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Please review the following information to ensure ** the GNU Free Documentation License version 1.3 requirements ** will be met: https://www.gnu.org/licenses/fdl-1.3.html. ** $QT_END_LICENSE$ ** ****************************************************************************/ /*! \class QFuture \inmodule QtCore \threadsafe \brief The QFuture class represents the result of an asynchronous computation. \since 4.4 \ingroup thread To start a computation, use one of the APIs in the \l {Qt Concurrent} framework. QFuture allows threads to be synchronized against one or more results which will be ready at a later point in time. The result can be of any type that has default, copy and possibly move constructors. If a result is not available at the time of calling the result(), resultAt(), results(), takeResult(), or takeResults() functions, QFuture will wait until the result becomes available. You can use the isResultReadyAt() function to determine if a result is ready or not. For QFuture objects that report more than one result, the resultCount() function returns the number of continuous results. This means that it is always safe to iterate through the results from 0 to resultCount(). takeResult() and takeResults() invalidate a future and any subsequent attempt to access result or results from the future leads to undefined behavior. isValid() tells you if results can be accessed. QFuture provides a \l{Java-style iterators}{Java-style iterator} (QFutureIterator) and an \l{STL-style iterators}{STL-style iterator} (QFuture::const_iterator). Using these iterators is another way to access results in the future. If the result of one asynchronous computation needs to be passed to another, QFuture provides a convenient way of chaining multiple sequential computations using then(). onCanceled() can be used for adding a handler to be called if the QFuture is canceled. Additionally, onFailed() can be used to handle any failures that occurred in the chain. Note that QFuture relies on exceptions for the error handling. If using exceptions is not an option, you can still indicate the error state of QFuture, by making the error type part of the QFuture type. For example, you can use std::variant, std::any or similar for keeping the result or failure or make your custom type. The example below demonstrates how the error handling can be done without using exceptions. Let's say we want to send a network request to obtain a large file from a network location. Then we want to write it to the file system and return its location in case of a success. Both of these operations may fail with different errors. So, we use std::variant to keep the result or error: \snippet code/src_corelib_thread_qfuture.cpp 3 And we combine the two operations using then(): \snippet code/src_corelib_thread_qfuture.cpp 4 It's possible to chain multiple continuations and handlers in any order. The first handler that can handle the state of its parent is invoked first. If there's no proper handler, the state is propagated to the next continuation or handler. For example: \snippet code/src_corelib_thread_qfuture.cpp 15 If \c testFuture is successfully fulfilled \c {Block 1} will be called. If it succeeds as well, the next then() (\c {Block 4}) is called. If \c testFuture gets canceled or fails with an exception, either \c {Block 2} or \c {Block 3} will be called respectively. The next then() will be called afterwards, and the story repeats. \note If \c {Block 2} is invoked and throws an exception, the following onFailed() (\c {Block 3}) will handle it. If the order of onFailed() and onCanceled() were reversed, the exception state would propagate to the next continuations and eventually would be caught in \c {Block 5}. In the next example the first onCanceled() (\c {Block 2}) is removed: \snippet code/src_corelib_thread_qfuture.cpp 16 If \c testFuture gets canceled, its state is propagated to the next then(), which will be also canceled. So in this case \c {Block 6} will be called. QFuture also offers ways to interact with a runnning computation. For instance, the computation can be canceled with the cancel() function. To suspend or resume the computation, use the setSuspended() function or one of the suspend(), resume(), or toggleSuspended() convenience functions. Be aware that not all running asynchronous computations can be canceled or suspended. For example, the future returned by QtConcurrent::run() cannot be canceled; but the future returned by QtConcurrent::mappedReduced() can. Progress information is provided by the progressValue(), progressMinimum(), progressMaximum(), and progressText() functions. The waitForFinished() function causes the calling thread to block and wait for the computation to finish, ensuring that all results are available. The state of the computation represented by a QFuture can be queried using the isCanceled(), isStarted(), isFinished(), isRunning(), isSuspending() or isSuspended() functions. QFuture is a lightweight reference counted class that can be passed