c++11 : Variadic Macros(變長參宏)

標籤：

Variadic macros are function-like macros that contain a variable number of arguments.Remarks 

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.

Example C++ 

// 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}

Output   

here are some varargs1(1)here are some varargs2(4)here are some varargs3(5)hello, worlderror

c++11 : Variadic Macros(變長參宏)