QVector COW thread-safety [Qt 4.8]

[JasonC]

Is the copy-on-write behavior of QVector threadsafe?

In particular, if I pass a copy-constructed QVector to a thread, the COW behavior indicates the data itself won't actually be copied until somebody does a write operation on one of instances. In a multi-threaded application is this transparent or does it require extra synchronization?

For example (representative only):

class Example : public QThread {
public:
    Example (QVector<int> data) : data_(data) { 
    }
    public void run () { 
      while (true)
        doReadOnlyThingsWith(data_);
    }
private: 
    QVector<int> data_;
}
 
void elsewhere () {
    QVector<int> data = ...;
    new Example(data)->start();
    new Example(data)->start();
    new Example(data)->start();
    new Example(data)->start();
    writeThingsToDataThatShouldntBeSeenByThreads(data);
}

class Example : public QThread {
public:
    Example (QVector<int> data) : data_(data) { 
    }
    public void run () { 
      while (true)
        doReadOnlyThingsWith(data_);
    }
private: 
    QVector<int> data_;
}

void elsewhere () {
    QVector<int> data = ...;
    new Example(data)->start();
    new Example(data)->start();
    new Example(data)->start();
    new Example(data)->start();
    writeThingsToDataThatShouldntBeSeenByThreads(data);
}

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In the above example, multiple threads are using the data (read-only) while the main thread may go on to modify it, but the goal is that the modifications have no effect on the threads (that's why I passed it by value in the first place). Intuitively this should work, but is that really OK? Is the underlying copy-on-write threadsafe?

The reason I ask is I'm trying to track down an infrequent seg fault and the debugger is not yielding helpful information. The only thing I noticed is that some of the threads are in QVector::realloc() when the program crashes, so I'm focusing on my usage of QVector.

Thanks,
J