c++11 : Variadic Macros(变长参宏)
Variadic macros are function-like macros that contain a variable number of arguments.
To use variadic macros, the ellipsis may be specified as the final formal argument in a macro definition, and the replacement identifier __VA_ARGS__ may be used in the definition to insert the extra arguments. __VA_ARGS__ is replaced by all of the arguments that match the ellipsis, including commas between them.
The C Standard specifies that at least one argument must be passed to the ellipsis, to ensure that the macro does not resolve to an expression with a trailing comma. The Visual C++ implementation will suppress a trailing comma if no arguments are passed to the ellipsis.
// variadic_macros.cpp #include <stdio.h> #define EMPTY #define CHECK1(x, ...) if (!(x)) { printf(__VA_ARGS__); } #define CHECK2(x, ...) if ((x)) { printf(__VA_ARGS__); } #define CHECK3(...) { printf(__VA_ARGS__); } #define MACRO(s, ...) printf(s, __VA_ARGS__) int main() { CHECK1(0, "here %s %s %s", "are", "some", "varargs1(1)\n"); CHECK1(1, "here %s %s %s", "are", "some", "varargs1(2)\n"); // won‘t print CHECK2(0, "here %s %s %s", "are", "some", "varargs2(3)\n"); // won‘t print CHECK2(1, "here %s %s %s", "are", "some", "varargs2(4)\n"); // always invokes printf in the macro CHECK3("here %s %s %s", "are", "some", "varargs3(5)\n"); MACRO("hello, world\n"); MACRO("error\n", EMPTY); // would cause error C2059, except VC++ // suppresses the trailing comma }
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