[Solved] Trouble with implementing Singleton

StephanWoebbeking

Thanks for the answer, I am aware of the shortcomings. Initially I had followed these concepts already, but while searching for the problem I tried to make the code more "open" to find the problem behind. Also the guide outlines pretty much exactly the same pattern that I have mentioned, so that's even more of a motivator to wonder what's wrong in the very short outline I have posted. So you see, where the syntactic error is located?

Stephan

sierdzio

You are using "this" to point to a static variable (which is available without object). I suspect that might make the compiler unhappy.

hskoglund

Not unhappy, I'd say instead "confusing", prefixing a static variable with this-> works the same as prefixing it with MainWindow:: or, if inside MainWindow class, not prefixing it at all.

The problem is that there's no instance of inst declared (it's just defined), that's why the linker returns the error. To get around it, for example declare it directly after the constructor, like this;
@
MainWindow* MainWindow::inst = 0;
@

StephanWoebbeking

AFAIK is a static variable always available in the instance context, just only - of course - not the other way around?!? Anyway, that's just one of the four "trials" to access the variable, if only that would be the location of protest from the compiler I would happily change it, but it complains about all 3 others as well. So how would I access that variable???

Stephan

hskoglund

Actually I think it's the linker that complains, because it can't find any location for your static inst variable (and why you need to declare an instance as in my example above).

Re. accessing the variable, so either of the 3 ways are fine: prefixing with MainWindow::, prefixing with this-> or not prefixing it at all.
(Of course only the first way is valid outside the MainWindow class :-)

StephanWoebbeking

If I change the definition according to my first post I only receive errors:

@
static MainWindow* MainWindow::inst;
@
-> Fehler: extra qualification 'MainWindow::' on member 'inst' [-fpermissive]
static MainWindow* MainWindow::inst;

@
static MainWindow* inst = 0;
@
-> Fehler: 'constexpr' needed for in-class initialization of static data member 'MainWindow* MainWindow::inst' of non-integral type [-fpermissive]

@
static MainWindow* MainWindow::inst = 0;
@
-> Fehler: extra qualification 'MainWindow::' on member 'inst' [-fpermissive]
-> Fehler: 'constexpr' needed for in-class initialization of static data member 'MainWindow* MainWindow::inst' of non-integral type [-fpermissive]

@hskoglund you recommend to declare it without "static", is that intentionelly? If you vote for static as well, then isn't it exactly the way I posted it at the beginning?

Stephan

hskoglund

Hi, to make it work you'll have to declare that variable inst 2 times, first inside your MainWindow class (in the .h file):
@
static MainWindow* inst; // My Singleton pointer as far as I understand my code...
@

Next, you'll have to write it one more time to create an instance of inst, easiest as I write above, insert it just after the constructor:
@
...
if ( !inst ) this->inst = this;
if ( !this->inst ) MainWindow::inst = this;
ui->setupUi(this);
}

MainWindow* MainWindow::inst = 0; // <--- good place

MainWindow::~MainWindow()
{
delete ui;
}
@

StephanWoebbeking

Wow, that works, thanks a lot for the hint! I must admit I haven't understood in the first place, I forgot why I have to declare it twice even I thought I knew the difference between declaration and definition, but here I missed out. Have to re-read some stuff I guess.

Thanks for your patience!
Stephan

SGaist

Hi,

Out of curiosity, are your callback called from another thread ?

StephanWoebbeking

Hoi,

yes, indeed, that's why I implemented the Singleton. I receive events from a dll which facilitates an own thread. This one wants to call back on a standard C function which was subject to another thread I opened here and it's also already solved. But from that function obviously I want to feed that information back into the "object-oriented system" anyhow, so at one point I need a static pointer to an instance. That's exactly this point we have discussed here.

I was actually missing the "double" definition described by hskoglung above, after I added that everything was fine.

Stephan