Re: signals/slots? RE: [boost] any_function (callback) library

From: Doug Gregor (gregod_at_[hidden])
Date: 2001-04-09 22:38:44

On Monday 09 April 2001 11:02, you wrote:
> From: Doug Gregor [mailto:gregod_at_[hidden]]
>
> > This has been discussed, and any_function (or "function" as it
> > will likely be renamed) is at the very least a starting point
> > for a full signals/slots library.
>
> Sorry if this is too big a question, but could you elaborate a bit on
> possible uses for a signals/slots library and why you'd use signals/slots
> over other methods?

Signals/slots are great when you need multicasting callbacks/events where the 
source(s) and/or target(s) may be connected or destroyed at any time - 
signals/slots manage all of this internally. For instance, if I have:

// at global scope
any_function<int, float, float> foo;

// later on, in a member function
foo = bind(this, &Bar::my_foo);

If the object represented by "this" is destroyed, the line:

foo(5.5, 33.3);.

will cause a segmentation fault because any_function does not know that the 
function object it is pointing to has disappeared. With a signal/slot system, 
the slot will notify any signals it is attached to when it disappears. So the 
code would be rewritten as:

// global scope
signal<int, float, float> foo;

// later
class Bar {
  slot<int, float, float> foo_slot;

  // in a member function
  foo_slot = bind(this, &Bar::my_foo);
  foo = foo_slot;
};

When the slot object foo_slot is destroyed on Bar's destruction, it will 
unregister itself from the signal "foo" automagically. Likewise, if the 
signal is destroyed it notifies all of the slots it points to. 

Additionally, signals will have the ability to decide how to combine results 
from any number of different slot targets. For instance, the "foo" signal 
might have 100 slots connected to it, so it must decide how to take the 100 
integer results and return a single result. 

The most often cited use of signals/slots (in my experience) is for GUI 
systems, which require many events and in which many widgets are created or 
destroyed. Tracing and updating lists of any_function objects would be a 
nightmare.

        Doug