replacing signals and slots with callback functions

[meena]

Hi

I wanted to replace the signals and slots in my application to callback functions.
example QObject::connect(c,SIGNAL(sendSignal2(void *)), c ,SLOT(Some_Function(void *)));
how to do the same functionality with callback... how to do this?

[antialias]

Why would you want to replace the signal/slot mechanism with callbacks?

[meena]

just wanted to check wether callbacks are faster than signals and slots.. as in my application they are taking much of a time..

is callback is faster than signals/slots? anybody have tested it? i wanted to test the same.. how to replace the code please help

[aamer4yu]

. as in my application they are taking much of a time..

Dont think they can be slow.. how much time are they taking ??

If you want to use callbacks,, it will be your design. I dont think Qt does that for you.

[meena]

They are taking around 550 mili seconds ... i understand that qt does not have anything for callbacks, but i wanted a help to replace those above statements.

[squidge]

In a way, signals/slots are a callback (direct connection), unless its not possible, in which case they use events (queued connection).

If you want to use callbacks you will have to implement the functionality yourself (eg, pass the functinos pointers to your relevent objects)

[antialias]

The real reason Qt got off the ground in the first place is because the whole callback issue got ugly in other frameworks.
By reintroducing them you'd destroy the thing Qt is best at.

Signals and slots are marginally slower than callbacks, but they are typesafe.

[mattc]

500 ms = half a second? That cannot be true, please measure again. The emit statement is just a function call, so I guess you are timing your own function.

As someone as pointed out, when you connect() the signals you can pass "Qt::QueuedConnection" to speed up the emit (the function will be called later by the event loop)

Connect: http://doc.trolltech.com/4.6/qobject.html#connect
Qt::QueuedConnection: http://doc.trolltech.com/4.6/qt.html...ctionType-enum

[SixDegrees]

The difference in execution speed between signals and slots is measured in a handful of milliseconds at most. In other words, it will be unnoticeable in all but the most pathological programs.

Rather than thrash around trying to replace an elegant software system with one that is error prone, buggy and semantically ugly - on a good day - and which is almost certainly not causing a significant slowdown - try running your application through a profiler to determine where it is spending it's time, and why.

[meena]

thanks everybody for replying, can u suggest/give some profiling tool and how to use them

[squidge]

That depends on your processor, some only work on Intel, some only on AMD, and some on both.

Intel used to provide a free one, but I think it's now commercial, called VTune.
AMD provide CodeAnalyst, which I think is still free.

then there's lot of third party ones which cost varying amounts.

If your happy with CLI, Visual studio comes with a free one, as does GCC.

[meena]

I am working on Linux, (32 bit fedora), My processor is AMD, I need to get the profiling data for qt project which i will be using for ARM 9 (I have arm cross compiler/arm tool chain , 32 bit), anything specific for me?

[tbscope]

Try valgrind or oprofile etc.
If you're really adventurous, systemtap.

These tools all come with a high learning curve though.

[yakin]

The Qt documentation says the following to the run time difference between callbacks and signal/slots:

"Compared to callbacks, signals and slots are slightly slower because of the increased flexibility they provide, although the difference for real applications is insignificant. In general, emitting a signal that is connected to some slots, is approximately ten times slower than calling the receivers directly, with non-virtual function calls. This is the overhead required to locate the connection object, to safely iterate over all connections (i.e. checking that subsequent receivers have not been destroyed during the emission), and to marshall any parameters in a generic fashion. While ten non-virtual function calls may sound like a lot, it's much less overhead than any new or delete operation, for example. As soon as you perform a string, vector or list operation that behind the scene requires new or delete, the signals and slots overhead is only responsible for a very small proportion of the complete function call costs.

The same is true whenever you do a system call in a slot; or indirectly call more than ten functions. On an i586-500, you can emit around 2,000,000 signals per second connected to one receiver, or around 1,200,000 per second connected to two receivers. The simplicity and flexibility of the signals and slots mechanism is well worth the overhead, which your users won't even notice."

http://doc.qt.nokia.com/4.6/signalsandslots.html

[squidge]

I am working on Linux, (32 bit fedora), My processor is AMD, I need to get the profiling data for qt project which i will be using for ARM 9 (I have arm cross compiler/arm tool chain , 32 bit), anything specific for me?

gprof - http://www.cs.utah.edu/dept/old/texi...prof.html#SEC2

[meena]

Thanks all of you, i will try to do profiling.

[meena]

gprof is working fine with qt projects, but the project where there are external libraries (for sensors which are connected to the board) included in the project there gmon.out is not getting created (Those libraries are compatible for arm, and i have arm tool chain, have arm-linux-gprof installed), in the project file i have included the lines
CONFIG += DEBUG
QMAKE_CXXFLAGS_DEBUG += -pg
QMAKE_LFLAGS_DEBUG += -pg
why gmon.out is not getting created..? are there anything extra i need to do?