from C to C++

from C to C++

[Peter Ingels (EBC)]

Hi,
I've inherited an old LAPD stack written in C and I'm trying to C++-ify it.
The old C stack uses a function pointer to keep track of the stack's state:

typedef (*Statptr)();
and the variable
Statptr Layer2State;

the Statptr variable points to functions that looks like e.g.:

Statptr  DLLpassive(MESSAGE* rx_sig_ptr);

That is the function itself returns a Statptr! I guess the C compiler isn't too fuzzy about type checking!? Any way, the "engine" of the state machine is then simply:

Layer2State = (Statptr) (*Layer2State) (mp);
where mp is the MESSAGE* arriving on the queue.

Now, the problem is that I can't port this scheme to C++. I'm thinking it's got be doable...

I ended up with:

typedef void* (LapdStack::*Statptr) (Message*);
the state functions (DLLpassive and so on) looks the same but the "engine" becomes:

Layer2State = (Statptr) ((this->*Layer2State)(mp));

This , however, doesn't compile. G++ 2.95.3 says:

/home/ebcinpe/clearcase/ebcinpe_view/mecs/egx/sw/dp/mg/mta/isdn_pri/lapd/src/Lap
dStack.cpp: In method `bool LapdStack::mainThreadWorker(IPC_Sig_t *)':
/home/ebcinpe/clearcase/ebcinpe_view/mecs/egx/sw/dp/mg/mta/isdn_pri/lapd/src/Lap
dStack.cpp:43: cannot convert `((LapdStack::Layer2State.void * (LapdStack::*)(Me
ssage *)::__index >= 0) ? (*(*(this + LapdStack::Layer2State.void * (LapdStack::
*)(Message *)::__pfn_or_delta2.{anonymous union}::__delta2) + ((LapdStack::Layer
2State.void * (LapdStack::*)(Message *)::__index * 4) + 0fffffffc))) : LapdStack
::Layer2State.void * (LapdStack::*)(Message *)::__pfn_or_delta2.{anonymous union
}::__pfn)((this + LapdStack::Layer2State.void * (LapdStack::*)(Message *)::__del
ta), mp_sig)' from type `void *' to type `void * (LapdStack::*)(Message *)'
make: *** [/home/ebcinpe/clearcase/ebcinpe_view/mecs/egx/sw/dp/mg/mta/isdn_pri/l
apd/obj/LapdStack.o] Error 1

The message is a bit suspicious looking but I guess the issue is 
"cannot convert from type `void *' to type `void * (LapdStack::*)(Message *)"

Can someone help me with this?
There are obviously some simple workarounds without function pointers, but I kind of like to keep them.

anyone?

/Peter

Re: from C to C++

[Ben Davis]

Hi,

On Monday 03 February 2003 2:55 pm, you wrote:
> typedef void* (LapdStack::*Statptr) (Message*);
> Layer2State = (Statptr) ((this->*Layer2State)(mp));

I'm no expert on C++, but try the following:

typedef void *(*LapdStack::Statptr)(Message *);
Layer2State = (LapdStack::Statptr)(*this->Layer2State)(mp);

Certainly in C, the idea is that the declaration (or the typedef) resembles 
the expressiong you can construct to get down to the primitive (or previously 
typedeffed) type. Notice how the function pointer 'this->Layer2State' is 
being dereferenced before you call it; that's what the weird (*)() syntax is 
doing.

This link may help a bit:
   http://dumb.sourceforge.net/index.php?r2actpage=index_docs_&doc=fnptr

Ben