Remove std::function from QProperty interface

std::function as a type is rather unfortunate for us, as its SSO buffer
makes it rather large, and we can ensure that the function is never
empty.
Considering that we do need to allocate memory for
QPropertyBindingPrivate anyway, we can get rid of the SSO buffer and
instead coalesce the allocations (similar to how std::make_shared works).
The memory looks then like
[--QPropertyBindingPrivate--][Functor]
and QPropertyBindingPrivate can get a pointer to the functor via
reinterpret_cast<std::byte>(this)+sizeof(QPropertyBindingPrivate).
To actually do anything with the functor, we do however need a "vtable"
which describes how we can call, destroy and move the functor. This is
done by creating a constexpr struct of function pointers, and storing a
pointer to it in QPropertyBindingPrivate.

As a consequence of those changes, we cannot use QESDP anymore, as we
now have to carefully deallocate the buffer we used for both the
QPropertyBindingPrivate and the functor. We introduce a custom
refcounting pointer for that. While we're at it, we make the refcount
non-atomic, as bindings do not work across threads to begin with.

Moreover, we can now make the class non-virtual, as that was only needed
to hack around limitations of QESDP in the context of exported symbols.

Change-Id: Idc5507e4c120e28df5bd5aea717fe69f15e540dc
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

1 files changed, 22 insertions, 0 deletions

diff --git a/tests/auto/corelib/kernel/qproperty/tst_qproperty.cpp b/tests/auto/corelib/kernel/qproperty/tst_qproperty.cpp
index 21e0955ba5..472ba6031f 100644
--- a/tests/auto/corelib/kernel/qproperty/tst_qproperty.cpp
+++ b/tests/auto/corelib/kernel/qproperty/tst_qproperty.cpp
@@ -33,6 +33,13 @@
 
 using namespace QtPrivate;
 
+
+struct DtorCounter {
+    static inline int counter = 0;
+    bool shouldIncrement = false;
+    ~DtorCounter() {if (shouldIncrement) ++counter;}
+};
+
 class tst_QProperty : public QObject
 {
     Q_OBJECT
@@ -43,6 +50,7 @@ private slots:
     void bindingWithDeletedDependency();
     void recursiveDependency();
     void bindingAfterUse();
+    void bindingFunctionDtorCalled();
     void switchBinding();
     void avoidDependencyAllocationAfterFirstEval();
     void boolProperty();
@@ -201,6 +209,20 @@ void tst_QProperty::bindingAfterUse()
     QCOMPARE(QPropertyBindingDataPointer::get(propWithBindingLater).observerCount(), 1);
 }
 
+void tst_QProperty::bindingFunctionDtorCalled()
+{
+    DtorCounter dc;
+    {
+        QProperty<int> prop;
+        prop.setBinding([dc]() mutable {
+                dc.shouldIncrement = true;
+                return 42;
+        });
+        QCOMPARE(prop.value(), 42);
+    }
+    QCOMPARE(DtorCounter::counter, 1);
+}
+
 void tst_QProperty::switchBinding()
 {
     QProperty<int> first(1);