C/C++ preprocessor: extract every second variadic parameter -


is there way extract every second parameter list of variadic parameters in c/c++ preprocessor?

i write macro generate boilerplate code interface methods in following way:

#define interface_fn(_name, _x, _y, _n_params, ...) \     void _name(__va_args__) \     { \         processing_function(_x, _y, _n_params, every_second(__va_args__); \     }  void processing_function(int x, int y, int count, ...) {     va_list params;     va_start(params, count);     while (count--) {         do_something(va_arg(params, parentclass*));     }     va_end(params);     // else }

which used as:

interface_fn(foo, 1, 2, 0); interface_fn(bar, 3, 4, 1, classx*, x); interface_fn(jar, 5, 6, 2, classy*, y, classz*, z);

the classes classx, classy, classz derivatives of parentclass;

i'm looking every_second macro.

here working example of asked for

#define every_second0(...)  #define every_second1_(second, ...) , second #define every_second1(first, ...) every_second1_(__va_args__)  #define every_second2_(second, ...) , second every_second1(__va_args__) #define every_second2(first, ...) every_second2_(__va_args__)  #define every_second3_(second, ...) , second every_second2(__va_args__) #define every_second3(first, ...) every_second3_(__va_args__)  #define every_second4_(second, ...) , second every_second3(__va_args__) #define every_second4(first, ...) every_second4_(__va_args__)  #define every_second5_(second, ...) , second every_second4(__va_args__) #define every_second5(first, ...) every_second5_(__va_args__)  #define count_every_second(_1,__1,_2,__2,_3,__3,_4,__4,_5,__5,num,...) every_second ## num #define every_second(...) count_every_second(__va_args__,5,5,4,4,3,3,2,2,1,0)(__va_args__)   #define interface_fn(_name, _x, _y, _n_params, ...) \     void _name(__va_args__) \     { \         processing_function(_x, _y, _n_params every_second(__va_args__)); \     }  class parentclass {}; class classx {}; class classy {}; class classz {};  void processing_function(int x, int y, int count, ...) {     va_list params;     va_start(params, count);     while (count--) {         do_something(va_arg(params, parentclass*));     }     va_end(params);     // else }  interface_fn(foo, 1, 2, 0); interface_fn(bar, 3, 4, 1, classx*, x); interface_fn(jar, 5, 6, 2, classy*, y, classz*, z);

the preprocessor output looks this

class parentclass {}; class classx {}; class classy {}; class classz {};  void processing_function(int x, int y, int count, ...) {  va_list params;  va_start(params, count);  while (count--) {   do_something(va_arg(params, parentclass*));  }  va_end(params);  }  void foo() { processing_function(1, 2, 0 ); }; void bar(classx*, x) { processing_function(3, 4, 1 , x); }; void jar(classy*, y, classz*, z) { processing_function(5, 6, 2 , y , z); };

though give obligatory warning there easier way without macros.

this have comma @ end if put odd number of arguments, though shouldn't happen use case. allows 5 pairs (10 total arguments) think can see pattern how add support more. there no way without explicitly defining each number of args due preprocessors limitations

have fun!

edit:

i note can improved never have specify _n_params , let macros count you, add macro

#define count_every_second_(_1,__1,_2,__2,_3,__3,_4,__4,_5,__5,num,...) num

and replace interface_fns definition 

#define interface_fn(_name, _x, _y, ...) \     void _name(__va_args__) \     { \         processing_function(_x, _y, count_every_second_(__va_args__,5,error,4,4,3,3,2,2,1,0) every_second(__va_args__)); \     }

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