问题
/* Debugging */
#ifdef DEBUG_THRU_UART0
# define DEBUG(...) printString (__VA_ARGS__)
#else
void dummyFunc(void);
# define DEBUG(...) dummyFunc()
#endif
I've seen this notation in different headers of C programming, I basically understood it's passing arguments, but I didn't understand what this "three dots notation" is called?
Can someone explain it with example or provide links also about VA Args?
回答1:
The dots are called, together with the __VA_ARGS__
, variadic macros
When the macro is invoked, all the tokens in its argument list [...], including any commas, become the variable argument. This sequence of tokens replaces the identifier VA_ARGS in the macro body wherever it appears.
source, bold emphasis of mine.
A sample of usage:
#ifdef DEBUG_THRU_UART0
# define DEBUG(...) printString (__VA_ARGS__)
#else
void dummyFunc(void);
# define DEBUG(...) dummyFunc()
#endif
DEBUG(1,2,3); //calls printString(1,2,3) or dummyFunc() depending on
//-DDEBUG_THRU_UART0 compiler define was given or not, when compiling.
回答2:
It's a varadic macro. It means you can call it with any number of arguments. The three ... is similar to the same construct used in a varadic function in C
That means you can use the macro like this
DEBUG("foo", "bar", "baz");
Or with any number of arguments.
The __VA_ARGS__ refers back again to the variable arguments in the macro itself.
#define DEBUG(...) printString (__VA_ARGS__)
^ ^
+-----<-refers to ----+
So DEBUG("foo", "bar", "baz");
would be replaced with printString ("foo", "bar", "baz")
来源:https://stackoverflow.com/questions/26053959/what-does-va-args-in-a-macro-mean