How can I delete a QSharedPointer

Question

I have written a wrapper around QTextEdit to use it like a QIODevice. I want to be able to use multiple wrapper with the same QTextEdit, so I may use different text color with each wrapper.

To make this wrapper thread-safe I added a QMutex to protect the usage of the QTextEdit. But I was thinking that I must use only one mutex to protect one QTextEdit.

I end up with the following implementation, using a QSharedPointer to protect QTextEdit.

texteditiodevice.h

class TextEditIODevice : public QIODevice
{
    Q_OBJECT

public:
    TextEditIODevice(QTextEdit * qTextEdit, QColor color, QObject * parent);

    virtual ~TextEditIODevice();

protected:
    qint64 readData(char *data, qint64 maxlen);

    qint64 writeData(const char *data, qint64 len);

private:
    /**
     * @brief Pointer to QTextEdit
     */
    QPointer<QTextEdit> textEdit;

    /**
     * @brief Text color
     */
    QColor color;

    /**
     * @brief Shared pointer to QTextEdit associated mutex
     */
    QSharedPointer<QMutex> mutex;

    /**
     * @brief Storage for QTextEdit associated mutexes
     */
    static QMap<QPointer<QTextEdit>, QSharedPointer<QMutex>> mutexes;
};

texteditiodevice.cpp

QMap<QPointer<QTextEdit>, QSharedPointer<QMutex>> TextEditIODevice::mutexes;

TextEditIODevice::TextEditIODevice(QTextEdit * qTextEdit, QColor color, QObject * parent) :
    QIODevice(parent),
    textEdit(qTextEdit),
    color(color)
{
    open(QIODevice::WriteOnly | QIODevice::Text);

    qRegisterMetaType<QTextCharFormat>("QTextCharFormat");
    qRegisterMetaType<QTextBlock>("QTextBlock");
    qRegisterMetaType<QTextCursor>("QTextCursor");

    if(mutexes.contains(textEdit))
        mutex = mutexes[textEdit];
    else
    {
        mutex = QSharedPointer<QMutex>(new QMutex());
        mutexes.insert(textEdit, mutex);
    }
}

TextEditIODevice::~TextEditIODevice()
{
}

qint64 TextEditIODevice::readData(char *data, qint64 maxlen)
{
    Q_UNUSED(data);
    Q_UNUSED(maxlen);
    return 0;
}



qint64 TextEditIODevice::writeData(const char *data, qint64 len)
{
    if(textEdit)
    {
        mutex->lock();
        const QColor lastColor = textEdit->textColor();
        textEdit->setTextColor(color);
        textEdit->append(QString(data));
        textEdit->setTextColor(lastColor);
        mutex->unlock();
    }

    return len;
}

I want to know where I can remove QSharedPointer instance from mutexes mapping so the QMutex will be deleted.

Thanks for your help

You have to rethink your architecture because you cannot do that. [QWidgets are not reentrant and must be used only from the main thread](http://doc.qt.io/qt-5/threads-qobject.html). Once you have fixed this you won't need mutexes anymore (only one thread) so the problem will be solved ! — dydil, Jul 04 '16 at 14:22
Write data is called in a slot, so it is called from the GUI Thread. But what if two signals are emitted at the same time ? — AMDG, Jul 04 '16 at 15:30
If your object is in the main thread and a signal is emitted from another thread then the signal is queued to be processed later by the main thread event loop, so no need to bother with a mutex :). This is however only if you connected your signal using auto (default) or queued connection. You can look at the section "Signals and Slots Across Threads" in the link I gave above for more information. — dydil, Jul 04 '16 at 15:41

score 1 · Answer 1 · answered Jul 04 '16 at 13:05

1

As long as the shared pointer is in static mutexes map, it will never be deallocated, and the lifetime of mutexes is the lifetime of the program.

If you want to actually delete a mutex, you have to remove it from the mutexes mapping.

answered Jul 04 '16 at 13:05

Some programmer dude

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Or use `QWeakPointer` in `mutexes` map. But I think `QWeakPointer` to null will still stay in map, even after object is deleted, you would have to clear out the null pointers. – Anže Jul 04 '16 at 13:15
I understand your point. I think i wasn't clear, I want some advice on when I should/can delete the mutex from mutexes mapping. – AMDG Jul 04 '16 at 13:30

How can I delete a QSharedPointer

texteditiodevice.h

texteditiodevice.cpp

1 Answers1