I have found that boost::signals2 uses sort of a lazy deletion of connected slots, which makes it difficult to use connections as something that manages lifetimes of objects. I am looking for a way to force slots to be deleted directly when disconnected. Any ideas on how to work around the problem by designing my code differently are also appreciated!
This is my scenario: I have a Command class responsible for doing something that takes time asynchronously, looking something like this (simplified):
class ActualWorker {
public:
boost::signals2<void ()> OnWorkComplete;
};
class Command : boost::enable_shared_from_this<Command> {
public:
...
void Execute() {
m_WorkerConnection = m_MyWorker.OnWorkDone.connect(boost::bind(&Command::Handle_OnWorkComplete, shared_from_this());
// launch asynchronous work here and return
}
boost::signals2<void ()> OnComplete;
private:
void Handle_OnWorkComplete() {
// get a shared_ptr to ourselves to make sure that we live through
// this function but don't keep ourselves alive if an exception occurs.
shared_ptr<Command> me = shared_from_this();
// Disconnect from the signal, ideally deleting the slot object
m_WorkerConnection.disconnect();
OnComplete();
// the shared_ptr now goes out of scope, ideally deleting this
}
ActualWorker m_MyWorker;
boost::signals2::connection m_WorkerConnection;
};
The class is invoked about like this:
...
boost::shared_ptr<Command> cmd(new Command);
cmd->OnComplete.connect( foo );
cmd->Execute();
// now go do something else, forget all about the cmd variable etcetera.
the Command class keeps itself alive by getting a shared_ptr to itself which is bound to the ActualWorker signal using boost::bind.
When the worker completes, the handler in Command is invoked. Now, since I would like the Command object to be destroyed, I disconnect from the signal as can be seen in the code above. The problem is that the actual slot object is not deleted when disconnected, it is only marked as invalid and then deleted at a later time. This in turn appears to depend on the signal to fire again, which it doesn't do in my case, leading to the slot never expiring. The boost::bind object thus never goes out of scope, holding a shared_ptr to my object that will never get deleted.
I can work around this by binding using the this pointer instead of a shared_ptr and then keeping my object alive using a member shared_ptr which I then release in the handler function, but it kind of makes the design feel a bit overcomplicated. Is there a way to force signals2 to delete the slot when disconnecting? Or is there something else I could do to simplify the design?
Any comments are appreciated!
I ended up doing my own (subset) implementation of a signal, the main requirement being that a slot should be destroyed by a call to connection::disconnect().
The implementation goes along the lines of the signal storing all slots in a map from slot implementation pointer to a shared_ptr for a slot implementation instead of a list/vector, thereby giving quick access to individual slots without having to iterate over all slots. A slot implementation is in my case basically a boost::function.
Connections have a weak_ptr to the internal implementation class for the signal and a weak_ptr to the slot implementation type to allow the signal to go out of scope and to use the slot pointer as the key into the signal map as well as an indication on whether the connection is still active (can't use a raw pointer as that could potentially be reused).
When disconnect is called, both of these weak pointers are converted to shared_ptrs and if both of these succeed, the signal implementation is asked to disconnect the slot given by the pointer. This is done by simple erasing it from the map.
The map is protected by a mutex to allow for multithreaded use. To prevent deadlocks, the mutex is not held while calling the slots, however this means that a slot may be disconnected from a different thread just prior to being called by the signal. This is also the case with regular boost::signals2 and in both of these scenarios one needs to be able to handle a callback from a signal even after one has disconnected.
To simplify the code for when the signal is fired, I am forcing all slots to be disconnected during this. This is different from boost::signals2, that does a copy of the list of slots before calling them in order to handle disconnections/connections while firing the signal.
The above works well for my scenario, where the signal of interest is fired very seldom (and in that case only once) but there are a lot of short-lived connections that otherwise use up a lot of memory even when using the trick outlined in the question.
For other scenarios, I've been able to replace the use of a signal with just a boost::function (thus requiring that there can only be a single connection) or just by sticking with the workaround in the question where the listener itself manages its lifetime.