Re: [Boost-users] [signals] thread-safe post-disconnect() cleanup

23 Feb 2007

      Timmo Stange wrote:
...
Peter Dimov wrote:
...
The slot_call_iterator idiom is another thing that I don't like in
the original Signals design; it feels too clever for its own good,
and is there to support a nice way of using combiners. I've nothing
against combiners per se, but the majority of the signal uses do not
involve slot returns or combinations thereof.
If I had to redesign the library, I'd explore a simple void
operator() that just calls the slots, and a separate
template<class Cm> void call_and_combine( Cm & cm, ... );
that calls cm( s( ... ) ) for each slot s.
Actually, a slightly better interface would be

    template St call_and_combine( Fn fn, St st, ... );

that calls st = fn( st, slot( ... ) ). The cm overload could make good use 
of an rvalue reference; in its absence, by-value seems a bit better since it 
enables bind() to be passed as cm.
...
That's what the documentation describes as the "push method" and
is probably also with what I had come up with as a combiner solution,
mostly because I personally wouldn't use combiners that often.
Indeed. According to the docs, there are two downsides to the push method: 
the need to keep state and the inability to abort the invocation.

The first is solved with using an 'accumulate-like' call_and_combine or with 
using bind().

int max( int x, int y ) // std::max<int> is overloaded :-/
{
    return x < y? y: x;
}

int r = signal.call_and_combine( max, INT_MIN );

// or

void max( int & x, int y )
{
    if( x > y ) x = y;
}

int r = INT_MIN;
signal.call_and_combine( bind( max, ref( r ), _1 ) );

The second can be solved with designating a "signal abort" exception.

void max_until_K( int & x, int y, int K )
{
    if( x > y ) x = y;
    if( x > K ) throw signal_abort();
}

int r = INT_MIN;
signal.call_and_combine( bind( max, ref( r ), _1, 100 ) );

This allows reusing the same signal with different combiners and parameters, 
which is probably an academic advantage.

Finally, there's always the option of just giving the users a copy of the 
slots and letting them obtain and combine the results at their own pace. :-)