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// Copyright (C) 2021 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
/*!
\macro Q_APPLICATION_STATIC(Type, VariableName, ...)
\since 6.3
\relates QGlobalStatic
This macro extends Q_GLOBAL_STATIC and creates a global and static object
of type \l QGlobalStatic, of name \a VariableName, initialized by the
variadic arguments, and that behaves as a pointer to \a Type, where the
actual lifetime of the type is bound to the QCoreApplication. The object
created by Q_APPLICATION_STATIC initializes itself on the first use, which
means that it will not increase the application or the library's load time.
Additionally, the object is initialized in a thread-safe manner on all
platforms.
In contrast to Q_GLOBAL_STATIC where the type is only meant to be destroyed at
program exit, here the actual lifetime of the type is bound to the lifetime of
the QCoreApplication. This makes it ideal to store semi-static QObjects, which
should also be destroyed once the QCoreApplication is destroyed. This means the
type will get deleted once the QCoreApplication emits the destroyed signal.
It is permitted for the object to be recreated when it's accessed again, if
a new QCoreApplication has also been created.
Since the value is bound to the QCoreApplication, it should only ever be
accessed if there is a valid QCoreApplication::instance(). Accessing this
object before QCoreApplication is created or after it's destroyed will
produce warnings and may have unpredictable behavior.
The typical use of this macro is as follows, in a global context (that is,
outside of any function bodies):
\code
Q_APPLICATION_STATIC(MyQObjectType, staticType, "some string", function())
\endcode
Do note that the arguments passed in variadic fashion to this macro are
evaluated every time the object is constructed, so in the above example,
the function \c{function} will be called more than once if the object is
recreated.
Aside from the value also being bound to the lifetime of the QCoreApplication,
this macro behaves identically to Q_GLOBAL_STATIC(). Please see that macro's
documentation for more information.
\section1 Threading guarantees
The Q_APPLICATION_STATIC macro ensures that the object is initialized only
once (per lifetime of a QCoreApplication), even if multiple threads try to
concurrently access the object. This is done by providing a per-object
mutex; application and library developers need to be aware that their
object will be constructed with this mutex locked and therefore must not
reenter the same object's initialization, or a deadlock will occur.
There is no thread-safety on the destruction of the object: user code must
not access this object once the QCoreApplication destructor starts to run.
User code must arrange to ensure this does not happen, such as by not
accessing it once the main thread's event loop has exited.
Like Q_GLOBAL_STATIC, Q_APPLICATION_STATIC provides no thread-safety
guarantees for accesses to the object once creation is finished. It is up
to user code to ensure that no racy data accesses happen.
In case the object created by this operation is a QObject, its associated
thread will be the one that succeeded in creating it. It will be destroyed
by the main thread, so a \l{QObject::}{moveToThread()} to the main thread
or to no thread before destruction is adviseable. Doing so from the
constructor of the class in question is a sensible solution if one can't
guarantee that the main thread will be the one to initialize the object.
\omit
\section1 Implementation details
See \l Q_GLOBAL_STATIC implementation details for an introduction.
Q_APPLICATION_STATIC uses the same \l QGlobalStatic public class that
Q_GLOBAL_STATIC does, but instead uses a QtGlobalStatic::ApplicationHolder
template class as the template parameter. The differences to
QtGlobalStatic::Holder are:
\list
\li The ApplicationHolder class is empty. Unlike Holder, the storage is
provided as a \c {static inline} member, simply so that the static
member reset() function can access it without having to save the
pointer in a lambda.
\li The ApplicationHolder constructor is trivial; initialization of the
type is instead deferred to the \c pointer() function. This means the
C++11 thread-safe initialization of statics does not protect the
object.
\li Instead, ApplicationHolder provides a mutex (implemented as a \c
{static inline} member of type \l QBasicMutex) and locks it before
constructing or destructing the object.
\li After constructing the object, it will QObject::connect() the
QCoreApplication::destroyed() signal to a function that will in turn
destroy the object.
\li The destructor will destroy the object if the application is
exiting without first destroying the QCoreApplication object (i.e., a
call to \c ::exit) or this Q_APPLICATION_STATIC is part of a plugin
that is being unloaded.
\endlist
\endomit
\sa Q_GLOBAL_STATIC, QGlobalStatic
*/
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