Struct qt_core::QSemaphoreReleaser

#[repr(C)]pub struct QSemaphoreReleaser { /* fields omitted */ }

The QSemaphoreReleaser class provides exception-safe deferral of a QSemaphore::release() call.

C++ class: QSemaphoreReleaser.

C++ documentation:

The QSemaphoreReleaser class provides exception-safe deferral of a QSemaphore::release() call.

QSemaphoreReleaser can be used wherever you would otherwise use QSemaphore::release(). Constructing a QSemaphoreReleaser defers the release() call on the semaphore until the QSemaphoreReleaser is destroyed (see RAII pattern).

You can use this to reliably release a semaphore to avoid dead-lock in the face of exceptions or early returns:

// ... do something that may throw or return early
sem.release();

If an early return is taken or an exception is thrown before the sem.release() call is reached, the semaphore is not released, possibly preventing the thread waiting in the corresponding sem.acquire() call from ever continuing execution.

When using RAII instead:

const QSemaphoreReleaser releaser(sem);
// ... do something that may throw or early return
// implicitly calls sem.release() here and at every other return in between

this can no longer happen, because the compiler will make sure that the QSemaphoreReleaser destructor is always called, and therefore the semaphore is always released.

QSemaphoreReleaser is move-enabled and can therefore be returned from functions to transfer responsibility for releasing a semaphore out of a function or a scope:

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

A QSemaphoreReleaser can be canceled by a call to cancel(). A canceled semaphore releaser will no longer call QSemaphore::release() in its destructor.

Methods

impl QSemaphoreReleaser

pub unsafe fn cancel(&self) -> Ptr<QSemaphore>

This is supported on cpp_lib_version="5.11.3" or cpp_lib_version="5.12.2" or cpp_lib_version="5.13.0" or cpp_lib_version="5.14.0" only.

Cancels this QSemaphoreReleaser such that the destructor will no longer call semaphore()->release(). Returns the value of semaphore() before this call. After this call, semaphore() will return nullptr.

Calls C++ function: QSemaphore* QSemaphoreReleaser::cancel().

C++ documentation:

Cancels this QSemaphoreReleaser such that the destructor will no longer call semaphore()->release(). Returns the value of semaphore() before this call. After this call, semaphore() will return nullptr.

To enable again, assign a new QSemaphoreReleaser:

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

pub unsafe fn new() -> CppBox<QSemaphoreReleaser>

This is supported on cpp_lib_version="5.11.3" or cpp_lib_version="5.12.2" or cpp_lib_version="5.13.0" or cpp_lib_version="5.14.0" only.

Default constructor. Creates a QSemaphoreReleaser that does nothing.

Calls C++ function: [constructor] void QSemaphoreReleaser::QSemaphoreReleaser().

C++ documentation:

Default constructor. Creates a QSemaphoreReleaser that does nothing.

pub unsafe fn from_q_semaphore_int(
    sem: impl CastInto<Ref<QSemaphore>>,
    n: c_int
) -> CppBox<QSemaphoreReleaser>

This is supported on cpp_lib_version="5.11.3" or cpp_lib_version="5.12.2" or cpp_lib_version="5.13.0" or cpp_lib_version="5.14.0" only.

Constructor. Stores the arguments and calls sem.release(n) in the destructor.

Calls C++ function: [constructor] void QSemaphoreReleaser::QSemaphoreReleaser(QSemaphore& sem, int n = …).

C++ documentation:

Constructor. Stores the arguments and calls sem.release(n) in the destructor.

pub unsafe fn from_q_semaphore_int2(
    sem: impl CastInto<Ptr<QSemaphore>>,
    n: c_int
) -> CppBox<QSemaphoreReleaser>

This is supported on cpp_lib_version="5.11.3" or cpp_lib_version="5.12.2" or cpp_lib_version="5.13.0" or cpp_lib_version="5.14.0" only.

Constructor. Stores the arguments and calls sem->release(n) in the destructor.

Calls C++ function: [constructor] void QSemaphoreReleaser::QSemaphoreReleaser(QSemaphore* sem, int n = …).

C++ documentation:

Constructor. Stores the arguments and calls sem->release(n) in the destructor.

pub unsafe fn from_q_semaphore(
    sem: impl CastInto<Ref<QSemaphore>>
) -> CppBox<QSemaphoreReleaser>

This is supported on cpp_lib_version="5.11.3" or cpp_lib_version="5.12.2" or cpp_lib_version="5.13.0" or cpp_lib_version="5.14.0" only.

Constructor. Stores the arguments and calls sem.release(n) in the destructor.

Calls C++ function: [constructor] void QSemaphoreReleaser::QSemaphoreReleaser(QSemaphore& sem).

C++ documentation:

Constructor. Stores the arguments and calls sem.release(n) in the destructor.

pub unsafe fn from_q_semaphore2(
    sem: impl CastInto<Ptr<QSemaphore>>
) -> CppBox<QSemaphoreReleaser>

This is supported on cpp_lib_version="5.11.3" or cpp_lib_version="5.12.2" or cpp_lib_version="5.13.0" or cpp_lib_version="5.14.0" only.

Constructor. Stores the arguments and calls sem->release(n) in the destructor.

Calls C++ function: [constructor] void QSemaphoreReleaser::QSemaphoreReleaser(QSemaphore* sem).

C++ documentation:

Constructor. Stores the arguments and calls sem->release(n) in the destructor.

pub unsafe fn semaphore(&self) -> Ptr<QSemaphore>

This is supported on cpp_lib_version="5.11.3" or cpp_lib_version="5.12.2" or cpp_lib_version="5.13.0" or cpp_lib_version="5.14.0" only.

Returns a pointer to the QSemaphore object provided to the constructor, or by the last move assignment, if any. Otherwise, returns nullptr.

Calls C++ function: QSemaphore* QSemaphoreReleaser::semaphore() const.

C++ documentation:

Returns a pointer to the QSemaphore object provided to the constructor, or by the last move assignment, if any. Otherwise, returns nullptr.

pub unsafe fn swap(&self, other: impl CastInto<Ref<QSemaphoreReleaser>>)

This is supported on cpp_lib_version="5.11.3" or cpp_lib_version="5.12.2" or cpp_lib_version="5.13.0" or cpp_lib_version="5.14.0" only.

Exchanges the responsibilites of *this and other.

Calls C++ function: void QSemaphoreReleaser::swap(QSemaphoreReleaser& other).

C++ documentation:

Exchanges the responsibilites of *this and other.

Unlike move assignment, neither of the two objects ever releases its semaphore, if any, as a consequence of swapping.

Therefore this function is very fast and never fails.

Trait Implementations

impl CppDeletable for QSemaphoreReleaser

unsafe fn delete(&self)

Unless canceled, calls QSemaphore::release() with the arguments provided to the constructor, or by the last move assignment.

Calls C++ function: [destructor] void QSemaphoreReleaser::~QSemaphoreReleaser().

C++ documentation:

Unless canceled, calls QSemaphore::release() with the arguments provided to the constructor, or by the last move assignment.