QtConcurrent::run() for a member function does not use the original instance

Question

I want to use QtConcurrent::run() for a member function, but it seems that it doesn't use the pointer to the instance. Instead, it looks like the default constructor is called

#include <QObject>
#include <QDebug>
#include <QtConcurrent>

class Foo : public QObject
{
    Q_OBJECT
public:
    Foo(int n = 0):n(n){}
    Foo(const Foo & f):Foo(f.n){}

    void foo(){qDebug() << "Foo " << n;}
    void bar(){QtConcurrent::run(this, &Foo::foo);}

private:
    int n;
};

void test(){
  Foo foo = Foo(2);
  foo.foo();
  foo.bar();
  QtConcurrent::run(&foo, &Foo::foo);
  QtConcurrent::run(&foo, &Foo::bar);
}

And the results of running test() are :

Foo  2
Foo  0 // Should be a 2
Foo  0 // Should be a 2
Foo  0 // Should be a 2

Edit : My instance indeed went out of scope. This code works fine

void test(){
    Foo * foo = new Foo(2);
    foo->foo();
    foo->bar();
    QtConcurrent::run(foo, &Foo::foo);
    QtConcurrent::run(foo, &Foo::bar);
}

`qDebug()` is NOT thread-safe. You must not use it from different threads. — Dmitry Sazonov, Sep 23 '15 at 10:00
@UmNyobe because my class in my real problem inherits from QObject — B. Decoster, Sep 23 '15 at 14:07
"does not use the original instance" ... because that instance isn't there anymore — Kuba hasn't forgotten Monica, Sep 23 '15 at 14:10
@KubaOber http://doc.qt.io/qt-5/qdebug.html#details - this class is not thread-safe or reentrant. And: http://stackoverflow.com/questions/22527253/is-qdebug-thread-safe — Dmitry Sazonov, Sep 23 '15 at 15:36
@SaZ The class doesn't have to be thread-safe nor reentrant, because you are not passing it anywhere. `qDebug()` gives you a fresh instance of the message logger, each and every time. It is perfectly fine to use such separate instances in multiple threads, in parallel. It is not defined whether such use introduces a synchronization point, so certainly you might want to assume that worst case there's a lock that `qDebug()` acquires. This is probably a pessimistic assumption, though. — Kuba hasn't forgotten Monica, Sep 23 '15 at 17:42

score 3 · Accepted Answer · answered Sep 23 '15 at 09:25

Making a call to an object which has been destructed is undefined behavior. What is happening is that when QtConcurrent::run effectively execute Foo::bar, the parameter foo has been destructed.

If I try to replicate your code I have:

Foo  2 
Foo  1730312062 
Foo  1730312062

Issue come from the fact that the object foo is on the stack and will be invalid as soon as test return. Provide a greater lifetime to your object.

QtConcurrent::run() for a member function does not use the original instance

1 Answers1