C宏用于更复杂的功能

Question

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.

Say I have the following function:

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

return a + b + c;  
}

How can I make this into a macro?

Answer 1

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)))

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

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

Answer 2

Easy - don't use macros - just use inline functions:

__inline int foo(int a, int b, int c)
{  
    if (a > 2)  
        c = a;  
    if (b > 3)  
        c = b;  
    return a + b + c;  
}

Answer 3

Whilst you could replace your function with a macro, in all likelihood, it will make no difference to the performance. Any reasonable compiler will have moved the function inline if it it thinks it will help.

Answer 4

If it looks like it should be a function, then make it a function.

Compiler technology has become good enough over time, so you don't have to worry about stuff like this. Have trust in your compiler and you should be fine.