问题
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? :)
回答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.
来源:https://stackoverflow.com/questions/27514116/variadic-macros-expansion-of-pasted-tokens