by value. QFuture is specialized to not contain any of the result fetching functions. Any QFuture can be assigned or copied into a QFuture as well. This is useful if only status or progress information is needed - not the actual result data. To interact with running tasks using signals and slots, use QFutureWatcher. You can also use QtFuture::connect to connect signals to a QFuture object which will be resolved when a signal is emitted. This allows working with signals like with QFuture objects. For example, if you combine it with then(), you can attach multiple continuations to a signal, which are invoked in the same thread or a new thread. \sa QtFuture::connect(), QFutureWatcher, {Qt Concurrent} */ /*! \fn template QFuture::QFuture() Constructs an empty, canceled future. */ /*! \fn template QFuture::QFuture(const QFuture &other) Constructs a copy of \a other. \sa operator=() */ /*! \fn template QFuture::QFuture(QFutureInterface *resultHolder) \internal */ /*! \fn template QFuture::~QFuture() Destroys the future. Note that this neither waits nor cancels the asynchronous computation. Use waitForFinished() or QFutureSynchronizer when you need to ensure that the computation is completed before the future is destroyed. */ /*! \fn template QFuture &QFuture::operator=(const QFuture &other) Assigns \a other to this future and returns a reference to this future. */ /*! \fn template bool QFuture::operator==(const QFuture &other) const Returns \c true if \a other is a copy of this future; otherwise returns \c false. */ /*! \fn template bool QFuture::operator!=(const QFuture &other) const Returns \c true if \a other is \e not a copy of this future; otherwise returns false. */ /*! \fn template void QFuture::cancel() Cancels the asynchronous computation represented by this future. Note that the cancelation is asynchronous. Use waitForFinished() after calling cancel() when you need synchronous cancelation. Results currently available may still be accessed on a canceled future, but new results will \e not become available after calling this function. Any QFutureWatcher object that is watching this future will not deliver progress and result ready signals on a canceled future. Be aware that not all running asynchronous computations can be canceled. For example, the future returned by QtConcurrent::run() cannot be canceled; but the future returned by QtConcurrent::mappedReduced() can. */ /*! \fn template bool QFuture::isCanceled() const Returns \c true if the asynchronous computation has been canceled with the cancel() function; otherwise returns \c false. Be aware that the computation may still be running even though this function returns \c true. See cancel() for more details. */ #if QT_DEPRECATED_SINCE(6, 0) /*! \fn template void QFuture::setPaused(bool paused) \obsolete Use setSuspended() instead. If \a paused is true, this function pauses the asynchronous computation represented by the future. If the computation is already paused, this function does nothing. Any QFutureWatcher object that is watching this future will stop delivering progress and result ready signals while the future is paused. Signal delivery will continue once the future is resumed. If \a paused is false, this function resumes the asynchronous computation. If the computation was not previously paused, this function does nothing. Be aware that not all computations can be paused. For example, the future returned by QtConcurrent::run() cannot be paused; but the future returned by QtConcurrent::mappedReduced() can. \sa pause(), resume(), togglePaused() */ /*! \fn template bool QFuture::isPaused() const \obsolete Use isSuspending() or isSuspended() instead. Returns \c true if the asynchronous computation has been paused with the pause() function; otherwise returns \c false. Be aware that the computation may still be running even though this function returns \c true. See setPaused() for more details. To check if pause actually took effect, use isSuspended() instead. \sa setPaused(), togglePaused(), isSuspended() */ /*! \fn template void QFuture::pause() \obsolete Use suspend() instead. Pauses the asynchronous computation represented by this future. This is a convenience method that simply calls setPaused(true). \sa resume() */ /*! \fn template void QFuture::togglePaused() \obsolete Use toggleSuspended() instead. Toggles the paused state of the asynchronous computation. In other words, if the computation is currently paused, calling this function resumes it; if the computation is running, it is paused. This is a convenience method for calling setPaused(!isPaused()). \sa setPaused(), pause(), resume() */ #endif // QT_DEPRECATED_SINCE(6, 0) /*! \fn template void QFuture::setSuspended(bool suspend) \since 6.0 If \a suspend is true, this function suspends the asynchronous computation represented by the future(). If the computation is already suspended, this function does nothing. QFutureWatcher will not immediately stop delivering progress and result ready signals when the future is suspended. At the moment of suspending there may still