Question

I am developing a custom exception, where I’d need a QString member. Something like:

class MyException
{
private:
    const QString fDescription;

public:
    MyException(QString desc);
};

MyException::MyException(QString desc) : fDescription(desc)
{}

When I try to use it:

if (isErrorEncountered)
{
    MyException e(QString("Descriptive message here..."));
    throw e;
}

I get a segmentation fault.
The symptoms are similar as described here:
Qt QString cloning Segmentation Fault

The SIGSEGV originates from QBasicAtomicInt::ref, coming from QString::QString(const QString &other).

It seems to me as if there is an attempt to copy an invalid QString within a copy constructor. It is my understanding, that the QString keeps a pointer to its content as long as there is a valid reference to it. If a copy of MyException instance is being made, is it not true that the temporary stack instance has not left the scope and the copy should succeed?

When I implement MyException without a QString member, everything work well.

Answer 1

The answer to the question is “Yes, a QString can safely be a member in an exception.”

The problems which I encountered were due to the fact that the object, acting as a source of data for the fDescription, had a bug. It implemented a getter method:

class Source
{
private: QString fData;
public:  QString GetData(void) { this->fData; }   // There is a missing return
}

Of course, the result of this call was the random value residing on the stack at the moment and hence the SIGSEGV.

The QString behaves as expected and with a valid reference to it, there are no problems using it in the exception.

The precise point where the SIGSEGV was raised was leaving the scope of the try block, where a copy of the exception is being constructed (remember: throw by value, catch by reference). The default copy constructor tries to create a shallow copy of all members and fails on accessing the invalid QString reference.