QObject: Extract Method q_choose_{assign,append} from doSetProperty()

We'll apply that doSetFoo() pattern in more places going forward, so
extract the central switch between lvalue and rvalue into a pair of
class helper functions for easier reuse.

Needed a new header for this, so started a qtclasshelper_p.h.

Amends 39cdf431f034121353e51768b4d1fec8b0dd35dc.

Pick-to: 6.10 6.9 6.8
Change-Id: I864b7faba41a74058e42d2ca96a0cc519751389f
Reviewed-by: Ivan Solovev <[email protected]>
Reviewed-by: Thiago Macieira <[email protected]>

3 files changed, 81 insertions, 8 deletions

diff --git a/src/corelib/CMakeLists.txt b/src/corelib/CMakeLists.txt
index af7173b72d5..4d83bbf794e 100644
--- a/src/corelib/CMakeLists.txt
+++ b/src/corelib/CMakeLists.txt
@@ -77,6 +77,7 @@ qt_internal_add_module(Core
         global/qsysinfo.cpp global/qsysinfo.h
         global/qsystemdetection.h
         global/qtclasshelpermacros.h
+        global/qtclasshelper_p.h
         global/qtconfiginclude.h
         global/qtconfigmacros.h
         global/qtcoreglobal.h global/qtcoreglobal_p.h
diff --git a/src/corelib/global/qtclasshelper_p.h b/src/corelib/global/qtclasshelper_p.h
new file mode 100644
index 00000000000..1e356726126
--- /dev/null
+++ b/src/corelib/global/qtclasshelper_p.h
@@ -0,0 +1,77 @@
+// Copyright (C) 2025 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
+// Qt-Security score:significant reason:default
+
+#ifndef QTCLASSHELPER_P_H
+#define QTCLASSHELPER_P_H
+
+#include <QtCore/private/qglobal_p.h>
+
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is not part of the Qt API.  It exists purely as an
+// implementation detail.  This header file may change from version to
+// version without notice, or even be removed.
+//
+// We mean it.
+//
+
+#include <type_traits>
+#include <utility>
+
+QT_BEGIN_NAMESPACE
+
+/*
+    Helper for setters overloaded on lvalue/rvalue. If the setter is doing a
+    lot of work around the actual assignment, a common pattern is to create a
+    private helper method
+
+        void doSetFoo(const Foo &lvalue, Foo *rvalue);
+
+    and implement the two setters inline by calling that function:
+
+        void setFoo(const Foo &f) { doSetFoo(f, nullptr); }
+        void setFoo(Foo &&f) { doSetFoo(f, &f); }
+
+    Then, in doSetFoo(), when assigning the argument to the member, use this
+    function:
+
+        q_choose_assign(m_foo, lvalue, rvalue);
+
+    or, when appending to a container,
+
+        q_choose_append(m_container, lvalue, rvalue);
+
+    The functions mandate (in the C++ sense) that all arguments are the same
+    type, so they deduce each argument separately and then static_assert that
+    they're the same. If we need std::exchange()-like mixed types later, it's
+    easy to relax. For now, avoid being overly general.
+*/
+template <typename T, typename U, typename V>
+decltype(auto) q_choose_assign(T &var, const U &lvalue, V *rvalue)
+{
+    static_assert(std::is_same_v<T, U>, "all arguments must be of the same type");
+    static_assert(std::is_same_v<U, V>, "all arguments must be of the same type");
+    if (rvalue)
+        return var = std::move(*rvalue);
+    else
+        return var = lvalue;
+}
+
+template <typename Container, typename U, typename V>
+decltype(auto) q_choose_append(Container &c, const U &lvalue, V *rvalue)
+{
+    static_assert(std::is_same_v<typename Container::value_type, U>, "arguments must match container");
+    static_assert(std::is_same_v<U, V>, "all arguments must be of the same type");
+    if (rvalue)
+        c.push_back(std::move(*rvalue));
+    else
+        c.push_back(lvalue);
+    return c.back();
+}
+
+QT_END_NAMESPACE
+
+#endif // QTCLASSHELPER_P_H
diff --git a/src/corelib/kernel/qobject.cpp b/src/corelib/kernel/qobject.cpp
index 0d2ec8efd7a..5a6da48d52c 100644
--- a/src/corelib/kernel/qobject.cpp
+++ b/src/corelib/kernel/qobject.cpp
@@ -8,6 +8,7 @@
 #include "qobject_p_p.h"
 #include "qmetaobject_p.h"
 
+#include <QtCore/private/qtclasshelper_p.h>
 #include "qabstracteventdispatcher.h"
 #include "qabstracteventdispatcher_p.h"
 #include "qcoreapplication.h"
@@ -4392,18 +4393,12 @@ bool QObject::doSetProperty(const char *name, const QVariant *lvalue, QVariant *
         } else {
             if (idx == -1) {
                 d->extraData->propertyNames.append(name);
-                if (rvalue)
-                    d->extraData->propertyValues.append(std::move(*rvalue));
-                else
-                    d->extraData->propertyValues.append(*lvalue);
+                q_choose_append(d->extraData->propertyValues, value, rvalue);
             } else {
                 if (value.userType() == d->extraData->propertyValues.at(idx).userType()
                         && value == d->extraData->propertyValues.at(idx))
                     return false;
-                if (rvalue)
-                    d->extraData->propertyValues[idx] = std::move(*rvalue);
-                else
-                    d->extraData->propertyValues[idx] = *lvalue;
+                q_choose_assign(d->extraData->propertyValues[idx], value, rvalue);
             }
         }