be computations that are in progress and cannot be stopped. Signals for such computations will still be delivered. If \a suspend is false, this function resumes the asynchronous computation. If the computation was not previously suspended, this function does nothing. Be aware that not all computations can be suspended. For example, the QFuture returned by QtConcurrent::run() cannot be suspended; but the QFuture returned by QtConcurrent::mappedReduced() can. \sa suspend(), resume(), toggleSuspended() */ /*! \fn template bool QFuture::isSuspending() const \since 6.0 Returns \c true if the asynchronous computation has been suspended with the suspend() function, but the work is not yet suspended, and computation is still running. Returns \c false otherwise. To check if suspension is actually in effect, use isSuspended() instead. \sa setSuspended(), toggleSuspended(), isSuspended() */ /*! \fn template bool QFuture::isSuspended() const \since 6.0 Returns \c true if a suspension of the asynchronous computation has been requested, and it is in effect, meaning that no more results or progress changes are expected. \sa setSuspended(), toggleSuspended(), isSuspending() */ /*! \fn template void QFuture::suspend() \since 6.0 Suspends the asynchronous computation represented by this future. This is a convenience method that simply calls setSuspended(true). \sa resume() */ /*! \fn template void QFuture::resume() Resumes the asynchronous computation represented by the future(). This is a convenience method that simply calls setSuspended(false). \sa suspend() */ /*! \fn template void QFuture::toggleSuspended() \since 6.0 Toggles the suspended state of the asynchronous computation. In other words, if the computation is currently suspending or suspended, calling this function resumes it; if the computation is running, it is suspended. This is a convenience method for calling setSuspended(!(isSuspending() || isSuspended())). \sa setSuspended(), suspend(), resume() */ /*! \fn template bool QFuture::isStarted() const Returns \c true if the asynchronous computation represented by this future has been started; otherwise returns \c false. */ /*! \fn template bool QFuture::isFinished() const Returns \c true if the asynchronous computation represented by this future has finished; otherwise returns \c false. */ /*! \fn template bool QFuture::isRunning() const Returns \c true if the asynchronous computation represented by this future is currently running; otherwise returns \c false. */ /*! \fn template int QFuture::resultCount() const Returns the number of continuous results available in this future. The real number of results stored might be different from this value, due to gaps in the result set. It is always safe to iterate through the results from 0 to resultCount(). \sa result(), resultAt(), results(), takeResult(), takeResults() */ /*! \fn template int QFuture::progressValue() const Returns the current progress value, which is between the progressMinimum() and progressMaximum(). \sa progressMinimum(), progressMaximum() */ /*! \fn template int QFuture::progressMinimum() const Returns the minimum progressValue(). \sa progressValue(), progressMaximum() */ /*! \fn template int QFuture::progressMaximum() const Returns the maximum progressValue(). \sa progressValue(), progressMinimum() */ /*! \fn template QString QFuture::progressText() const Returns the (optional) textual representation of the progress as reported by the asynchronous computation. Be aware that not all computations provide a textual representation of the progress, and as such, this function may return an empty string. */ /*! \fn template void QFuture::waitForFinished() Waits for the asynchronous computation to finish (including cancel()ed computations). */ /*! \fn template T QFuture::result() const Returns the first result in the future. If the result is not immediately available, this function will block and wait for the result to become available. This is a convenience method for calling resultAt(0). \note Calling result() leads to undefined behavior if isValid() returns \c false for this QFuture. \sa resultAt(), results(), takeResult(), takeResults() */ /*! \fn template T QFuture::resultAt(int index) const Returns the result at \a index in the future. If the result is not immediately available, this function will block and wait for the result to become available. \note Calling resultAt() leads to undefined behavior if isValid() returns \c false for this QFuture. \sa result(), results(), takeResult(), takeResults(), resultCount() */ /*! \fn template bool QFuture::isResultReadyAt(int index) const Returns \c true if the result at \a index is immediately available; otherwise returns \c false. \note Calling isResultReadyAt() leads to undefined behavior if isValid() returns \c false for this QFuture. \sa resultAt(), resultCount(), takeResult(), takeResults() */ /*! \fn template QFuture::operator T() const Returns the first result in the future. If the result is not immediately available, this function will block and wait for the result to become