How to inform data-changed in static function?

[Bayesky]

I have suffered from some trouble for a long time when I used hiredis in my app.
In order to get server response, I have to write a static function as a callback function. In the function, I change some data and emit signal to inform my main thread to update display.
The question is the emit signal must be static in callback function, but I couldn't use signal&slot(connect) when I declare the signal static.So how to inform outside in static function?

[J.Hilk]

hi @Bayesky

take look at his rather old thread
https://forum.qt.io/topic/87302/sending-static-signals-is-it-possible/5

the possible solution, the one from @VRonin with the big warning, may be of use.

[VRonin]

Actually there is a much cleaner solution. Say the static callback function must have signature void callBackFunction(int argument); then you declare a static function

void fullCallBack(int argument, MyObject* emitter){
emitter->mySignal(argument);
}

and then use std::bind to create a function that matches the required signature:

MyObject* object = new MyObject;
server->sendSomething("hello world",std::bind(fullCallBack,std::placeholders::_1,object));

[Bayesky]

@VRonin Thank you very much.
I have define a static function with 4 parameters

void Widget::fullCallBack(redisAsyncContext *c, void *r, void *priv, Widget* emitter){
    //print something and get some values
    emitter->mySignal(); //emit signal
}

And then I use the following codes to shrink parameters to 3.

Widget *w = new Widget;
std::function<void(redisAsyncContext *c, void *r, void *priv)> fun2;
fun2 = std::bind(fullCallBack, std::placeholders::_1, std::placeholders::_2, std::placeholders::_3, w);

When I use fun2 to replace subCallback,

redisAsyncCommand(c, fun2,  (char*) "sub"*, "SUBSCRIBE foo");//send SUB command

an error occur which said:

Widget.cpp:43:5: error: no matching function for call to 'redisAsyncCommand'
async.h:132:5: note: candidate function not viable: 
no known conversion from 'std::function<void (redisAsyncContext *, void *, void *)>' 
(aka 'function<void (redisAsyncContext *, void *, void *)>') to 'redisCallbackFn *'
(aka 'void (*)(struct redisAsyncContext *, void *, void *)') for 2nd argument

Are there any mistakes in use? Thank you.

[VRonin]

@Bayesky said in How to inform data-changed in static function?:

redisAsyncCommand

Can you show us the signature of redisAsyncCommand? Is it a C function?

[Bayesky]

@VRonin Yes, it is a C function.

int redisAsyncCommand(
  redisAsyncContext *ac, redisCallbackFn *fn, void *privdata,
  const char *format, ...);

The function redisAsyncConnect can be used to establish a non-blocking connection to Redis. It returns a pointer to the newly created redisAsyncContext struct.
The privdata argument can be used to curry arbitrary data to the callback from the point where the command is initially queued for execution.

[VRonin]

EDIT: See @SGaist answer below for the correct solution

Yeah, the problem is that C doesn't do objects (aka functors) unfortunately. You'll have to do something ugly, declare a static Widget* emitter; and

void fullCallBack(redisAsyncContext *c, void *r, void *priv){
    //print something and get some values
    emitter->mySignal(); //emit signal
}

then you can use

emitter=w;
redisAsyncCommand(c, fullCallBack,  (char*) "sub"*, "SUBSCRIBE foo");

It looks super ugly in C++ but you need to adapt the style to what C can handle

[Bayesky]

@VRonin I attempt to use a static Widget* emitter; as a private variable. And then

void fullCallBack(redisAsyncContext *c, void *r, void *priv){
    //print something and get some values
    //no matter declare emitter = new Widget; or not
    emitter->mySignal(); //emit signal
}

Another error occur which says

:-1: error: symbol(s) not found for architecture x86_64
:-1: error: linker command failed with exit code 1 (use -v to see invocation)

[SGaist]

Hi,

The hiredis library has a Qt example.

Hope it helps

[Bayesky]

@SGaist @VRonin @J-Hilk
Thanks a lot. It works.