Macroized Parameters

c, c-preprocessor, macros, variadic-macros, visual-c++

Solution

This is a bug in the Visual C++ compiler. The compiler does not correctly expand the variadic argument pack when `__VA_ARGS__` appears as an argument to another macro.

The workaround is to use an additional layer of indirection to force the variadic argument pack to be expanded before `MACRO_1` is invoked:

#define MACRO_1(a, b, c) fn(a, b, c)
#define MACRO_1_(args_list) MACRO_1 args_list
#define MACRO(...) MACRO_1_((__VA_ARGS__))

There is a Microsoft Connect bug for this issue: Variadic Macro Replacement (Wayback Machine). If this issue is important to you, please upvote that bug report.

Problem

I have a system with many parameter sets "macroized" (macros of the form "#define name value,value,value). I would like to pass these to a macro, but when I do I get an error. example: ``` void fn(int a, int b, int c){ return; } #define MACRO_1(a, b, c) fn(a, b, c) #define MACRO(...) MACRO_1(__VA_ARGS__) #define PARAM1 1 #define PARAM2 2, 2 #define PARAM3 3, 3, 3 int main(int argc, char * argv[]){ MACRO(0,0,0); MACRO(PARAM1,1, 1); MACRO(PARAM2,2); MACRO(PARAM3); return 0; } ``` in Visual C I get: ``` 1>c:\main.c(10): warning C4003: not enough actual parameters for macro 'MACRO_1' 1>c:\main.c(10): error C2059: syntax error : ',' 1>c:\main.c(11): warning C4003: not enough actual parameters for macro 'MACRO_1' 1>c:\main.c(11): error C2059: syntax error : ',' 1>c:\main.c(12): warning C4003: not enough actual parameters for macro 'MACRO_1' 1>c:\main.c(12): error C2059: syntax error : ',' 1>c:\main.c(13): warning C4003: not enough actual parameters for macro 'MACRO_1' 1>c:\main.c(13): error C2059: syntax error : ',' ```

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