“#ifdef” inside a macro [duplicate]

问题

Possible Duplicate:
#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              \

回答1:

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.

回答2:

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

回答3:

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

回答4:

You can do the same thing like this:

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

回答5:

How about:

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

来源：https://stackoverflow.com/questions/7246512/ifdef-inside-a-macro

标签

c++

c-preprocessor