Can macros be overloaded by number of arguments?

Question

How does this work? How can a C99/C++11 variadic macro be implemented to expand to different things on the sole basis of how many arguments are given to it?

Answer 1

(Edit: See the end for a ready-made solution.)

To get an overloaded macro, first we need a macro which selects between several implementations. This part doesn't use a variadic macro. Then a variadic macro which generically counts its arguments produces a selector. Plugging the argument count into a dispatcher produces an overloaded macro.

Caveat: This system cannot tell the difference between zero and one arguments because there is no difference between no argument, and a single empty argument. They both look like MACRO().

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To select between implementations, use the macro catenation operator with a series of function-like macros.

#define select( selector, ... ) impl ## _ ## selector( __VA_ARGS__ )
#define impl_1() meh
#define impl_2( abc, xyz ) # abc "wizza" xyz()
//etc

// usage: select( 1 ) => impl_1() => meh
//        select( 2, huz, bar ) => impl_2( huzza, bar ) => "huz" "wizza" bar()

Because the ## operator suppresses macro expansion of its arguments, it's better to wrap it in another macro.

#define CAT( A, B ) A ## B
#define SELECT( NAME, NUM ) CAT( NAME ## _, NUM )

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To count arguments, use __VA_ARGS__ to shift arguments like so (this is the clever part):

#define GET_COUNT( _1, _2, _3, _4, _5, _6 /* ad nauseam */, COUNT, ... ) COUNT
#define VA_SIZE( ... ) GET_COUNT( __VA_ARGS__, 6, 5, 4, 3, 2, 1 )

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Library code:

#define CAT( A, B ) A ## B
#define SELECT( NAME, NUM ) CAT( NAME ## _, NUM )

#define GET_COUNT( _1, _2, _3, _4, _5, _6 /* ad nauseam */, COUNT, ... ) COUNT
#define VA_SIZE( ... ) GET_COUNT( __VA_ARGS__, 6, 5, 4, 3, 2, 1 )

#define VA_SELECT( NAME, ... ) SELECT( NAME, VA_SIZE(__VA_ARGS__) )(__VA_ARGS__)

Usage:

#define MY_OVERLOADED( ... ) VA_SELECT( MY_OVERLOADED, __VA_ARGS__ )
#define MY_OVERLOADED_1( X ) foo< X >
#define MY_OVERLOADED_2( X, Y ) bar< X >( Y )
#define MY_OVERLOADED_3( X, Y, Z ) bang_ ## X< Y >.Z()