Qt Connections: Using Lambda Expression Resets Passed int Variable (Weird)

Question

Consider the following code:

int counter = 0;

QTimer* timer = new QTimer(this);

connect(timer, &QTimer::timeout, [this, &counter]() mutable {
    counter++;
    qDebug() << counter;
});

timer->start(500);

Expected:

1
2
3
4
...

Output:

32766 (a random number)
...

Is there some undefined stuff going on here? I can't find anything about this effect.

You capture a reference to a local variable. Once that local variable dies (because you leave the scope it was defined in) you have a dangling reference. — Max Langhof
– Max Langhof, Commented Jul 31, 2019 at 16:36
That was the solution! After making "counter" into a class member variable, it never died, and no more dangling reference. Thanks! — cid
– cid, Commented Jul 31, 2019 at 16:42

Yakk - Adam Nevraumont · Accepted Answer · 2019-07-31 16:42:46Z

4

&counter in the [] means you are capturing counter by-reference in the lambda.

If the variable int counter has gone out of scope (as local variables are wont to do), then this means you have a dangling reference; using it is undefined behavior.

The easy way to fix this is to just capture counter by value -- [this, counter] instead of [this, &counter]. Then the lambda will own its own copy of the state of counter. As it is mutable, it will have permission to edit its own state.

answered Jul 31, 2019 at 16:42

Yakk - Adam Nevraumont

280k31 gold badges367 silver badges585 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Qt Connections: Using Lambda Expression Resets Passed int Variable (Weird)

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related