#ifdef vs #if - which is better/safer as a method for enabling/disabling compilation of particular sections of code?

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)

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They're both hideous. Instead, do this:

#ifdef DEBUG
#define D(x) do { x } while(0)
#else
#define D(x) do { } while(0)
#endif

Then whenever you need debug code, put it inside D();. And your program isn't polluted with hideous mazes of #ifdef.

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#ifdef just checks if a token is defined, given

#define FOO 0

then

#ifdef FOO // is true
#if FOO // is false, because it evaluates to "#if 0"

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We have had this same problem across multiple files and there is always the problem with people forgetting to include a "features flag" file (With a codebase of > 41,000 files it is easy to do).

If you had feature.h:

#ifndef FEATURE_H
#define FEATURE_H

// turn on cool new feature
#define COOL_FEATURE 1

#endif // FEATURE_H

But then You forgot to include the header file in file.cpp:

#if COOL_FEATURE
    // definitely awesome stuff here...
#endif

Then you have a problem, the compiler interprets COOL_FEATURE being undefined as a "false" in this case and fails to include the code. Yes gcc does support a flag that causes a error for undefined macros... but most 3rd party code either defines or does not define features so this would not be that portable.

We have adopted a portable way of correcting for this case as well as testing for a feature's state: function macros.

if you changed the above feature.h to:

#ifndef FEATURE_H
#define FEATURE_H

// turn on cool new feature
#define COOL_FEATURE() 1

#endif // FEATURE_H

But then you again forgot to include the header file in file.cpp:

#if COOL_FEATURE()
    // definitely awseome stuff here...
#endif

The preprocessor would have errored out because of the use of an undefined function macro.

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For the purposes of performing conditional compilation, #if and #ifdef are almost the same, but not quite. If your conditional compilation depends on two symbols then #ifdef will not work as well. For example, suppose you have two conditional compilation symbols, PRO_VERSION and TRIAL_VERSION, you might have something like this:

#if defined(PRO_VERSION) && !defined(TRIAL_VERSION)
...
#else
...
#endif

Using #ifdef the above becomes much more complicated, especially getting the #else part to work.

I work on code that uses conditional compilation extensively and we have a mixture of #if & #ifdef. We tend to use #ifdef/#ifndef for the simple case and #if whenever two or more symbols are being evaluation.

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I think it's entirely a question of style. Neither really has an obvious advantage over the other.

Consistency is more important than either particular choice, so I'd recommend that you get together with your team and pick one style, and stick to it.

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I myself prefer:

#if defined(DEBUG_ENABLED)

Since it makes it easier to create code that looks for the opposite condition much easier to spot:

#if !defined(DEBUG_ENABLED)

vs.

#ifndef(DEBUG_ENABLED)

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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);

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#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

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#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. :-)

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That is not a matter of style at all. Also the question is unfortunately wrong. You cannot compare these preprocessor directives in the sense of better or safer.

#ifdef macro

means "if macro is defined" or "if macro exists". The value of macro does not matter here. It can be whatever.

#if macro

if always compare to a value. In the above example it is the standard implicit comparison:

#if macro !=0

example for the usage of #if

#if CFLAG_EDITION == 0
    return EDITION_FREE;
#elif CFLAG_EDITION == 1
    return EDITION_BASIC;
#else
    return EDITION_PRO;
#endif

you now can either put the definition of CFLAG_EDITION either in your code

#define CFLAG_EDITION 1 

or you can set the macro as compiler flag. Also see here.

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

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Both are exactly equivalent. In idiomatic use, #ifdef is used just to check for definedness (and what I'd use in your example), whereas #if is used in more complex expressions, such as #if defined(A) && !defined(B).

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There is a difference in case of different way to specify a conditional define to the driver:

diff <( echo | g++ -DA= -dM -E - ) <( echo | g++ -DA -dM -E - )

output:

344c344
< #define A 
---
> #define A 1

This means, that -DA is synonym for -DA=1 and if value is omitted, then it may lead to problems in case of #if A usage.

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A little OT, but turning on/off logging with the preprocessor is definitely sub-optimal in C++. There are nice logging tools like Apache's log4cxx which are open-source and don't restrict how you distribute your application. They also allow you to change logging levels without recompilation, have very low overhead if you turn logging off, and give you the chance to turn logging off completely in production.

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I used to use #ifdef, but when I switched to Doxygen for documentation, I found that commented-out macros cannot be documented (or, at least, Doxygen produces a warning). This means I cannot document the feature-switch macros that are not currently enabled.

Although it is possible to define the macros only for Doxygen, this means that the macros in the non-active portions of the code will be documented, too. I personally want to show the feature switches and otherwise only document what is currently selected. Furthermore, it makes the code quite messy if there are many macros that have to be defined only when Doxygen processes the file.

Therefore, in this case, it is better to always define the macros and use #if.

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I've always used #ifdef and compiler flags to define it...

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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.

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I like #define DEBUG_ENABLED (0) when you might want multiple levels of debug. For example:

#define DEBUG_RELEASE (0)
#define DEBUG_ERROR (1)
#define DEBUG_WARN (2)
#define DEBUG_MEM (3)
#ifndef DEBUG_LEVEL
#define DEBUG_LEVEL (DEBUG_RELEASE)
#endif
//...

//now not only
#if (DEBUG_LEVEL)
//...
#endif

//but also
#if (DEBUG_LEVEL >= DEBUG_MEM)
LOG("malloc'd %d bytes at %s:%d\n", size, __FILE__, __LINE__);
#endif

Makes it easier to debug memory leaks, without having all those log lines in your way of debugging other things.

Also the #ifndef around the define makes it easier to pick a specific debug level at the commandline:

make -DDEBUG_LEVEL=2
cmake -DDEBUG_LEVEL=2
etc

If not for this, I would give advantage to #ifdef because the compiler/make flag would be overridden by the one in the file. So you don't have to worry about changing back the header before doing the commit.

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As with many things, the answer depends. #ifdef is great for things that are guaranteed to be defined or not defined in a particular unit. Include guards for example. If the include file is present at least once, the symbol is guaranteed to be defined, otherwise not.

However, some things don't have that guarantee. Think about the symbol HAS_FEATURE_X. How many states exist?

Undefined Defined Defined with a value (say 0 or 1).

So, if you're writing code, especially shared code, where some may #define HAS_FEATURE_X 0 to mean feature X isn't present and others may just not define it, you need to handle all those cases.

#if !defined(HAS_FEATURE_X) || HAS_FEATURE_X == 1

Using just an #ifdef could allow for a subtle error where something is switched in (or out) unexpectedly because someone or some team has a convention of defining unused things to 0. In some ways, I like this #if approach because it means the programmer actively made a decision. Leaving something undefined is passive and from an external point of view, it can sometimes be unclear whether that was intentional or an oversight.

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