overlay class over memory

[lromanus@ftml.net]

I am relatively new to C++ so this is probably a basic question: Can I
overlay a class over a chuck of memory just like one can overlay a
struct over memory. I need to parse a frame comming from the network
hardware.

Thanks,

Piotr

[Victor Bazarov]

Re: overlay class over memory

lromanus@ftml.n et wrote:[color=blue]
> I am relatively new to C++ so this is probably a basic question: Can I
> overlay a class over a chuck of memory just like one can overlay a
> struct over memory. I need to parse a frame comming from the network
> hardware.[/color]

You should take a look at "placement new".

V
--
Please remove capital As from my address when replying by mail

[Frank Schmidt]

Re: overlay class over memory


<lromanus@ftml. net> schrieb im Newsbeitrag
news:1140920987 .611858.215690@ u72g2000cwu.goo glegroups.com.. .[color=blue]
>I am relatively new to C++ so this is probably a basic question: Can I
> overlay a class over a chuck of memory just like one can overlay a
> struct over memory. I need to parse a frame comming from the network
> hardware.[/color]

Only if your class looks like a classic struct. If you add "hidden"
varialbles to the class, e.g. a a pointer to the virtual function table do
to a virtual function in the class, ... it will mess up the memory
alignment.

[lromanus@ftml.net]

Re: overlay class over memory

Is placement new fixing the problem of virtual tables?

Piotr

[Victor Bazarov]

Re: overlay class over memory

lromanus@ftml.n et wrote:[color=blue]
> Is placement new fixing the problem of virtual tables?[/color]

What book are you reading that doesn't explain 'new'? "Placement
new" is not different at all in the procedure of constructing the
object. The only difference is actual memory allocation, the
"regular" 'new' allocates, the "placement" doesn't.

V
--
Please remove capital As from my address when replying by mail

[Gianni Mariani]

Re: overlay class over memory

lromanus@ftml.n et wrote:[color=blue]
> I am relatively new to C++ so this is probably a basic question: Can I
> overlay a class over a chuck of memory just like one can overlay a
> struct over memory. I need to parse a frame comming from the network
> hardware.[/color]

Simply casting the memory *might* work except for alignment and byte order.

This is a posting I did a while ago - might help.

http://groups.google.com/group/comp.lang.c++/msg/d733c7ff9e4a6bda?&hl=en