Singleton initializing twice, std::call_once also running twice

Question

I have following simple singleton, the std::call_once is just to make sure that things are broken, I intend to remove it as soon as I have this working.

std::once_flag flag1;

ProfileModel* ProfileModel::instance()
{
    std::call_once(flag1, [](){ qDebug() << "Simple example: called once\n"; });
    static ProfileModel self;
    qDebug() << &self ;
    return &self;
}

Looks simple, and from what I know from C++ this should work. but I'm having two pointers returned by this call, and call once also prints twice.

Simple example: called once
Constructing Konsole::ProfileModel(0x7ff7378a31f0)
Simple example: called once
Constructing Konsole::ProfileModel(0x5592aeb87160)

The only thing I did that I'm not sure it matters, is that I have this inside of a static library, and I only call this twice, once inside of this library, once outside - in the main application.

Anyone can share a bit of light?

It's only defined once, right? It's only defined in the library, not also in the application? — user253751, Aug 20 '20 at 17:33
I don't see `qDebug() << &self` in the output. The output doesn't look like it was generated by the code shown. — François Andrieux, Aug 20 '20 at 17:36
just put a breakpoint in ProfileModel::ProfileModel and see where it is called — Jeffrey, Aug 20 '20 at 17:46
@FrançoisAndrieux the output was manually written, the original call was qDebug() "Constructing" << &self; I'll edit. — Tomaz Canabrava, Aug 20 '20 at 17:57
@Jeffrey I did, it's being called twice but shouldn't. I think this is the same issue as https://stackoverflow.com/questions/26547454/static-variable-is-initialized-twice . — Tomaz Canabrava, Aug 20 '20 at 17:57

score 1 · Answer 1 · answered Aug 20 '20 at 17:59

1

It's a duplicate of Static variable is initialized twice , with the difference that here it's not a static variable, but a method that has a static variable.

answered Aug 20 '20 at 17:59

Tomaz Canabrava

2,320
15
20

Singleton initializing twice, std::call_once also running twice

1 Answers1