qt-everywhere-src-5.14.1/qtbase/src/corelib/doc/snippets/code/src_corelib_thread_qsemaphore.cpp

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//! [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]

//! [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]

//! [6]
{ // some scope
    QSemaphoreReleaser releaser; // does nothing
    // ...
    if (someCondition) {
        releaser = QSemaphoreReleaser(sem);
        // ...
    }
    // ...
} // conditionally calls sem.release(), depending on someCondition
//! [6]

//! [7]
releaser.cancel(); // avoid releasing old semaphore()
releaser = QSemaphoreReleaser(sem, 42);
// now will call sem.release(42) when 'releaser' is destroyed
//! [7]