available. This is a convenience method for calling result() or resultAt(0). \note Calling this function leads to undefined behavior if isValid() returns \c false for this QFuture. \sa result(), resultAt(), results(), takeResult(), takeResults(), isValid() */ /*! \fn template QList QFuture::results() const Returns all results from the future. If the results are not immediately available, this function will block and wait for them to become available. \note Calling results() leads to undefined behavior if isValid() returns \c false for this QFuture. \sa result(), resultAt(), takeResult(), takeResults(), resultCount(), isValid() */ /*! \fn template std::vector QFuture::takeResults() If isValid() returns \c false, calling this function leads to undefined behavior. takeResults() takes all results from the QFuture object and invalidates it (isValid() will return \c false for this future). If the results are not immediately available, this function will block and wait for them to become available. This function tries to use move semantics for the results if available and falls back to copy construction if the type is not movable. \note QFuture in general allows sharing the results between different QFuture objects (and potentially between different threads). takeResults() was introduced to make QFuture also work with move-only types (like std::unique_ptr), so it assumes that only one thread can move the results out of the future, and only once. \sa takeResult(), result(), resultAt(), results(), resultCount(), isValid() */ /*! \fn template std::vector QFuture::takeResult() Call this function only if isValid() returns \c true, otherwise the behavior is undefined. This function takes the first result from the QFuture object, for convenience when only one result is expected. If there are any other results, they are discarded after taking the first one (if such behavior is undesired, use takeResults() instead). If the result is not immediately available, this function will block and wait for the result to become available. The QFuture will try to use move semantics if possible, and will fall back to copy construction if the type is not movable. After the result was taken, isValid() will evaluate as \c false. \note QFuture in general allows sharing the results between different QFuture objects (and potentially between different threads). takeResult() was introduced to make QFuture also work with move-only types (like std::unique_ptr), so it assumes that only one thread can move the results out of the future, and do it only once. \sa takeResults(), result(), results(), resultAt(), isValid() */ /*! \fn template bool QFuture::isValid() const Returns \c true if a result or results can be accessed or taken from this QFuture object. Returns false after the result was taken from the future. \sa takeResults(), takeResult(), result(), results(), resultAt() */ /*! \fn template QFuture::const_iterator QFuture::begin() const Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the first result in the future. \sa constBegin(), end() */ /*! \fn template QFuture::const_iterator QFuture::end() const Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the imaginary result after the last result in the future. \sa begin(), constEnd() */ /*! \fn template QFuture::const_iterator QFuture::constBegin() const Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the first result in the future. \sa begin(), constEnd() */ /*! \fn template QFuture::const_iterator QFuture::constEnd() const Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the imaginary result after the last result in the future. \sa constBegin(), end() */ /*! \class QFuture::const_iterator \reentrant \since 4.4 \inmodule QtCore \brief The QFuture::const_iterator class provides an STL-style const iterator for QFuture. QFuture provides both \l{STL-style iterators} and \l{Java-style iterators}. The STL-style iterators are more low-level and more cumbersome to use; on the other hand, they are slightly faster and, for developers who already know STL, have the advantage of familiarity. The default QFuture::const_iterator constructor creates an uninitialized iterator. You must initialize it using a QFuture function like QFuture::constBegin() or QFuture::constEnd() before you start iterating. Here's a typical loop that prints all the results available in a future: \snippet code/src_corelib_thread_qfuture.cpp 0 \sa QFutureIterator, QFuture */ /*! \typedef QFuture::const_iterator::iterator_category Typedef for std::bidirectional_iterator_tag. Provided for STL compatibility. */ /*! \typedef QFuture::const_iterator::difference_type Typedef for ptrdiff_t. Provided for STL compatibility. */ /*! \typedef QFuture::const_iterator::value_type Typedef for T. Provided for STL compatibility. */ /*! \typedef QFuture::const_iterator::pointer Typedef for const T *. Provided for STL compatibility. */ /*! \typedef QFuture::const_iterator::reference Typedef for const T &. Provided for STL compatibility. */ /*! \fn template QFuture::const_iterator::const_iterator() Constructs an