Question

I want to create macros that will insert a parameter into a function call. For example, I have function Action() declared below. Action takes as it’s inputs an enum for the state number and a formatted string with optional args for the string.

I want to define macros so that instead of callingAction( ActionState1, "someText %d", &arg) with ActionState1 for the state parameter, I can just call State1("someText %d", &arg) instead. That way the macro would stick in the ActionState1 for the state param for me. I’m thinking of something like the following:

#define State1(formatedString, ...) Action(ActionState1, formatedString, ...)
#define State2(formatedString, ...) Action(ActionState2, formatedString, ...)
#define State3(formatedString, ...) Action(ActionState3, formatedString, ...)

enum {
  ActionState1,
  ActionState2,
  ActionState3
}

static void Action( State state, String formatedString, ...);

Does anyone know what the proper format for this would be?

Answer 1

I believe that __VA_ARGS__ is what you’re looking for:

#define State1(formattedString, ...) Action(1, (formattedString), __VA_ARGS__)
 .
 .
 .

This is a C99 feature, and Wikipedia claims that they are not part of any official C++ standard (I note this because you use the C++ tag), but a fairly popular extension. There is some good discussion over at this question.