can a class member function be used as a callback function?

[JDT]

Hi,

Can we pass a member function in a class as a callback function?
Someone instucted me that I can only use a static functon or a global
function as a callback. Your help is appreciated.

JD

 

[Ian Collins]

Hi,

Can we pass a member function in a class as a callback function? Someone
instucted me that I can only use a static functon or a global function
as a callback. Your help is appreciated.

Yes, you can. But you have to pass an instance of the class as well in
order to call the function.

 

[David Harmon]

On Wed, 28 Mar 2007 01:29:42 GMT in comp.lang.c++, JDT

Can we pass a member function in a class as a callback function?

In general, no.

This issue is covered in Marshall Cline's C++ FAQ. See the topic
"# [33.2] How do I pass a pointer-to-member-function to a signal
handler, X event callback, system call that starts a thread/task, etc?"
It is always good to check the FAQ before posting. You can get the FAQ
at:
http://www.parashift.com/c++-faq-lite/

 

[Juha Nieminen]

On Wed, 28 Mar 2007 01:29:42 GMT in comp.lang.c++, JDT


In general, no.

What do you mean "in general"? I think that's confusing.

You can, of course, get a pointer to a member function and pass
it around all you like. It is perfectly possible to construct a
system where a member function is called as a callback.

Of course the catch is that this function pointer alone is not
enough. Member functions implicitly take a pointer to an instance
of the class (that's the 'this' pointer seen inside the function)
and thus to call the function such an instance (well, a pointer to
one) is needed. The function cannot be called alone. If you want
the callback system to call a member function of a specific instance
of the class, you have to give it a pointer to that instance too,
besides the pointer to the member function.

Naturally if the callback system only takes a function pointer
and nothing else, and you can't modify the callback system, then
you are out of luck.

 

[David Harmon]

On Wed, 28 Mar 2007 01:29:42 GMT in comp.lang.c++, JDT


In general, no.

What do you mean "in general"? I think that's confusing.[/QUOTE]

It means, don't bother nitpicking.

 

[Christopher Pisz]

Hi,

Can we pass a member function in a class as a callback function? Someone
instucted me that I can only use a static functon or a global function as
a callback. Your help is appreciated.

JD

I've has success using static methods of a class as callback also. In on I
implemented myself, the callback took an instance as an argument, then the
one static callback could dispatch to the particular instance. In some
cases, maybe you don't care about the instance and just want one callback,
but then should it really be a member of a class? so, to answer your
question, it is possible, but how it is possible really depends on how the
callback mechanism was designed.

 

[James Kanze]

Can we pass a member function in a class as a callback function?

You can pass whatever the interface requires, and nothing else.
I've even seen some interfaces (in a Windows manager for Sun)
which used pointers to member function for the callbacks. It's
rather exceptional, however. Typically, two cases occur:

-- The interface was designed with C++ in mind. In that case,
most of the time, you pass pointers to functional objects as
the callback. Traditionally, those objects must derive from
a common base class, and override a specific function in
that class, but some newer interfaces use more or less fancy
template tricks to allow you to pass pretty much anything
which will support a function call operator.

-- The interface is designed for C or C/C++. In that case,
you'll almost certainly have to pass the address of an
`extern "C"' function: static members don't cut it, nor do
normal global functions in C++. Hopefully, you'll also get
the chance to pass a void* with the address of "user data";
your user data will be a functional object, as above, which
gets called in the function whose address you pass.

Someone instucted me that I can only use a static functon or a global
function as a callback.

To date, I've never seen an interface where a static member
function could be used (except perhaps some of those using fancy
templates). See above: it very much depends on the interface,
and it's possible for an interface to use just about anything.
What you do have to do is conform to the interface: if it
expects a pointer to a member function of class X, you have to
pass it a pointer to a (non-static) member fucntion of class X.
If it has a C compatible interface, you must pass it a function
declared `extern "C"'. And if it expects a pointer to a class
CallBack, you have to derive from CallBack, and pass it a
pointer to an object of the derived type.