vPtr for an empty class

[slamps]

Hi, i have this code:

struct MenuChan
{
virtual void draw()=0;
};
struct MenuData_01
{
int nEast;
int nNorth;
char cId;
};
struct MenuChan_01: public PstarMenuChan, public MenuData_01
{
virtual void draw();
};


Im trying to send MenuData_01 structure data thru tcp and read it in the other side in order to convert it to MenuChan_01, the problem is that the size of thus two structure is different.

i get this results:
sizeof(MenuData_01)=12
sizeof(MenuChan_01)=16

i know that this space is populated by the vPtr but i don't know how to convert
the data i get to the MenuChan_01 class.

please help.

[weaknessforcats]

You do not want to send the sizeof(some struct). That sizeof includes, as you note, the VTBL pointer plus any slack bytes caused a word alignment. The target machine cannot use the vtbl address and possibly the slack bytes are different also dependingupon the target processor.

Send the the data members individually and build your object anew on the other side.

[slamps]

You do not want to send the sizeof(some struct). That sizeof includes, as you note, the VTBL pointer plus any slack bytes caused a word alignment. The target machine cannot use the vtbl address and possibly the slack bytes are different also dependingupon the target processor.

Send the the data members individually and build your object anew on the other side.

hi,
my problem is that i have many structures as the one mentioned above and I don't want to build specific code for each one of them.
is there any way to calculate the position of the VTABLE inside the structure?

[weaknessforcats]

is there any way to calculate the position of the VTABLE inside the structure?

I believe it is implementation dependent. Visual C++ uses the first 4 bytes of the object.

Microsoft uses a BSTR for stuff like this. A BSTR is a series of C strings separated by \0 with a final \0\0 marking the end of the string. The first part of the BSTR is the length. That way a receiver can read the first part, get the length, allocate the correct amount of memory,and then read the rest of that BSTR into to the allocation.

I suggest you convert your structs into a communications format object and that you work your communication in that format. So, you Unpack() to create a communications format object and the receiver calls a constructor using the communications format object to convert back to the original object.

[slamps]

I believe it is implementation dependent. Visual C++ uses the first 4 bytes of the object.

Microsoft uses a BSTR for stuff like this. A BSTR is a series of C strings separated by \0 with a final \0\0 marking the end of the string. The first part of the BSTR is the length. That way a receiver can read the first part, get the length, allocate the correct amount of memory,and then read the rest of that BSTR into to the allocation.

I suggest you convert your structs into a communications format object and that you work your communication in that format. So, you Unpack() to create a communications format object and the receiver calls a constructor using the communications format object to convert back to the original object.

I took your advise, thanks.