Passing function parameters to a function through a macro

Question

I want to create a macro in which one of the parameters is the parameters for a function used within the macro.

#define Macro(PERAMS, OTHER) \
    functionBeingUsed(PERAMS); \
    OTHER; 


Macro(1,2,3,4, int i = 0);

As you can see you can see the commas are being used by both the macro and the function. which results in broken code.

I was wondering if there was a way to achieve the result that I need so the code can be interpreted like the following.

Macro((1,2,3,4), int i = 0);

Please note I am not using C++11.

Answer 1

Whenever you need to do anything nontrivial with the preprocessor, you should immediately jump to the Boost.PP library. Here, the BOOST_PP_REMOVE_PARENS macro seems relevant:

#define Macro(PARAMS, OTHER) \
    functionBeingUsed(BOOST_PP_REMOVE_PARENS(PARAMS)); \
    OTHER; 

Macro((1,2,3,4), int i = 0);

Alternatively, you could just write functionBeingUsed PARAMS and let the passed parentheses be used.

Answer 2

It is not possible to do what you have described. However, you can do something similar with variadic macros . You would need to put the PARAMS part at the end and it would look like the following:

#define Macro(OTHER, ...) \
    functionBeingUsed(__VA_ARGS__); \
    OTHER;

Macro(int i = 0, 1, 2, 3, 4);

Answer 3

I believe the only answer is to use multiple macros

#define MacroBegin \
   \\PreFunction

#define MacroEnd \
   \\PostFunction

MacroBegin
functionBeingCalled(1,2,3,4);
MacroEnd

Answer 4

To differentiate which commas belong to the macro itself and which belong to the function being called, you have to wrap the function parameters in parenthesis, eg:

Macro((1,2,3,4), int i = 0);

You would then have to remove the parenthesis from the function call inside the macro, since they will be coming from the macro parameter instead, eg:

#define Macro(PERAMS, OTHER) \
    functionBeingUsed PERAMS; \
    OTHER; 

So, something like this:

Macro((1,2,3,4), int i = 0);

Will resolve to this:

functionBeingUsed (1,2,3,4);
int i = 0; 

Just remember the extra parenthesis even if the function being called doesn't take any input parameters:

Macro((), ...);

functionBeingUsed ();
...; 

Answer 5

To keep your syntax, you may do something like:

#define COUNT_N(_1, _2, _3, _4, _5, _6, _7, _8, _9, _10, N, ...)    N
#define COUNT(...)   COUNT_N(__VA_ARGS__, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)
// Warning: COUNT() return 1 (as COUNT(A)) :-/

#define IDENTITY(N) N
#define APPLY(macro, ...) IDENTITY(macro(__VA_ARGS__))

#define F_1(a) functionBeingUsed(); a;
#define F_2(a, b) functionBeingUsed(a); b;
#define F_3(a, b, c) functionBeingUsed(a, b); c;
#define F_4(a, b, c, d) functionBeingUsed(a, b, c); d;
#define F_5(a, b, c, d, e) functionBeingUsed(a, b, c, d); e;
#define F_6(a, b, c, d, e, f) functionBeingUsed(a, b, c, d, e); f;
#define F_7(a, b, c, d, e, f, g) functionBeingUsed(a, b, c, d, e, f); g;
#define F_8(a, b, c, d, e, f, g, h) functionBeingUsed(a, b, c, d, e, f, g); h;

#define DISPATCH(N) F_ ## N

#define Macro(...) IDENTITY(APPLY(DISPATCH, COUNT(__VA_ARGS__)))(__VA_ARGS__)

Demo

but simpler to change your syntax to:

#define Macro(PARAMS, OTHER)  \
    functionBeingUsed PARAMS; \
    OTHER;

Macro((1,2,3,4), int i = 0);