Objective-C中的消息发送成本 [英] Cost of message dispatch in Objective-C

问题描述

我很想知道在各种情况下Objective-C中的消息发送成本。特别是我想指导我选择的程序设计，所以我不想通过避免消息发送来过早优化，因为它们可以实现更好的设计。

I'm curious to know about the cost of message dispatch in Objective-C in various situations. Particularly I want to guide my choice of program design so I'm not tempted to prematurely optimize by avoiding message dispatches when they would make for a better design.

我当前的项目是我有一个带有实例变量的类：offsetX和offsetY。我经常想要绝对偏移量，目前我在这个地方都有这行代码： -

A case in my current project is that I have a class with instance variables: offsetX and offsetY. I often want the absolute offset and at the moment I have this line of code all over the place:-

int absOffset = ((offsetX < 0.0) ? -offsetX : offsetX) + 
                 ((offsetY < 0.0) ? -offsetY : offsetY);

现在如果这是C ++，我会创建一个返回absOffset值的内联函数。即使在Java / C＃中，我也可以将这样的函数定义为final / sealed，并且非常确定它将被内联。

Now if this was C++ I would create an inline function that returned the value for absOffset. Even in Java/C# I could define such a function as final/sealed and be pretty sure it would be inlined.

目标-C将是： -

The objective-C would be:-

-(int)absOffset {
    return ((offsetX < 0.0) ? -offsetX : offsetX) + 
            ((offsetY < 0.0) ? -offsetY : offsetY);
}

我会这样称呼： -

and I would call it like so:-

int ao = [self absOffset];

现在，编译器能够内联吗？我假设它至少能够将它修复为直接函数调用并避免动态消息调度（我假设）objective-c必须使用因为它的类型系统。

Now, is the compiler able to inline that? I assume it is able at least fix it to a direct function call and avoid the dynamic message dispatch that (I assume) objective-c must use because of it's type system.

另外，一般来说，客观C中的消息发送费用是多少？通过'id'和指向具体类的指针调用它有什么不同？

Also, in general, how much does message dispatch cost in objective-C? Does it differ when calling through an 'id' versus a pointer to a concrete class?

推荐答案

目标C消息非常快。速度可比到C ++虚拟方法调用，虽然不是那么快。避免消息传递肯定是过早优化。您可能不希望在内部循环中执行大量操作，但您选择的算法和其他因素将对代码的速度有更大的影响。如果它太慢，请使用分析器并从那里开始。

Objective C messages are very fast. The speed is comparable to C++ virtual method calls, although not quite as fast. Avoiding message passing is definitely premature optimization. You might not want to do a lot of it in an inner loop, but the algorithms you choose and other factors will have a much bigger factor on how fast your code is. If it is too slow, use a profiler and go from there.

这篇关于Objective-C中的消息发送成本的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！