() - start));

如果这两个是相同的，除了表示法，它们应该采取的时间完全一致量。但正如你所看到的，这种情况并非如此。在我的机器：

If the two were the same, except for notation, they should take exactly the same amount of time. But as you can see, this is not the case. On my machine:

Time with dot syntax:      3937
Time with bracket syntax:  9857

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