Variadic macro wrapper that expands to format string with characters corresponding to number of arguments

Question

Question

I am looking for a variadic C preprocessor macro that passes its argument and a corresponding format string to a function, repeating a character depending on the number of arguments. For example, I would like a macro FOO which expands as follows (or to equivalent C code):

• FOO(1) → bar("d",1)
• FOO(1,2) → bar("dd",1,2),
• FOO(1,2,3) → bar("ddd",1,2,3)
• bonus: FOO() → bar("")

While I can combine the solutions to C preprocessor macro for returning a string repeated a certain number of times and C++ preprocessor __VA_ARGS__ number of arguments (or similar questions) or use variadic macros, these have several drawbacks such as:

• requiring special libraries, such as Boost (which would be an issue for me),
• being compiler-dependent,
• only working at runtime,
• being extremely complicated.

My hope is that some better solutions emerge when these problems are not regarded separately.

Background

I want to callback Python functions in a C extension of Python in automatically generated code. So, for example, I need foo(1,2,3) to expand to:

PyObject_CallObject( callback_foo, Py_Build_Value("(Oddd)",Y,1,2,3) )

I know that all arguments of foo are doubles, but I do not know their number. (The above example is somewhat simplified. I am aware that it is missing a few Py_DECREFs.)

Answer 1

The best I could come up with so far is to take this answer and simplify it:

# define EXPAND(x) x

# define FORMATSTRING(...) EXPAND(ELEVENTHARG1(__VA_ARGS__ __VA_OPT__(,) RSEQ()))
# define ELEVENTHARG1(...) EXPAND(ELEVENTHARG2(__VA_ARGS__))
# define ELEVENTHARG2(_1,_2,_3,_4,_5,_6,_7,_8,_9,_10,N,...) N
# define RSEQ() "dddddddddd","ddddddddd","dddddddd", \
    "ddddddd","dddddd","ddddd","dddd","ddd","dd","d",""

# define FOO(...) bar( FORMATSTRING(__VA_ARGS__) __VA_OPT__(,) __VA_ARGS__ )

FOO()      // expands to: bar( "" )
FOO(1)     // expands to: bar( "d" , 1 )
FOO(1,2,3) // expands to: bar( "ddd" , 1,2,3 )

This works with GCC and Clang (with -std=c++2a) and up to ten arguments (but can be expanded).

The biggest compatibility issue are the two instances of __VA_OPT__(,), which are only required for handling the zero-argument case. Otherwise, they can be replaced by a simple ,.