I'm currently using GCC 4.4, and I'm having quite the headache casting between void*
and a pointer to member function. I'm trying to write an easy-to-use library for binding C++ objects to a Lua interpreter, like so:
LuaObject<Foo> lobj = registerObject(L, "foo", fooObject);
lobj.addField(L, "bar", &Foo::bar);
I've got most of it done, except for the following function (which is specific to a certain function signature until I have a chance to generalize it):
template <class T>
int call_int_function(lua_State *L)
{
// this next line is problematic
void (T::*method)(int, int) = reinterpret_cast<void (T::*)(int, int)>(lua_touserdata(L, lua_upvalueindex(1)));
T *obj = reinterpret_cast<T *>(lua_touserdata(L, 1));
(obj->*method)(lua_tointeger(L, 2), lua_tointeger(L, 3));
return 0;
}
For those of you unfamiliar with Lua, lua_touserdata(L, lua_upvalueindex(1))
gets the first value associated with a closure (in this case, it's the pointer to member function) and returns it as a void*
. GCC complains that void*
-> void (T::*)(int, int)
is an invalid cast. Any ideas on how to get around this?
You cannot cast a pointer-to-member to void *
or to any other "regular" pointer type. Pointers-to-members are not addresses the way regular pointers are. What you most likely will need to do is wrap your member function in a regular function. The C++ FAQ Lite explains this in some detail. The main issue is that the data needed to implement a pointer-to-member is not just an address, and in fact varies tremendously based on the compiler implementation.
I presume you have control over what the user data lua_touserdata
is returning. It can't be a pointer-to-member since there isn't a legal way to get this information back out. But you do have some other choices:
The simplest choice is probably to wrap your member function in a free function and return that. That free function should take the object as its first argument. See the code sample below.
Use a technique similar to that of Boost.Bind's mem_fun to return a function object, which you can template on appropriately. I don't see that this is easier, but it would let you associate the more state with the function return if you needed to.
Here's a rewrite of your function using the first way:
template <class T>
int call_int_function(lua_State *L)
{
void (*method)(T*, int, int) = reinterpret_cast<void (*)(T*, int, int)>(lua_touserdata(L, lua_upvalueindex(1)));
T *obj = reinterpret_cast<T *>(lua_touserdata(L, 1));
method(obj, lua_tointeger(L, 2), lua_tointeger(L, 3));
return 0;
}