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c++cmacrosc-preprocessorifndef

Do I check the existence of a macro function with or without the parens?

Before I define a macro function, I can check that it doesn't already exist
(this avoids overwriting a previous definition).

I can implement the check and definition like this: 

#ifndef MACRO(X)
#define MACRO(X) FUNCTION(X)
#endif

Or like this:

#ifndef MACRO
#define MACRO(X) FUNCTION(X)
#endif

Both appear to work when the function is already defined.
So, which is correct? Which is preferred?

Solution

  • Without.

    The standard specifies that #ifndef is equivalent to #if !defined, and that the argument to defined must be a (possibly parenthesized) identifier. You can't have parens in an identifier, so defined MACRO(X) is not an allowed form. This use of defined causes undefined behaviour, so it is not portable.