C++ lambda const value reference got deleted in thread

Question

I have encountered the following situation:

void Plugin::sendMessage(const QString& jid, const QString &message) {
    qDebug() << "SENDING TO JID1: " << jid;
    QtConcurrent::run([&, this]() {
        qDebug() << "SENDING TO JID2: " << jid;
    }
}

produces next output:

SENDING TO JID1: "test@xmpp.org"

SENDING TO JID2: "

and then it crashes. It looks like jid is no longer exists in the lambda, but why? How then can I use the variables by references in this code?

The problem is most likely that you have no control over when the lambda function in the `QConcurrent` is executed, and that might mean that the string that is referenced by `jid` have been destructed. Capture by value instead. — Some programmer dude, Jan 24 '15 at 12:19
Yes, it works if I capture by value, just wondered why compiler can't prolong existence of the reference until lambda execution. — VP., Jan 24 '15 at 12:25
Because the compiler have no idea what's happening, or when (if ever) the reference will be needed. — Some programmer dude, Jan 24 '15 at 12:32
Okay, you may answer the question and I'll accept it. If you don't want too I may delete the question. — VP., Jan 24 '15 at 12:33

score 0 · Accepted Answer · edited May 23 '17 at 12:10

If you are coming from Java background, note that references in C++ and Java mean something very different. In Java references are garbage collected and therefore it is easy to prolong their lifetime as needed. In C++ there is no garbage collection and a reference variable lifetime has little relation to the lifetime of the referenced object.

See also C++ References and Java References or Programmers Why do C++ and Java both use the notion of “reference” but not in the same sense? for some more explanation.

C++ lambda const value reference got deleted in thread

1 Answers1