uninitialized iterator. Functions like operator*() and operator++() should not be called on an uninitialized iterartor. Use operator=() to assign a value to it before using it. \sa QFuture::constBegin(), QFuture::constEnd() */ /*! \fn template QFuture::const_iterator::const_iterator(QFuture const * const future, int index) \internal */ /*! \fn template QFuture::const_iterator::const_iterator(const const_iterator &other) Constructs a copy of \a other. */ /*! \fn template QFuture::const_iterator &QFuture::const_iterator::operator=(const const_iterator &other) Assigns \a other to this iterator. */ /*! \fn template const T &QFuture::const_iterator::operator*() const Returns the current result. */ /*! \fn template const T *QFuture::const_iterator::operator->() const Returns a pointer to the current result. */ /*! \fn template bool QFuture::const_iterator::operator!=(const const_iterator &other) const Returns \c true if \a other points to a different result than this iterator; otherwise returns \c false. \sa operator==() */ /*! \fn template bool QFuture::const_iterator::operator==(const const_iterator &other) const Returns \c true if \a other points to the same result as this iterator; otherwise returns \c false. \sa operator!=() */ /*! \fn template QFuture::const_iterator &QFuture::const_iterator::operator++() The prefix ++ operator (\c{++it}) advances the iterator to the next result in the future and returns an iterator to the new current result. Calling this function on QFuture::constEnd() leads to undefined results. \sa operator--() */ /*! \fn template QFuture::const_iterator QFuture::const_iterator::operator++(int) \overload The postfix ++ operator (\c{it++}) advances the iterator to the next result in the future and returns an iterator to the previously current result. */ /*! \fn template QFuture::const_iterator &QFuture::const_iterator::operator--() The prefix -- operator (\c{--it}) makes the preceding result current and returns an iterator to the new current result. Calling this function on QFuture::constBegin() leads to undefined results. \sa operator++() */ /*! \fn template QFuture::const_iterator QFuture::const_iterator::operator--(int) \overload The postfix -- operator (\c{it--}) makes the preceding result current and returns an iterator to the previously current result. */ /*! \fn template QFuture::const_iterator &QFuture::const_iterator::operator+=(int j) Advances the iterator by \a j results. (If \a j is negative, the iterator goes backward.) \sa operator-=(), operator+() */ /*! \fn template QFuture::const_iterator &QFuture::const_iterator::operator-=(int j) Makes the iterator go back by \a j results. (If \a j is negative, the iterator goes forward.) \sa operator+=(), operator-() */ /*! \fn template QFuture::const_iterator QFuture::const_iterator::operator+(int j) const Returns an iterator to the results at \a j positions forward from this iterator. (If \a j is negative, the iterator goes backward.) \sa operator-(), operator+=() */ /*! \fn template QFuture::const_iterator QFuture::const_iterator::operator-(int j) const Returns an iterator to the result at \a j positions backward from this iterator. (If \a j is negative, the iterator goes forward.) \sa operator+(), operator-=() */ /*! \typedef QFuture::ConstIterator Qt-style synonym for QFuture::const_iterator. */ /*! \class QFutureIterator \reentrant \since 4.4 \inmodule QtCore \brief The QFutureIterator class provides a Java-style const iterator for QFuture. QFuture has both \l{Java-style iterators} and \l{STL-style iterators}. The Java-style iterators are more high-level and easier to use than the STL-style iterators; on the other hand, they are slightly less efficient. An alternative to using iterators is to use index positions. Some QFuture member functions take an index as their first parameter, making it possible to access results without using iterators. QFutureIterator\ allows you to iterate over a QFuture\. Note that there is no mutable iterator for QFuture (unlike the other Java-style iterators). The QFutureIterator constructor takes a QFuture as its argument. After construction, the iterator is located at the very beginning of the result list (i.e. before the first result). Here's how to iterate over all the results sequentially: \snippet code/src_corelib_thread_qfuture.cpp 1 The next() function returns the next result (waiting for it to become available, if necessary) from the future and advances the iterator. Unlike STL-style iterators, Java-style iterators point \e between results rather than directly \e at results. The first call to next() advances the iterator to the position between the first and second result, and returns the first result; the second call to next() advances the iterator to the position between the second and third result, and returns the second result; and so on. \image javaiterators1.png Here's how to iterate over the elements in reverse order: \snippet code/src_corelib_thread_qfuture.cpp 2 If you want to find all occurrences of a particular value, use findNext() or findPrevious() in a