Re: [Boost-users] boost & c-style calback issue

2 Jun 2008

      ...
...
...
i'm facing this problem may be someone on the list do have an elegant
solution
class A;
class B;
class C {
int operator () (A*,B*)
};
typedef int (*foobar)(A*,B*);
void do_something(foobar cb);
C obj;
do_something(obj);	*ERROR*
Object of type C is not convertible to function
pointer type foobar.
...
did try boost::function  ftor = obj;
do_something(ftor);	*ERROR*
compiler generates in both cases the following error messages
cannot convert parameter 2 from 'boost::function<Signature>' to 'int
(__cdecl
*)(A *,B *)'
with
1>         [
1>             Signature=int (A *, B *)
1>         ]
1>         No user-defined-conversion operator available that can
perform
this
conversion, or the operator cannot be called
I can't make the corresponding class B member function static due to
side-effects.
Any clue how to get rid of this?
You got it backwards - raw function pointer is convertible
to boost::function, not the other way around (compiler is
trying to tell you exactly that.)
Make 'do_something' take boost::function instead.
The problem is that "do_something" is a legacy code written in plain C and
it
expect to get a function pointer with the signature
typedef int (*foobar)(A*,B*);
where A and B are struct.
Making the member function that has that signature static helps solve the
problem but then we do have undesirable side-effect since this operation
should
be instance based and not object-bases.
May be someone can come out with a better approach?!
With such restriction I would go with making C a singleton
(or pooled object - same idea) and doing something like:

  class C {
  public:
    int DoStuff( A*, B* ); // or op() - doesn't matter
    static C& GetInstance( /*XXX*/ ); // or pointer
  private:
    // singleton stuff ...
  };

  // extern "C" ?
  int CallbackFunc( A* a, B* b ) {
    return C::GetInstance( /*XXX*/ ).DoStuff( a, b );
  }

  do_something( CallbackFunc );

This is of course just one way of doing things and very
much depends on the context. Generally, you got a
subsystem boundary and need to figure out a clean interface.

Hope this helps.
--
 Nikolai

Re: [Boost-users] boost & c-style calback issue

Nikolai N Fetissov