C-preprocessor recursive macro

Question

#define PP_ARG0_(arg0, ...) arg0
#define PP_REST_(arg0, ...) __VA_ARGS__
#define PP_ARG0(args) PP_ARG0_ args
#define PP_REST(args) PP_REST_ args

#define FUNCTION(name) void name();
#define FUNCTION_TABLE(...)                   \
    FUNCTION(PP_ARG0((__VA_ARGS__)))          \
    FUNCTION_TABLE(PP_REST((__VA_ARGS__)))    \

test code:

FUNCTION_TABLE(f1, f2,f3,testA,testB,testC);

Obviously, because of recursive expansion it will only declare void f1(); and the rest won't be expanded:

void f1(); FUNCTION_TABLE(f2,f3,testA,testB,testC);

What kind of trick can I use to achieve recursive expansion in this case? The problem is that I need to support MANY arguments (up 100) and I absolutely cannot use boost.

Answer 1

The simplest solution is to use sequence iteration like this:

#define CAT(x, y) PRIMITIVE_CAT(x, y)
#define PRIMITIVE_CAT(x, y) x ## y

#define FUNCTION(name) void name();
#define FUNCTION_TABLE(seq) CAT(FUNCTION_TABLE_1 seq, _END)
#define FUNCTION_TABLE_1(x) FUNCTION(x) FUNCTION_TABLE_2
#define FUNCTION_TABLE_2(x) FUNCTION(x) FUNCTION_TABLE_1
#define FUNCTION_TABLE_1_END
#define FUNCTION_TABLE_2_END

Then you call FUNCTION_TABLE with a preprocessor sequence instead of varidiac arguments:

FUNCTION_TABLE((f1)(f2)(f3)(testA)(testB)(testC))

Not only is this much simpler, its also faster(ie faster compilation) than using the recursive solution(like the one you've shown or like this one here).

Answer 2

Here's the answer in case somebody wants to do the same.

#define _PP_0(_1, ...) _1            // (a,b,c,d) => a
#define _PP_X(_1, ...) (__VA_ARGS__) // (a,b,c,d) => (b,c,d)

//for each a in __VA_ARGS__ do f(a,x) 
//where x is some parameter passed to PP_TRANSFORM
#define PP_TRANSFORM(f,x,...) \
    PP_JOIN(PP_TRANSFORM_,PP_NARG(__VA_ARGS__))(f,x,(__VA_ARGS__))

#define PP_TRANSFORM_0(...)
#define PP_TRANSFORM_1( f,x,a) f(_PP_0 a,x) PP_TRANSFORM_0( f,x,_PP_X a)
#define PP_TRANSFORM_2( f,x,a) f(_PP_0 a,x) PP_TRANSFORM_1( f,x,_PP_X a)
...
#define PP_TRANSFORM_51(f,x,a) f(_PP_0 a,x) PP_TRANSFORM_50( f,x,_PP_X a)
...
#define PP_TRANSFORM_99(f,x,a) f(_PP_0 a,x) PP_TRANSFORM_98(f,x,_PP_X a)
#define PP_TRANSFORM_100(f,x,a)f(_PP_0 a,x) PP_TRANSFORM_99(f,x,_PP_X a)

where PP_NARG is the macro that counts number of arguments and PP_JOIN is the macro that joins tokens (that is PP_JOIN(a,b) => ab). You'll also need to patch that PP_NARG if you want to be able to process more than 64 arguments.

Now, back to the original question. Solution using the PP_TRANSFORM is:

#define FUNCTION(name, dummy) void name();
#define FUNCTION_TABLE(...) PP_TRANSFORM(FUNCTION,dummy,__VA_ARGS__)

if you want to generate c++ implementation functions then you can use that opaque x parameter of PP_TRANSFORM:

#define FUNCTION_CPP(name, class) void class::name(){}
#define FUNCTION_TABLE_CPP(...) PP_TRANSFORM(FUNCTION_CPP,MyClass,__VA_ARGS__)

All this works equally well with GCC and MSVC preprocessors; PP_TRANSFORM_NN doesn't use __VA_ARGS__ to avoid separate implementations of 100 defines for GCC and MSVC