loop. Multiple iterators can be used on the same future. If the future is modified while a QFutureIterator is active, the QFutureIterator will continue iterating over the original future, ignoring the modified copy. \sa QFuture::const_iterator, QFuture */ /*! \fn template QFutureIterator::QFutureIterator(const QFuture &future) Constructs an iterator for traversing \a future. The iterator is set to be at the front of the result list (before the first result). \sa operator=() */ /*! \fn template QFutureIterator &QFutureIterator::operator=(const QFuture &future) Makes the iterator operate on \a future. The iterator is set to be at the front of the result list (before the first result). \sa toFront(), toBack() */ /*! \fn template void QFutureIterator::toFront() Moves the iterator to the front of the result list (before the first result). \sa toBack(), next() */ /*! \fn template void QFutureIterator::toBack() Moves the iterator to the back of the result list (after the last result). \sa toFront(), previous() */ /*! \fn template bool QFutureIterator::hasNext() const Returns \c true if there is at least one result ahead of the iterator, e.g., the iterator is \e not at the back of the result list; otherwise returns false. \sa hasPrevious(), next() */ /*! \fn template const T &QFutureIterator::next() Returns the next result and advances the iterator by one position. Calling this function on an iterator located at the back of the result list leads to undefined results. \sa hasNext(), peekNext(), previous() */ /*! \fn template const T &QFutureIterator::peekNext() const Returns the next result without moving the iterator. Calling this function on an iterator located at the back of the result list leads to undefined results. \sa hasNext(), next(), peekPrevious() */ /*! \fn template bool QFutureIterator::hasPrevious() const Returns \c true if there is at least one result ahead of the iterator, e.g., the iterator is \e not at the front of the result list; otherwise returns false. \sa hasNext(), previous() */ /*! \fn template const T &QFutureIterator::previous() Returns the previous result and moves the iterator back by one position. Calling this function on an iterator located at the front of the result list leads to undefined results. \sa hasPrevious(), peekPrevious(), next() */ /*! \fn template const T &QFutureIterator::peekPrevious() const Returns the previous result without moving the iterator. Calling this function on an iterator located at the front of the result list leads to undefined results. \sa hasPrevious(), previous(), peekNext() */ /*! \fn template bool QFutureIterator::findNext(const T &value) Searches for \a value starting from the current iterator position forward. Returns \c true if \a value is found; otherwise returns \c false. After the call, if \a value was found, the iterator is positioned just after the matching result; otherwise, the iterator is positioned at the back of the result list. \sa findPrevious() */ /*! \fn template bool QFutureIterator::findPrevious(const T &value) Searches for \a value starting from the current iterator position backward. Returns \c true if \a value is found; otherwise returns \c false. After the call, if \a value was found, the iterator is positioned just before the matching result; otherwise, the iterator is positioned at the front of the result list. \sa findNext() */ /*! \namespace QtFuture \inmodule QtCore \brief Contains miscellaneous identifiers used by the QFuture class. */ /*! \enum QtFuture::Launch \since 6.0 Represents execution policies for running a QFuture continuation. \value Sync The continuation will be launched in the same thread in which the parent has been executing. \value Async The continuation will be launched in in a separate thread taken from the global QThreadPool. \value Inherit The continuation will inherit the launch policy of the parent or its thread pool, if it was using a custom one. \sa QFuture::then(), QThreadPool::globalInstance() */ /*! \fn template static QFuture> QtFuture::connect(Sender *sender, Signal signal) Creates and returns a QFuture which will become available when the \a sender emits the \a signal. If the \a signal takes no arguments, a QFuture is returned. If the \a signal takes a single argument, the resulted QFuture will be filled with the signal's argument value. If the \a signal takes multiple arguments, the resulted QFuture is filled with std::tuple storing the values of signal's arguments. If the \a sender is destroyed before the \a signal is emitted, the resulted QFuture will be canceled. For example, let's say we have the following object: \snippet code/src_corelib_thread_qfuture.cpp 10 We can connect its signals to QFuture objects in the following way: \snippet code/src_corelib_thread_qfuture.cpp 11 We can also chain continuations to be run when a signal is emitted: \snippet code/src_corelib_thread_qfuture.cpp 12 You can also start the continuation in a new thread or a custom thread pool using QtFuture::Launch policies. For example: \snippet code/src_corelib_thread_qfuture.cpp 13 Throwing an