Re: [boost] [signals] making Assignable and CopyConstructable

From: Ulrich Eckhardt (uli_at_[hidden])
Date: 2005-07-27 13:36:21

On Tuesday 26 July 2005 22:45, Rebooted wrote:
> In my program I need to store a map of signals, with an integer id
> corresponding to a signal in the map.

Some kind of map<id_type, signal_type>.

> Objects can request to be informed when another a specific message is
> emitted by connecting a member function to the signal in this map at
> the index corresponding to that message type.

IOW, objects can subscribe to events.

> However, since boost::signal inherits noncopyable, it cannot be stored
> in a map. What are the reasons for signal being noncopyable, and is
> there any chance this could be changed in the future?

(I'm walking on unknown grounds here, since I use libsigC++ instead, but last 
time I looked, they were sufficiently similar - maybe some names are 
different.) 
The problem is that when you connect a slot to a signal, you get a reference 
to that connection. Using that reference, you can also separate the 
connection again. Now, what would happen when a signal was copyable? You 
could end up with several connections, so which one would you disconnect? 
Also, copying is not a trivial operation, so you'd rather avoid it.

Ideas:
Use Boost's function instead, which is copyable. Using bind, you can achieve 
almost the same as with slots/signals. The exception is the automatic 
disconnecting when the target object is destroyed, but that can also be 
achieved by using weak_ptr and a wrapper function (maybe lambda would 
eliminate even that).

Store a smart pointer instead of the signal itself. This would even speed up 
operations on the map.

Uli