Variadic macros: expansion of pasted tokens

Question

I'm wondering if it's possible to "nest" variadic macro invocations. I'm only truly concerned with GCC and Clang. My macro definition looks like this:

/**
 * @brief Invoke an instance method.
 */
#define $(obj, method, ...) \
    ({ \
        typeof(obj) _obj = obj; \
        _obj->interface->method(_obj, ## __VA_ARGS__); \
    })

I use this to conveniently call "instance methods" in my OO framework (https://github.com/jdolan/objectively):

$(array, addObject, obj);

Works boss. Unfortunately, I haven't yet figured out a way to allow nesting of these calls, which would be very useful in some situations; e.g.:

/**
 * @see MutableSetInterface::addObjectsFromArray(MutableSet *, const Array *)
 */
static void addObjectsFromArray(MutableSet *self, const Array *array) {

    if (array) {
        for (size_t i = 0; i < array->count; i++) {
            $(self, addObject, $(array, objectAtIndex, i));
        }
    }
}

The nested variadic macro invocation above fails to compile because the inner invocation is never expanded. Is it possible to fix this, or have I already abused the preprocessor to its limits? :)

Answer 1

This is a common problem with nested preprocessor macros. Preprocessor expansion rules are fairly arcane; the relevant tl;dr is that macros are expanded in layers. The workaround is to add a layer of indirection in which the parameter can be expanded:

#define MI(obj, method, ...) \
  ({ \
    typeof(obj) _obj = obj; \
    _obj->interface->method(_obj, ## __VA_ARGS__); \
  })

#define M(obj, method, ...) MI(obj, method, __VA_ARGS__)

// This will now expand properly.
M(self, addObject, M(array, objectAtIndex, M(foo, bar, i)))

Side note: be aware that $ is not part of C's basic source character set; using it will probably not be portable.