I've been working with libffi lately, and since it uses a C API, any abstraction is done by using void pointers (good ol' C). I'm creating a class (with variadic templates) which utilizes this API. The class declaration is the following: (where Ret
= return value and Args
= function arguments)
template <typename Ret, typename... Args>
class Function
Within this class I have two different functions declared as well (simplified):
Ret Call(Args... args); // Calls the wrapped function
void CallbackBind(Ret * ret, void * args[]); // The libffi callback function (it's actually static...)
I want to be able to use Call
from CallbackBind
; and that is my problem. I have no idea how I am supposed to convert the void*
array to the templated argument list. This is what I want more or less:
CallbackBind(Ret * ret, void * args[])
{
// I want to somehow expand the array of void pointers and convert each
// one of them to the corresponding template type/argument. The length
// of the 'void*' vector equals sizeof...(Args) (variadic template argument count)
// Cast each of one of the pointers to their original type
*ret = Call(*((typeof(Args[0])*) args[0]), *((typeof(Args[1])*) args[1]), ... /* and so on */);
}
If this is not achievable, are there any workarounds or different solutions available?
You don't want to iterate over the types, you want to create a parameter pack and expand it in a variadic template. You have an array, so the pack you want is a pack of integers 0,1,2... to serve as array indices.
#include <redi/index_tuple.h>
template<typename Ret, typename... Args>
struct Function
{
Ret (*wrapped_function)(Args...);
template<unsigned... I>
Ret dispatch(void* args[], redi::index_tuple<I...>)
{
return wrapped_function(*static_cast<Args*>(args[I])...);
}
void CallbackBind(Ret * ret, void * args[])
{
*ret = dispatch(args, to_index_tuple<Args...>());
}
};
Something like that, using index_tuple.h
The trick is that CallbackBind creates an index_tuple
of integers representing the arg positions, and dispatches to another function which deduces the integers and expands the pack into a list of cast expressions to use as arguments to the wrapped function.
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