Re: [boost] Determining interest in Delegates submission

Subject: Re: [boost] Determining interest in Delegates submission
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2011-09-08 11:29:45

On Thu, Sep 8, 2011 at 7:46 AM, Jared McKee <jared.mckee_at_[hidden]> wrote:

> On Thu, Sep 8, 2011 at 10:19 AM, Jeffrey Lee Hellrung, Jr.
> <jeffrey.hellrung_at_[hidden]> wrote:
> >
> > On Wed, Sep 7, 2011 at 7:48 PM, Jared McKee <jared.mckee_at_[hidden]>
> wrote:
> >
> > > I would like to determine interest in a possible library submission.
> > >
> > > I have written some code which converts a member function and object
> > > pointer
> > > combination to a free function for use in C style callbacks. It has
> some
> > > limitations which I will describe shortly.
> > >
> > > The basic idea is to be able to convert:
> > >
> > > R (B::*)(T0,T1,…) and B*
> > >
> > > to
> > >
> > > R (*)(T0,T1,…)
> > >
> > > Here's an example usage:
> > >
> > > class foo {
> > >       int x;
> > >       int test(int v) {return x + v;}
> > > };
> > >
> > > foo* a = new foo;
> > > a->x = 3;
> > > int (*f)(int) = delegate(a, &foo::inc);
> > > cout << f(4) << endl; // outputs "7"
> > >
> > [...snip implementation notes...]
> >
> > I admit I only glossed over your implementation notes, but it looks like
> > this would be very similar to (or...the same as...?) a bound (e.g., via
> > Boost.Bind) boost::mem_fn:
> >
> > http://www.boost.org/doc/libs/1_47_0/libs/bind/mem_fn.html
> >
> > Is that accurate?
>
> Good question. I believe the approach in boost::mem_fn is to create
> C++ functor objects which are structs with operator() overloaded.
> These functors are not compatible with straight C function pointers.
> My implementation returns C function pointers, not functors.
>
> Here's a discussion about the topic:
>
> http://stackoverflow.com/questions/1840029/passing-functor-as-function-pointer
>

Oh...dear.

I can see the use for a small module that converts C++ functors into a kind
of (function pointer, void*) struct for use with interfacing with C-style
APIs, but trying to get a lonely function pointer to call a functor seems
wrought with peril (e.g., "gotchas" and safety issues)...  I assume you're
trying to provide a general solution to the 3rd case within the first answer
to the referenced stackoverflow link?

- Jeff