Is passing of data with one signal to multiple slots safe in Qt?

Question

My Qt application is continuously receiving data packets from server side and on basis of data, it is constructing object. I have to pass these objects(one at a time) to three different widgets. So, I have connected one signal to slots of all three widgets but I doubt if it is safe to pass objects like that? Suppose the data packets arrive at fast rate, will these get overwritten with new data packet? Can the objects get corrupted due to this or can I loose some packets or data while I am processing the older packet.

Is there a mechanism to ensure that overrun does not happen. Also, will there be some throttling if the data packets at a very fast rate.

Answer 1

Connecting a single signal to multiple slots is a design feature of Qt.

All (almost?) Qt value classes are reference counted, so it's safe to pass them around as much as you like. They are mostly also "copy on write" so you can freely make copies of them where ever you want with no real performance penalty.

Like any other multithreaded code, you need to be careful if multiple threads are writing to the same object at the same time.

Answer 2

Qt's signals and slots are handled in the event loop of the program. When a signal is emitted, calls to any of the connected slots are queued and will be handled during the event loop's tick. Overhead for emitting a signal is quite low; you can emit around 2,000,000 signals a second when connected to a single slot, and 1,200,000 when connected to 2 slots on an i586-500, according to Qt's docs. Handling of the incoming data is likely to be a much bigger part of your program's response time compared to the time spent emitting signals. As long as your data object construction can keep up with the rate of the info coming in from the network, you will be able to emit signals with no problem. The signals will be added to the receiving object's queue to be handled through the next pass of the event loop, so no signals will be lost or overwritten.

If you're worried about multiple receivers modifying the packet data, as long as you are emitting a unique copy of the incoming network data by passing the data object by value to the signal as opposed to a reference or a pointer, you can connect to multiple slots without worrying about your slots modifying the data.

I have used the signals and slots mechanism to handle incoming network data to great effect for a group school project. It uses a UDP protocol to communicate with clients to implement a multiplayer shooter reminiscent of asteroids. I encountered no problems using signals and slots to deliver packet data around the various classes of the server. Most of the problems I did encounter were due to the algorithmic simplicity of the game's netcode. (Everything is updated every packet, no client-side lag compensation, etc) If you're interested, the code's public for viewing here: http://foxx.arksaw.com/svn/tis/ Of interest are the "netlib" "server" and "client" directories.

Answer 3

I'm not sure with the network programming part of this, but as far as signals and slots go you should be able to connect one signal to as many slots as you need.