QMetaType & QVariant: "load" and "save" std::nullptr_t

We don't load and save pointers usually because the pointer value cannot
be guaranteed to remain across program invocations. However, nullptr is
an exception: a null pointer is always a null pointer.

We don't actually have to read or write anything: there's only one value
possible for a std::nullptr_t and it is nullptr.

[ChangeLog][Important Behavior Changes] A QVariant containing a
std::nullptr_t is now streamable to/from QDataStream.

Task-number: QTBUG-59391
Change-Id: Iae839f6a131a4f0784bffffd14aa374f6475d283
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

2 files changed, 12 insertions, 5 deletions

diff --git a/src/corelib/kernel/qmetatype.cpp b/src/corelib/kernel/qmetatype.cpp
index 232a134718..718c42ba55 100644
--- a/src/corelib/kernel/qmetatype.cpp
+++ b/src/corelib/kernel/qmetatype.cpp
@@ -1255,7 +1255,6 @@ bool QMetaType::save(QDataStream &stream, int type, const void *data)
     case QMetaType::UnknownType:
     case QMetaType::Void:
     case QMetaType::VoidStar:
-    case QMetaType::Nullptr:
     case QMetaType::QObjectStar:
     case QMetaType::QModelIndex:
     case QMetaType::QPersistentModelIndex:
@@ -1264,6 +1263,8 @@ bool QMetaType::save(QDataStream &stream, int type, const void *data)
     case QMetaType::QJsonArray:
     case QMetaType::QJsonDocument:
         return false;
+    case QMetaType::Nullptr:
+        return true;
     case QMetaType::Long:
         stream << qlonglong(*static_cast<const long *>(data));
         break;
@@ -1477,7 +1478,6 @@ bool QMetaType::load(QDataStream &stream, int type, void *data)
     case QMetaType::UnknownType:
     case QMetaType::Void:
     case QMetaType::VoidStar:
-    case QMetaType::Nullptr:
     case QMetaType::QObjectStar:
     case QMetaType::QModelIndex:
     case QMetaType::QPersistentModelIndex:
@@ -1486,6 +1486,8 @@ bool QMetaType::load(QDataStream &stream, int type, void *data)
     case QMetaType::QJsonArray:
     case QMetaType::QJsonDocument:
         return false;
+    case QMetaType::Nullptr:
+        return true;
     case QMetaType::Long: {
         qlonglong l;
         stream >> l;
diff --git a/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp b/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
index 7d9f56ef38..7e1979dfb3 100644
--- a/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
+++ b/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
@@ -1559,7 +1559,6 @@ DECLARE_NONSTREAMABLE(QJsonArray)
 DECLARE_NONSTREAMABLE(QJsonDocument)
 DECLARE_NONSTREAMABLE(QObject*)
 DECLARE_NONSTREAMABLE(QWidget*)
-DECLARE_NONSTREAMABLE(std::nullptr_t)
 
 #define DECLARE_GUI_CLASS_NONSTREAMABLE(MetaTypeName, MetaTypeId, RealType) \
     DECLARE_NONSTREAMABLE(RealType)
@@ -1598,7 +1597,10 @@ void tst_QMetaType::saveAndLoadBuiltin()
 
     if (isStreamable) {
         QVERIFY(QMetaType::load(stream, type, value)); // Hmmm, shouldn't it return false?
-        QCOMPARE(stream.status(), QDataStream::ReadPastEnd);
+
+        // std::nullptr_t is nullary: it doesn't actually read anything
+        if (type != QMetaType::Nullptr)
+            QCOMPARE(stream.status(), QDataStream::ReadPastEnd);
     }
 
     stream.device()->seek(0);
@@ -1608,7 +1610,10 @@ void tst_QMetaType::saveAndLoadBuiltin()
 
     if (isStreamable) {
         QVERIFY(QMetaType::load(stream, type, value)); // Hmmm, shouldn't it return false?
-        QCOMPARE(stream.status(), QDataStream::ReadPastEnd);
+
+        // std::nullptr_t is nullary: it doesn't actually read anything
+        if (type != QMetaType::Nullptr)
+            QCOMPARE(stream.status(), QDataStream::ReadPastEnd);
     }
 
     QMetaType::destroy(type, value);