Qt atomic int get and immediately set

Question

I need to get current value stored in atomic int (QAtomicInteger I suppose) and immedeately reset it to 0. This to be the counter. While one thread incrementing this counter, I am to get the value in another thread at some moment and immediately reset this counter to be sure every increment is counted.

How can I do this?

Answer 1

Solution

In order to get the value stored in a QAtomicInt and immediately assign another value to it, use QAtomicInteger::fetchAndStoreRelaxed.

Note: You could also use another of the available fetchAndStore methods, depending on the memory ordering you need.

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Example

As for the bigger picture of how you want to use this value, I have prepared an example for you.

#include "Counter.h"
#include <QTimer>

Counter::Counter(QObject *parent) :
    QObject(parent),
    m_counter(new QAtomicInt()),
    m_counting(false)
{

}

int Counter::setValue(int value) const
{
    return m_counter->fetchAndStoreRelaxed(value);
}

void Counter::countFrom(int value, int delay)
{
    if (m_counting)
        return;

    m_counting = true;
    m_counter->store(value);

    auto *timer = new QTimer();

    timer->start(delay);

    connect(timer, &QTimer::timeout, [this](){
        m_counter->operator ++();
        changed(m_counter->load());
    });
}

The full example consisits of one thread incrementing a counter. It also allows to read the value of the counter on a click of a button, immediately resetting the counter to zero.

The complete code of this example could be downloaded from GitHub.