#ifdef vs #if - which is better/safer as a method

Question

This may be a matter of style, but there's a bit of a divide in our dev team and I wondered if anyone else had any ideas on the matter...

Basically, we have some debug print statements which we turn off during normal development. Personally I prefer to do the following:

//---- SomeSourceFile.cpp ----

#define DEBUG_ENABLED (0)

...

SomeFunction()
{
    int someVariable = 5;

#if(DEBUG_ENABLED)
    printf("Debugging: someVariable == %d", someVariable);
#endif
}

Some of the team prefer the following though:

// #define DEBUG_ENABLED

...

SomeFunction()
{
    int someVariable = 5;

#ifdef DEBUG_ENABLED
    printf("Debugging: someVariable == %d", someVariable);
#endif
}

...which of those methods sounds better to you and why? My feeling is that the first is safer because there is always something defined and there's no danger it could destroy other defines elsewhere.

Answer 1

It's a matter of style. But I recommend a more concise way of doing this:

#ifdef USE_DEBUG
#define debug_print printf
#else
#define debug_print
#endif

debug_print("i=%d\n", i);

You do this once, then always use debug_print() to either print or do nothing. (Yes, this will compile in both cases.) This way, your code won't be garbled with preprocessor directives.

If you get the warning "expression has no effect" and want to get rid of it, here's an alternative:

void dummy(const char*, ...)
{}

#ifdef USE_DEBUG
#define debug_print printf
#else
#define debug_print dummy
#endif

debug_print("i=%d\n", i);

Answer 2

Alternatively, you can declare a global constant, and use the C++ if, instead of the preprocessor #if. The compiler should optimize the unused branches away for you, and your code will be cleaner.

Here is what C++ Gotchas by Stephen C. Dewhurst says about using #if's.

Answer 3

#if and #define MY_MACRO (0)

Using #if means that you created a "define" macro, i.e., something that will be searched in the code to be replaced by "(0)". This is the "macro hell" I hate to see in C++, because it pollutes the code with potential code modifications.

For example:

#define MY_MACRO (0)

int doSomething(int p_iValue)
{
   return p_iValue + 1 ;
}

int main(int argc, char **argv)
{
   int MY_MACRO = 25 ;
   doSomething(MY_MACRO) ;

   return 0;
}

gives the following error on g++:

main.cpp|408|error: lvalue required as left operand of assignment|
||=== Build finished: 1 errors, 0 warnings ===|

Only one error.

Which means that your macro successfully interacted with your C++ code: The call to the function was successful. In this simple case, it is amusing. But my own experience with macros playing silently with my code is not full of joy and fullfilment, so...

#ifdef and #define MY_MACRO

Using #ifdef means you "define" something. Not that you give it a value. It is still polluting, but at least, it will be "replaced by nothing", and not seen by C++ code as lagitimate code statement. The same code above, with a simple define, it:

#define MY_MACRO

int doSomething(int p_iValue)
{
   return p_iValue + 1 ;
}

int main(int argc, char **argv)
{
   int MY_MACRO = 25 ;
   doSomething(MY_MACRO) ;

   return 0;
}

Gives the following warnings:

main.cpp||In function ‘int main(int, char**)’:|
main.cpp|406|error: expected unqualified-id before ‘=’ token|
main.cpp|399|error: too few arguments to function ‘int doSomething(int)’|
main.cpp|407|error: at this point in file|
||=== Build finished: 3 errors, 0 warnings ===|

So...

Conclusion

I'd rather live without macros in my code, but for multiple reasons (defining header guards, or debug macros), I can't.

But at least, I like to make them the least interactive possible with my legitimate C++ code. Which means using #define without value, using #ifdef and #ifndef (or even #if defined as suggested by Jim Buck), and most of all, giving them names so long and so alien no one in his/her right mind will use it "by chance", and that in no way it will affect legitimate C++ code.

Post Scriptum

Now, as I'm re-reading my post, I wonder if I should not try to find some value that won't ever ever be correct C++ to add to my define. Something like

#define MY_MACRO @@@@@@@@@@@@@@@@@@

that could be used with #ifdef and #ifndef, but not let code compile if used inside a function... I tried this successfully on g++, and it gave the error:

main.cpp|410|error: stray ‘@’ in program|

Interesting. :-)

Answer 4

#if gives you the option of setting it to 0 to turn off the functionality, while still detecting that the switch is there.
Personally I always #define DEBUG 1 so I can catch it with either an #if or #ifdef

Answer 5

The first seems clearer to me. It seems more natural make it a flag as compared to defined/not defined.

Answer 6

My initial reaction was #ifdef, of course, but I think #if actually has some significant advantages for this - here's why:

First, you can use DEBUG_ENABLED in preprocessor and compiled tests. Example - Often, I want longer timeouts when debug is enabled, so using #if, I can write this

  DoSomethingSlowWithTimeout(DEBUG_ENABLED? 5000 : 1000);

... instead of ...

#ifdef DEBUG_MODE
  DoSomethingSlowWithTimeout(5000);
#else
  DoSomethingSlowWithTimeout(1000);
#endif

Second, you're in a better position if you want to migrate from a #define to a global constant. #defines are usually frowned on by most C++ programmers.

And, Third, you say you've a divide in your team. My guess is this means different members have already adopted different approaches, and you need to standardise. Ruling that #if is the preferred choice means that code using #ifdef will compile -and run- even when DEBUG_ENABLED is false. And it's much easier to track down and remove debug output that is produced when it shouldn't be than vice-versa.

Oh, and a minor readability point. You should be able to use true/false rather than 0/1 in your #define, and because the value is a single lexical token, it's the one time you don't need parentheses around it.

#define DEBUG_ENABLED true

instead of

#define DEBUG_ENABLED (1)