问题描述
如果我写的话，性能会有所提高
Is there any performance gain if I write
- (NSString *)helloStringWithName:(NSString *)name
static NSString *formatString = @"Hello %@!";
return [NSString stringWithFormat:formatString, name];
}
代替
- (NSString *)helloStringWithName:(NSString *)name
return [NSString stringWithFormat:@"Hello %@!", name];
}
??
如果我猜到了，我会认为每次运行代码时都会创建并自动发布后者，但是我猜编译器足够聪明，可以知道在这里做什么.
If I were to guess I would think that the latter one is created and autoreleased each time the code is running, but I guess the compiler is smart enough to know what do here..
推荐答案
Objective-C中的字符串文字是在编译时分配的，因此，您在性能上不会获得任何好处.
String literals in Objective-C are allocated at compile time, therefore you won't reasonably gain anything in performance.
考虑这个
NSString * str = @"Hello";
NSString * str2 = @"Hello";
NSLog(@"%p", str); // => 0x860358
NSLog(@"%p", str2); // => 0x860358
因此，如果您的意图是说类似这样的话:
Therefore if your intent is to say something like:
"嘿，编译器，格式字符串始终是相同的，因此不必费心分配多次"
答案将是:
"太好了，告诉我一些我不知道的东西"
注意
如果您仍然持怀疑态度，可以看一下程序集(请查看Dirk的答案)，但是让我在这里给您一些建议:不要为此类性能问题而烦恼.
If you are still skeptical you can take a look at the assembly (check out Dirk's answer), but let me give you a piece of advice here: don't bang your head against such performance issues.
使用NSString和高级抽象作为对象的开销通常在程序性能中绝对占主导地位，因此，即使每个字符串获得十亿分之一秒的时间，您也不会合理地注意到这一点.正如您理所当然地怀疑的那样，编译器已经足够聪明，可以处理这些细节.
The overhead of using NSString and a high level abstraction as objects in general is definitely predominant in your program performances, so even if you gain a nanosecond per string, you won't reasonably even notice that.And as you rightfully suspect, the compiler is already smart enough to take care of such details.
总结一下:让编译器完成工作，然后由您完成，即编写可读且可维护的代码.
To wrap it up: let the compiler do its job and you do yours, i.e. writing readable and maintainable code.
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