cmacrosc-preprocessor

Overloading Macro on Number of Arguments

I have two macros FOO2 and FOO3:

#define FOO2(x,y) ...
#define FOO3(x,y,z) ...

I want to define a new macro FOO as follows:

#define FOO(x,y) FOO2(x,y)
#define FOO(x,y,z) FOO3(x,y,z)

But this doesn't work because macros do not overload on number of arguments.

Without modifying FOO2 and FOO3, is there some way to define a macro FOO (using __VA_ARGS__ or otherwise) to get the same effect of dispatching FOO(x,y) to FOO2, and FOO(x,y,z) to FOO3?

Solution

  • Simple as:

    #define EXPAND(x)                           x
#define GET_MACRO(_1, _2, _3, name, ...)    name
#define FOO(...)    EXPAND( GET_MACRO(__VA_ARGS__, FOO3, FOO2)(__VA_ARGS__) )

    With the above definitions the "overloaded" FOO macro expands like this:

    FOO(a, b)           // expands to FOO2(a, b)
FOO(a, b, c)        // expands to FOO3(a, b, c)

    Live demo.