C宏功能越来越复杂 [英] C Macros for more complex functions

问题描述

我见过的宏简单的例子，但想了解更复杂的东西，比方说用if语句和重新分配给变量。可以更复杂的前pressions这样在宏来完成？我有一个会运行数十亿次的函数，因此它会很高兴有preprocessor只是扔code在那里，而不是传递变量来回。

I've seen simple examples of macros, but wondering about something more complex, say with if statements and reassigning given variables. Can more complex expressions like this be done in a macro? I've got a function that'll be run billions of times, so it would nice to have the preprocessor just throw the code in there rather than passing variables back and forth.

说我有以下功能：

int foo(int a, int b, int c){  
if (a > 2)  
  c = a;  
if (b > 3)  
  c = b;  

return a + b + c;  
}

我怎样才能使这成为宏？

How can I make this into a macro?

推荐答案

注意宏不一样的函数调用 - 宏实际上取代C源$ C ​​$ c其中它们用于调用和返回，而不是 - 所以可以有很多意想不到的行为！您的例子可以做成一个宏像这样：

Note that macros aren't the same as function calls - macros actually replace the C source code where they are used instead of calling and returning - so there can be a lot of unexpected behavior! Your example could be made into a macro like so:

#define FOO(a,b,c)((a)+(b)+(((b)>3)?(b):((a)>2)?(a):(c)))

但同样，也有<一个href=\"http://www.brainbell.com/tutors/c/Advice_and_Warnings_for_C/Macros_and_Miscellaneous_Pitfalls.html\"相对=使用复杂的宏时，nofollow的>很多陷阱的，如不清楚运营商precedence和重复自动递增/递减操作符。

But again, there are many pitfalls when using complex macros, such as unclear operator precedence and duplicated auto-increment/decrement operators.

这也可能是最好的倾听其他答案的建议，使用替代战略不是复杂的宏。

It's probably best to heed the advice of other answers and use alternative strategies than complex macros.

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