#define TRACE(...) doesn't work in C++

Question

I have the following preprocessor divective:

#ifndef NDEBUG
#define TRACE printf
#else
#define TRACE(...)
#endif

and example of usage is:

TRACE("TRACE: some parameter = %i\n", param);

In C all works perfectly well when I build both debug and release versions, but in C++ compiler emits the following:

warning: invalid character in macro parameter name

error: badly punctuated parameter list in `#define'

and points these warning and error to the 'TRACE(...)' directive.

How to write this in C++ correctly?

Answer 1

You could do:

inline void TRACE(...) {}

Answer 2

What compiler? For what it's worth, G++ 4.3.1 does not have any problem with that code.

Answer 3

#define TRACE false ||

This turns TRACE(x,y,z) into false || (x,y,z). Since x,y and z will be expressions, (x,y,z) evaluates to z (comma operator). z must be a built-in type (to be legally passed to a printf-style function) so it should be valid on the right side of ||. It won't be evaluated, but it must be a legal expression (e.g. you can't reference class members which only exist in DEBUG builds)

Vararg macros are a C99 invention, they're not in C++98 but might very well be in C++0x

[edit] - Using || to guarantee non-evaluation