Re: ? Function Pointer to a Method of Unknown Class

From:

"Igor Tandetnik" <itandetnik@mvps.org>

Newsgroups:

microsoft.public.vc.language

Date:

Sat, 16 Sep 2006 08:08:46 -0400

Message-ID:

<u7cZKjY2GHA.2036@TK2MSFTNGP05.phx.gbl>

"Alec S." <@> wrote in message
news:eG0CB1W2GHA.4172@TK2MSFTNGP05.phx.gbl

I've got a class which needs to store a pointer to a function. The
class will use that as a callback to pass some data to the specified
function when the data changes.

I need to be able to pass a method of the required format from an
unknown class to it.

For example:

typedef void(*Z)(int);
class A {
   int i;
   Z fp;
   void ReadI();
}
class B {
   void init();
   void Foo(int t=0;);
   A m_a;
}

void B::init() {
   m_a.fp=Foo;
}

Z is defined to be a plain non-member function. You are trying assign a
member function pointer to it. This is not going to work - mainly
because A does not know which object to call the member function on.

You can make B::Foo static, then the assignment will work. Of couse Foo
won't then be able to access B's data members. Alternatively, consider
something like this:

class ICallback {
public:
    virtual void HaveI(int i) = 0;
};

class A {
    ICallback* callback;
public:
    A(ICallback* ?) : callback(c) {}
    void ReadI() {int i = something(); callback->HaveI(i);}
};

class B : public ICallback {
    A m_a;
    B() : m_a(this) {}
    void HaveI(int i);
};

--
With best wishes,
    Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925