• Russ Lewis (45/45) Aug 20 2001 I didn't see any spec for member function pointers, so I thought I'd

Russ Lewis <russ deming-os.org> writes:

I didn't see any spec for member function pointers, so I thought I'd
throw out an idea:

You cannot take a pointer to a member function of a class; however, you
can take a pointer to a member function of an *object*.  The pointer
returned contains both a pointer to the object and a pointer to the
function, as found in the virtual function table.  When you dereference
the pointer, it automatically calls it as a member function.  This means
you won't have to write static wrappers for all of your callbacks.



class Foo
{
    int myFunc(int);
};

int (Foo::myFuncPtr)(int);

myFuncPtr = Foo.myFunc;  // illegal
Foo bar;
myFuncPtr = bar.myFunc; // legal
....
int baz = myFuncPtr(3);



Also, I would hope that you could use member function pointers to
interchange with C.  My understanding is that in at least some C++
implementations, the this pointer is just passed as the first
"invisible" argument; C could use this to call member functions.  D
would allow you to cast member function pointers to equivalent C
structures.  This means that you could have C code call member function
callbacks:

***C API***
typedef int (*Callback)(void *,int);
typedef struct
{
     void *obj;
     Callback func;
} CallbackPointer;

***D Code***
class Foo
{
   extern C: int myCallback(int);
};
Foo bar;
CallbackPointer baz;
baz = (CallbackPointer*)(bar.myCallback);
// the D code then passes baz to the C code

***C Code***
/* time to call the callback */
CallbackPointer baz;
int retCode = (baz.func)(baz.obj,3);