Macro overloading

问题:

Is it possible to define something like this:

#define FOO(x, y) BAR()
#define FOO(x, sth, y) BAR(sth)

so that this:

FOO("daf", sfdas);
FOO("fdsfs", something, 5);

is translated to this:

BAR();
BAR(something);

?

Edit: Actually, BAR's are methods of my class. Sorry for not saying that before (didn't think it was relevant).

Answering DyP's question:

class Testy
{
    public:
    void TestFunction(std::string one, std::string two, std::string three)
    {
        std::cout << one << two << three;
    }
    void AnotherOne(std::string one)
    {
        std::cout << one;
    }
    void AnotherOne(void)
    {
        std::cout << "";
    }
};

#define PP_NARG(...) PP_NARG_(__VA_ARGS__,PP_RSEQ_N())
#define PP_NARG_(...) PP_ARG_N(__VA_ARGS__)
#define PP_ARG_N(_1, _2, _3, N, ...) N
#define PP_RSEQ_N() 3, 2, 1, 0

// macro for exactly 2 arguments
#define FOO_2(_1, _2) AnotherOne()
// macro for exactly 3 arguments
#define FOO_3(_1, _2, _3) AnotherOne(_2)

// macro selection by number of arguments
#define FOO_(N) FOO_##N
#define FOO_EVAL(N) FOO_(N)
#define TestFunction(...) FOO_EVAL(PP_NARG(__VA_ARGS__))(__VA_ARGS__)

And call:

Testy testy;
testy.TestFunction("one", "two", "three");   // line 9

Compiler output:

Warning 1 warning C4003: not enough actual parameters for macro 'PP_ARG_N' main.cpp 9

Warning 2 warning C4003: not enough actual parameters for macro 'FOO_' main.cpp 9

Error 3 error C2039: 'FOO_' : is not a member of 'Testy' main.cpp 9

回答1:

Edit ------------------------------------------------look here----------------------------------------------------------------->

(Overloading macro on number of arguments)

// functions, or macros, ....
void bar(){}
void bar(int){}

#define EXPAND(X) X    // for MSVC10 compatibility

// compute number of (variadic) macro arguments
// from http://groups.google.com/group/comp.std.c/browse_thread/thread/77ee8c8f92e4a3fb/346fc464319b1ee5?pli=1
#define PP_NARG(...) EXPAND( PP_NARG_(__VA_ARGS__, PP_RSEQ_N()) )
#define PP_NARG_(...) EXPAND( PP_ARG_N(__VA_ARGS__) )
#define PP_ARG_N(_1, _2, _3, N, ...) N
#define PP_RSEQ_N() 3, 2, 1, 0


// macro for exactly 2 arguments
#define FOO_2(_1, _2) bar()
// macro for exactly 3 arguments
#define FOO_3(_1, _2, _3) bar(_2)

// macro selection by number of arguments
#define FOO_(N) FOO_##N
#define FOO_EVAL(N) FOO_(N)
#define FOO(...) EXPAND( FOO_EVAL(EXPAND( PP_NARG(__VA_ARGS__) ))(__VA_ARGS__) )

int main()
{
    int something = 42;
    FOO("daf", sfdas);
    FOO("fdsfs", something, 5);
}

Preprocessor output:

void bar(){}
void bar(int){}

int main()
{
    int something = 42;
    bar();
    bar(something);
}

---

Edit2: Seems like VS2010 has some issues with __VA_ARGS__ and macro replacement.

UPDATE: It's ... a bug?? (also see this SO question):

#define MACRO2(PARAM0, PARAM1, ...) arg 0: >PARAM0<    arg 1: >PARAM1<    \
  additional args: >__VA_ARGS__<
#define MACRO1(...) MACRO2(__VA_ARGS__, OTHERARG_0, OTHERARG_1)

MACRO1(ARG0, ARG1);

Preprocessor output:

arg 0: >ARG0, ARG1<    arg 1: >OTHERARG_0<    additional args: >OTHERARG_1<;

For a workaround, see the linked SO question. I've updated the original answer (code) above and tested it with MSVC10 -> works now.