On Sep 7, 2004, at 1:36 PM, Frank Maddin wrote:

If I add a function to both Hello and World, from the “Hello World” example, called alternate() and set up signals like so:

 

            boost::signal sig_h;

            boost::signal< void (World*) > sig_w;

 

            sig_h.connect(boost::mem_fn(&Hello::alternate)); 

            sig_w.connect(boost::mem_fn(&World::alternate));

 

I have to define two different signals since the object type is part of the signal definition.  Is there an easy way to get a single signal to call the alternate() function of both Hello and World?

Without Hello and World being related (e.g., sharing a base class declaring a virtual "alternate" function), I don't see a way. Still, when you call these signals you have to know whether you have a "Hello" in hand or a "World"; perhaps you wanted to store the "Hello" and "World" objects within the signal, and pass it no arguments? For instance, boost::signal sig; sig.connect(boost::bind(&Hello::alternate, &my_hello)); sig.connect(boost::bind(&World::alternate, &my_world));

I recently found this signal slot solution.  http://www.codeproject.com/cpp/ElmueSignalsandSlots.asp

There is an extra level of indirection here, in that you define a slot that points to the function.  I’d be interested to see what people on this forum think about this solution. If I understand things correctly, this solution doesn’t seem to fit into the categories listed in the “Choice of Slot Definitions” docs.

I don't see much benefit to extra (explicit) level of indirection here. Most slots are short bind expressions that don't ever need to be revisited, so it seems that it could be a bit challenging to determine where some slots should reside: always in the class containing the member function that is called (assuming one only uses member functions!)?

 Also a general signal slot question.  What are the main reasons why developers don’t use an inherited pure interface for every callback type?  A signal would be a template class with a list of pointers to the interface and some methods to operate on the list.  Not flexible enough?

You often want only one event out of five, so it's a pain to drop in all of the stubs needed to ignore the other events. Also, you don't get a choice of names to use: if the writer of the interface says it's "on_click", you must write a function named "on_click" even though you'd rather write something like "submit_current_query" that describes the action taken when the event fires, not the event itself (which is essentially irrelevant).

  Doesn’t scale well to larger uses?  Time spent calling virtual functions?  I saw this in the boost signal docs that might apply - “the use of a large number of small adaptor classes containing virtual functions has been found to cause an unacceptable increase in the size of executables (polymorphic class types require more code than non-polymorphic types)”.

This is more of an implementation detail; using function pointers instead of virtual functions tends to reduce the amount of code generated for libraries such as Boost.Signals or Boost.Function. Doug