#define, #ifdef #undef #endif

问题

I have the following code

#define PROC_ADD

void main(void)
{
    while(1)
    {
#ifdef PROC_ADD
// Do this code here then undefined it to run the code in the else
// processing work
#undef PROC_ADD
#else
// now that PROC_ADD has been undefined run this code
// processing work
#endif
    }
}

However, it will run the code. But it won't run the code in the else after the PROC_ADD has been undefined.

I think the reason could be that you can only define and undefine at compile time, and not at run-time. However, I am not really sure.

回答1:

What you are doing is the build time equivalent of:

int x = 1;

int main()
{
    if (x)
    {
        ...
        x = 0;
    }
    else
    {
        ...
    }
}

ifdef, etc. happen at build time, but for your example, that's not an issue. Once you evaluate the if (either the runtime or build-time form), the decision about which branch to take it made. Changing something after the decision has been made does not change that decision.

回答2:

The ifdef condition is evaluated when the preprocessor gets to it. When you undef PROC_ADD inside the ifdef'd code, the preprocessor has already decided which section of code to include and which to ignore.

Furthermore, yes: ifdef, undef, etc are processed at pre-processing time -- the compiler never even sees these so-called directives. This of course means run-time code never sees these directives either.

Edit: The preprocessor works by taking a single pass through the text file. The preprocessor does not even care that your text file happens to contain C code! It has zero knowledge that your ifdefs and elses and whatnot happen to be inside a while loop.

回答3:

#defines only work during preprocessing. So

#define PROC_ADD 
void main(void) 
{
#ifdef PROC_ADD 
// Do this code here then undefined it to run the code in the else 
// processing work 
#undef PROC_ADD 
#else 
// now that PROC_ADD has been undefined run this code 
// processing work 
#endif 
}

will be processed the following way: since PROC_ADDR is defined the preprocessor will completely exclude the #else branch and then execute #undef, so the #else branch code never survives preprocessing and never reaches the compiler.

回答4:

In just about every programming language or syntax, once execution has entered one branch of a conditional (in this case, the conditional being #ifdef, even if the condition changes during execution of the branch, other branches will never be executed.

I'm sure you wouldn't expect this to print "Hello", would you?

if (i == 1)
    i = 0;
else
    printf("Hello\n");

Basically what you're saying is that the code under the else branch should always execute, then just take it out of a branch, and put it directly in the code.

Both the compiler and the execution only make one pass through conditionals, once a match has been found they look no further.

回答5:

So you want the code represented by the second comment section to always run? Why not just do

#ifdef PROC_ADD
// Do the stuff to be done if PROC_ADD is defined
#undef PROC_ADD
#endif
// Do the stuff to always be done

edit

OK - if you want run-time behaviour changes you must use run-time constructs (such as a variable to serve as a flag). As we are all saying ;), pre-processor directives are evaluated once only, at compile time.

回答6:

Think about it this way: else portion of the following code is not executed even though x has been set to false in the if section.

The condition is checked in the if(x) line itself - once it enters that block, it doesn't recalculate each of the subsequent else sections - compiler has already made a decision on that.

bool x = true;
if(x)
{
  //do something
  x = false;
}
else
{
  //else code
}

来源：https://stackoverflow.com/questions/1802107/define-ifdef-undef-endif

标签

visual-studio-2008

c-preprocessor

preprocessor-directive