Do signal of global object could be connect to slot of other object?

cokefish

Do signal of global object could be connect to slot of other object?
I found that the signal emitted could not trigger slot in other object.

JKSH

Hi @cokefish, and welcome to the Qt Dev Net!

@cokefish said in Do signal of global object could be connect to slot of other object?:

Do signal of global object could be connect to slot of other object?
I found that the signal emitted could not trigger slot in other object.

Please describe how you create the global object.

Remember: you must create the QCoreApplication/QGuiApplication/QApplication before you create any other QObject.

cokefish

@JKSH
I defined it out of main().

JKSH

@cokefish said in Do signal of global object could be connect to slot of other object?:

@JKSH
I defined it out of main().

That's too vague.

Please post your code -- show us how you created the global object, and how you connected the signals.

VRonin

connect is a static method so you can invoke QObject::connect() directly in the main

kshegunov

Yes, but I'm pretty sure JKSH is worried about the order of creation of QObjects instead. :)

VRonin

Ok, thanks, got it.

So the solution is declare a global QObject pointer instead of QObject instance and create the object on the heap from main

Are you 110% sure you need a global object? it's almost never a good thing

kshegunov

Well, the "cleanest" solution I've come up with is to do what QCoreApplication does - set the reference in the constructor, e.g:

class MyGlobalObject
{
public:
    MyGlobalObject()
    {
        Q_ASSERT(!reference);
        reference = this;
    }

    static MyGlobalObject * instance()
    {
        return reference;
    }
private:
    static MyGlobalObject * reference;
};

MyGlobalObject * MyGlobalObject::reference = nullptr; //< This goes in the cpp obviously.

Then one can create the object wherever convenient (i.e. main()'s stack comes to mind) and get the instance everywhere:

int main(int argc, char ** argv)
{
     QApplication app(argc, argv);
     MyGlobalObject global; //< Ensures the global object is created after the root QObject, and destroyed before

     // ...
     return QApplication::exec();
}

However using global objects is grossly discouraged for a number of reasons, discussed numerous times in this forum and elsewhere.