How to single out the first parameter sent to a macro taking only a variadic parameter

问题

I try to get at the first actual parameter sent to a variadic macro. This is what I tried, and which does not work in VS2010:

#define FIRST_ARG(N, ...) N
#define MY_MACRO(...) decltype(FIRST_ARG(__VA_ARGS__))

When I look at the preprocessor output I see that FIRST_ARG returns the entire argument list sent to MY_MACRO...

On the other hand when I try with:

FIRST_ARG(1,2,3)

it expands to 1 as intended.

This seems to be somehow the inverse of the problem solved by the infamous two level concat macros. I know that "macro parameters are fully expanded before inserted in the macro body" but this does not seem to help me here as I don't understand what this means in the context of ... and __VA_ARGS__

Obviously __VA_ARGS__ binds to N and is only evaluated later. I tried several ways with extra macro levels but it didn't help.

回答1:

This is a bug in the Visual C++ preprocessor. The workaround listed there works. This:

#define FIRST_ARG_(N, ...) N
#define FIRST_ARG(args) FIRST_ARG_ args
#define MY_MACRO(...) decltype(FIRST_ARG((__VA_ARGS__)))

MY_MACRO(x, y, z)

expands to:

decltype(x)

来源：https://stackoverflow.com/questions/4750688/how-to-single-out-the-first-parameter-sent-to-a-macro-taking-only-a-variadic-par