Re: [Openslp-devel] [Openslp-users] callbacks in c++

[John C. <jca...@no...>]

Gaurang,

C++ member functions have a hidden first parameter - a reference to
"this". By doing this, the language can allow transparent access to the
associated object's data. However, this makes the function signature
wrong for the callback. 

What you need to do is declare your private callback member function
"static". Then the signature will match, but you won't be able to get at
the associated object's data because static members are associated with
the class, not the individual object. That is, they act like global
functions with private scope. To solve this problem, you can use the
void * cookie parameter as a sort of pass-through for the object itself:

int MyClass::SomeMember(...)
{
   ...
   SomeSLPFunction(..., MyClass::MyPrivateStaticCallback, this);
   ...
}

Then, inside your MyPrivateStaticCallback method, you just cast the
cookie back to your object like this:

int MyClass::MyPrivateStaticCallback(..., void * cookie)
{
   MyClass * thisObject = static_cast<MyClass *>(cookie);
   // now you can use thisObject to call members and reference even
private data
   thisObject.somePrivateData = 1;
   return 0;
}

John

>>> "Gaurang Sardesai" <gsa...@vm...> 06/26/06 4:48 PM >>>
After successfully implementing what i'd set out to do in C, i'm trying
to
"port" my solution to an more object oriented approach. I created a few
classes, for the url, the discovery etc, and tried including the
callbacks as
private functions. however, the project just refuses to compile, with a
"could not find matching function" error. the moment i remove the
callback
definitions and declarations from the class, i have no problems and it
runs
fine. 
 
i haven't done too much c++ coding, so i was wondering if this is some
peculiarity of callbacks and c++. i heard the words "functors"
brandished
around, and before i delve in and try to figure out what those are, is
there
anything i should know w.r.t implementing openslp in c++? any
quirks/tips/tricks?
 
thanks,
 
cheers,
gaurang