"#ifdef" inside a macro

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#ifdef inside #define

How do I use the character "#" successfully inside a Macro? It screams when I do something like that:

#define DO(WHAT)        \
#ifdef DEBUG        \                           
  MyObj->WHAT()         \       
#endif              \

score 72 · Accepted Answer · edited May 23 '17 at 12:34

72

You can't do that. You have to do something like this:

#ifdef DEBUG
#define DO(WHAT) MyObj->WHAT()
#else
#define DO(WHAT) do { } while(0)
#endif

The do { } while(0) avoids empty statements. See this question, for example.

edited May 23 '17 at 12:34

Community

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1

answered Aug 30 '11 at 16:29

Roger Lipscombe

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So with an empty statement I could run into trouble on some c++ compilers? My app is cross platform, OSX, Windows and Ubuntu. Is this the safe thing to do? – JasonGenX Aug 30 '11 at 16:35
2

+1: The `do {} while(0)` construct is something I forgot to mention in my answer. – Oliver Charlesworth Aug 30 '11 at 16:40
@Ron: This actually lets you write : `if(x) DO(m); else DO(n);`. while @Oli's solution will give compilation error. – Nawaz Aug 30 '11 at 16:40
3

This is one reason to use inline functions rather than macros! – Bo Persson Aug 30 '11 at 16:43
1

@Bo Persson: What solution do you suggest using inline functions? Please post. – Nawaz Aug 30 '11 at 16:46
DO(m) and DO(n) can easily be function calls. Why bother with tricky macros? – Bo Persson Aug 30 '11 at 16:49
1

Well, in this case, `MyObj` would have to be passed as an argument to those functions, so it's not a drop in replacement. Moreover, you'd then have to mess about with pointer-to-member parameters, which could get ugly quite quickly. That said, I don't actually like this use of macros to hide what's going on, but the question was asked... – Roger Lipscombe Aug 31 '11 at 08:34
what about simply `#define DO(WHAT) ;` in the #else branch? – Kristóf Szalay Jun 09 '17 at 13:10

score 13 · Answer 2 · answered Aug 30 '11 at 16:27

13

It screams because you can't do that.

I suggest the following as an alternative:

#ifdef DEBUG
#define DO(WHAT) MyObj->WHAT()
#else
#define DO(WHAT)
#endif

answered Aug 30 '11 at 16:27

Oliver Charlesworth

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score 5 · Answer 3 · answered Aug 30 '11 at 16:33

It seems that what you want to do can be achieved like this, without running into any problems:

#ifdef DEBUG
#    define DO(WHAT) MyObj->WHAT()
#else
#    define DO(WHAT) while(false)
#endif

Btw, better use the NDEBUG macro, unless you have a more specific reason not to. NDEBUG is more widely used as a macro that means no-debugging. For example the standard assert macro can be disabled by defining NDEBUG. Your code would become:

#ifndef NDEBUG
#    define DO(WHAT) MyObj->WHAT()
#else
#    define DO(WHAT) while(false)
#endif

You should emphasize that since `NDEBUG` means no-debugging, the `#ifdef` becomes `#ifndef`. — ilpelle, Jan 19 '18 at 16:05

score 2 · Answer 4 · answered Aug 30 '11 at 16:30

2

You can do the same thing like this:

#ifdef DEBUG
#define DO(WHAT) MyObj->WHAT()
#else
#define DO(WHAT)
#endif

answered Aug 30 '11 at 16:30

antlersoft

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See other for correct way to avoid empty statements... – antlersoft Aug 30 '11 at 16:31

score 2 · Answer 5 · answered Aug 30 '11 at 16:31

2

How about:

#ifdef DEBUG
#define DO(WHAT) MyObj->WHAT()
#else
#define DO(WHAT)
#endif

answered Aug 30 '11 at 16:31

Miguel Grinberg

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"#ifdef" inside a macro

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