src_corelib_thread_qsemaphore.cpp

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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
 
#include <QSemaphore>
#include <QSemaphoreReleaser>
#include <chrono>
 
void examples()
{
    {
        //! [0]
        QSemaphore sem(5);      // sem.available() == 5
 
        sem.acquire(3);         // sem.available() == 2
        sem.acquire(2);         // sem.available() == 0
        sem.release(5);         // sem.available() == 5
        sem.release(5);         // sem.available() == 10
 
        sem.tryAcquire(1);      // sem.available() == 9, returns true
        sem.tryAcquire(250);    // sem.available() == 9, returns false
        //! [0]
    }
 
    {
        //! [1]
        QSemaphore sem(5);      // a semaphore that guards 5 resources
        sem.acquire(5);         // acquire all 5 resources
        sem.release(5);         // release the 5 resources
        sem.release(10);        // "create" 10 new resources
        //! [1]
    }
 
    {
        //! [2]
        QSemaphore sem(5);      // sem.available() == 5
        sem.tryAcquire(250);    // sem.available() == 5, returns false
        sem.tryAcquire(3);      // sem.available() == 2, returns true
        //! [2]
    }
 
    {
        //! [3]
        QSemaphore sem(5);            // sem.available() == 5
        sem.tryAcquire(250, 1000);    // sem.available() == 5, waits 1000 milliseconds and returns false
        sem.tryAcquire(3, 30000);     // sem.available() == 2, returns true without waiting
        //! [3]
    }
 
    {
        using namespace std::chrono_literals;
 
        //! [tryAcquire-QDeadlineTimer]
        QSemaphore sem(5);                          // sem.available() == 5
        sem.tryAcquire(250, QDeadlineTimer(1000));  // sem.available() == 5, waits 1000 milliseconds and returns false
        sem.tryAcquire(3, QDeadlineTimer(30s));     // sem.available() == 2, returns true without waiting
        //! [tryAcquire-QDeadlineTimer]
    }
 
    QSemaphore sem(5);
 
    {
        //! [4]
        // ... do something that may throw or return early
        sem.release();
        //! [4]
    }
 
    {
        //! [5]
        const QSemaphoreReleaser releaser(sem);
        // ... do something that may throw or early return
        // implicitly calls sem.release() here and at every other return in between
        //! [5]
    }
 
    {
        bool someCondition;
 
        //! [6]
        { // some scope
            QSemaphoreReleaser releaser; // does nothing
            // ...
            if (someCondition) {
                releaser = QSemaphoreReleaser(sem);
                // ...
            }
            // ...
        } // conditionally calls sem.release(), depending on someCondition
        //! [6]
    }
 
    {
        QSemaphoreReleaser releaser;
 
        //! [7]
        releaser.cancel(); // avoid releasing old semaphore()
        releaser = QSemaphoreReleaser(sem, 42);
        // now will call sem.release(42) when 'releaser' is destroyed
        //! [7]
    }
}