exception from a slot invoked by Qt's signal-slot connection is considered to be an undefined behavior, if it is not handled within the slot. But with QFuture::connect(), you can throw and handle exceptions from the continuations: \snippet code/src_corelib_thread_qfuture.cpp 14 \note The connected future will be fulfilled only once, when the signal is emitted for the first time. \sa QFuture, QFuture::then() */ /*! \fn template template QFuture::ResultType> QFuture::then(Function &&function) \since 6.0 \overload Attaches a continuation to this future, allowing to chain multiple asynchronous computations if desired. When the asynchronous computation represented by this future finishes, \a function will be invoked in the same thread in which this future has been running. A new QFuture representing the result of the continuation is returned. \note Use other overloads of this method if you need to launch the continuation in a separate thread. If this future has a result (is not a QFuture), \a function takes the result of this future as its argument. You can chain multiple operations like this: \code QFuture future = ...; future.then([](int res1){ ... }).then([](int res2){ ... })... \endcode Or: \code QFuture future = ...; future.then([](){ ... }).then([](){ ... })... \endcode The continuation can also take a QFuture argument (instead of its value), representing the previous future. This can be useful if, for example, QFuture has multiple results, and the user wants to access them inside the continuation. Or the user needs to handle the exception of the previous future inside the continuation, to not interrupt the chain of multiple continuations. For example: \snippet code/src_corelib_thread_qfuture.cpp 5 If the previous future throws an exception and it is not handled inside the continuation, the exception will be propagated to the continuation future, to allow the caller to handle it: \snippet code/src_corelib_thread_qfuture.cpp 6 In this case the whole chain of continuations will be interrupted. \note If the parent future gets canceled, its continuations will also be canceled. \sa onFailed(), onCanceled() */ /*! \fn template template QFuture::ResultType> QFuture::then(QtFuture::Launch policy, Function &&function) \since 6.0 \overload Attaches a continuation to this future, allowing to chain multiple asynchronous computations. When the asynchronous computation represented by this future finishes, \a function will be invoked according to the given launch \a policy. A new QFuture representing the result of the continuation is returned. Depending on the \a policy, continuation will run in the same thread as the parent, run in a new thread, or inherit the launch policy and thread pool of the parent. In the following example both continuations will run in a new thread (but in the same one). \code QFuture future = ...; future.then(QtFuture::Launch::Async, [](int res){ ... }).then([](int res2){ ... }); \endcode In the following example both continuations will run in new threads using the same thread pool. \code QFuture future = ...; future.then(QtFuture::Launch::Async, [](int res){ ... }) .then(QtFuture::Launch::Inherit, [](int res2){ ... }); \endcode \sa onFailed(), onCanceled() */ /*! \fn template template QFuture::ResultType> QFuture::then(QThreadPool *pool, Function &&function) \since 6.0 \overload Attaches a continuation to this future, allowing to chain multiple asynchronous computations if desired. When the asynchronous computation represented by this future finishes, \a function will be invoked in a separate thread taken from the QThreadPool \a pool. \sa onFailed(), onCanceled() */ /*! \fn template template QFuture QFuture::onFailed(Function &&handler) \since 6.0 Attaches a failure handler to this future, to handle any exceptions that may have been generated. Returns a QFuture of the parent type. The handler will be invoked only in case of an exception, in the same thread as the parent future has been running. \a handler is a callable which takes either no argument or one argument, to filter by specific error types similar to \l {https://en.cppreference.com/w/cpp/language/try_catch} {catch} statement. For example: \snippet code/src_corelib_thread_qfuture.cpp 7 If there are multiple handlers attached, the first handler that matches with the thrown exception type will be invoked. For example: \snippet code/src_corelib_thread_qfuture.cpp 8 If none of the handlers matches with the thrown exception type, the exception will be propagated to the resulted future: \snippet code/src_corelib_thread_qfuture.cpp 9 \note You can always attach a handler taking no argument, to handle all exception types and avoid writing the try-catch block. \sa then(), onCanceled() */ /*! \fn template template QFuture QFuture::onCanceled(Function &&handler) \since 6.0 Attaches a cancellation \a handler to this future, to be called when the future is canceled. The \a handler is a callable which doesn't take any arguments. It will be invoked in the same thread in which this future has been running. \sa then(), onFailed() */