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[Doc] QObject: Sync warnings of isSignalConnected() and receivers()

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Extends e75c1a00e31723f1c9deb8427725fa0a58fae2a8 because calling
receivers() in disconnectNotify() can deadlock too.

As a drive-by, some statements are generalized further:
* "expensive initialization" -> "expensive operations" since that is
  what the relevant snippets actually show.
* The potential race condition is not limited to "after this function
  returns and before the signal gets emitted"

Task-number: QTBUG-106025
Change-Id: Iff014706b9e8d8147e3bbb9ac51542197eec5db3
Pick-to: 6.8 6.5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit f5b874fd2d393b783dad85c95a158383420e481a)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
This commit is contained in:
Sze Howe Koh 2025-02-26 15:59:52 +08:00 committed by Qt Cherry-pick Bot
parent 54ef8bf929
commit 206a8fec2c
1 changed files with 16 additions and 8 deletions
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24
src/corelib/kernel/qobject.cpp
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@ -2732,10 +2732,16 @@ int QObject::senderSignalIndex() const
\snippet code/src_corelib_kernel_qobject.cpp 21
As the code snippet above illustrates, you can use this function to avoid
expensive operations or emitting a signal that nobody listens to.
\warning In a multithreaded application, consecutive calls to this
function are not guaranteed to yield the same results.
\warning This function violates the object-oriented principle of
modularity. However, it might be useful when you need to perform
expensive initialization only if something is connected to a
signal.
modularity. In particular, this function must not be called from an
override of connectNotify() or disconnectNotify(), as those might get
called from any thread.
\sa isSignalConnected()
*/
@ -2792,14 +2798,17 @@ int QObject::receivers(const char *signal) const
\snippet code/src_corelib_kernel_qobject.cpp 49
As the code snippet above illustrates, you can use this function to avoid
expensive initialization or emitting a signal that nobody listens to.
However, in a multithreaded application, connections might change after
this function returns and before the signal gets emitted.
expensive operations or emitting a signal that nobody listens to.
\warning In a multithreaded application, consecutive calls to this
function are not guaranteed to yield the same results.
\warning This function violates the object-oriented principle of
modularity. In particular, this function must not be called from an
override of connectNotify() or disconnectNotify(), as those might get
called from any thread.
\sa receivers()
*/
bool QObject::isSignalConnected(const QMetaMethod &signal) const
{
@ -3483,8 +3492,7 @@ bool QObject::disconnect(const QObject *sender, const QMetaMethod &signal,
\warning This function violates the object-oriented principle of
modularity. However, it might be useful when you need to perform
expensive initialization only if something is connected to a
signal.
an expensive operation only if something is connected to a signal.
\warning This function is called from the thread which performs the
connection, which may be a different thread from the thread in which