QDataStream: add operator<< and >> for std::nullptr_t

std::nullptr_t is nullary: it accepts only one value, nullptr. So we
don't need to read or write anything. This commit simply adds the two
operators that allow generic code to operate on std::nullptr_t if
required.

This commit also adds the actual use to QMetaType::load/save, even
though there's no change in behavior.

[ChangeLog][QtCore][QDataStream] Added operator<< and operator>>
overloads that take std::nullptr_t, to facilitate generic code.

Change-Id: Iae839f6a131a4f0784bffffd14aa37e7f62d2740
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/io/qdatastream.cpp

@@ -770,6 +770,17 @@ int QDataStream::readBlock(char *data, int len)
     return readResult;
 }
 
+/*!
+    \fn QDataStream &QDataStream::operator>>(std::nullptr &ptr)
+    \since 5.9
+    \overload
+
+    Simulates reading a \c{std::nullptr_t} from the stream into \a ptr and
+    returns a reference to the stream. This function does not actually read
+    anything from the stream, as \c{std::nullptr_t} values are stored as 0
+    bytes.
+*/
+
 /*!
     \fn QDataStream &QDataStream::operator>>(quint8 &i)
     \overload
@@ -1084,6 +1095,15 @@ int QDataStream::readRawData(char *s, int len)
   QDataStream write functions
  *****************************************************************************/
 
+/*!
+    \fn QDataStream &QDataStream::operator<<(std::nullptr ptr)
+    \since 5.9
+    \overload
+
+    Simulates writing a \c{std::nullptr_t}, \a ptr, to the stream and returns a
+    reference to the stream. This function does not actually write anything to
+    the stream, as \c{std::nullptr_t} values are stored as 0 bytes.
+*/
 
 /*!
     \fn QDataStream &QDataStream::operator<<(quint8 i)

src/corelib/io/qdatastream.h

@@ -152,6 +152,7 @@ public:
     QDataStream &operator>>(quint32 &i);
     QDataStream &operator>>(qint64 &i);
     QDataStream &operator>>(quint64 &i);
+    QDataStream &operator>>(std::nullptr_t &ptr) { ptr = nullptr; return *this; }
 
     QDataStream &operator>>(bool &i);
     QDataStream &operator>>(qfloat16 &f);
@@ -167,6 +168,7 @@ public:
     QDataStream &operator<<(quint32 i);
     QDataStream &operator<<(qint64 i);
     QDataStream &operator<<(quint64 i);
+    QDataStream &operator<<(std::nullptr_t) { return *this; }
     QDataStream &operator<<(bool i);
     QDataStream &operator<<(qfloat16 f);
     QDataStream &operator<<(float f);

src/corelib/kernel/qmetatype.cpp

@@ -1350,6 +1350,7 @@ bool QMetaType::save(QDataStream &stream, int type, const void *data)
     case QMetaType::QJsonDocument:
         return false;
     case QMetaType::Nullptr:
+        stream << *static_cast<const std::nullptr_t *>(data);
         return true;
     case QMetaType::Long:
         stream << qlonglong(*static_cast<const long *>(data));
@@ -1573,6 +1574,7 @@ bool QMetaType::load(QDataStream &stream, int type, void *data)
     case QMetaType::QJsonDocument:
         return false;
     case QMetaType::Nullptr:
+        stream >> *static_cast<std::nullptr_t *>(data);
         return true;
     case QMetaType::Long: {
         qlonglong l;

tests/auto/corelib/io/qdatastream/tst_qdatastream.cpp

@@ -77,6 +77,9 @@ private slots:
     void stream_QDateTime_data();
     void stream_QDateTime();
 
+    void stream_nullptr_t_data();
+    void stream_nullptr_t();
+
     void stream_QFont_data();
     void stream_QFont();
 
@@ -187,6 +190,7 @@ private:
     void writeQBrush(QDataStream *s);
     void writeQColor(QDataStream *s);
     void writeQByteArray(QDataStream *s);
+    void writenullptr_t(QDataStream *s);
 #ifndef QT_NO_CURSOR
     void writeQCursor(QDataStream *s);
 #endif
@@ -216,6 +220,7 @@ private:
     void readQBrush(QDataStream *s);
     void readQColor(QDataStream *s);
     void readQByteArray(QDataStream *s);
+    void readnullptr_t(QDataStream *s);
 #ifndef QT_NO_CURSOR
     void readQCursor(QDataStream *s);
 #endif
@@ -1008,6 +1013,11 @@ void tst_QDataStream::writeQByteArray(QDataStream *s)
     *s << d4;
 }
 
+void tst_QDataStream::writenullptr_t(QDataStream *s)
+{
+    *s << nullptr;
+}
+
 void tst_QDataStream::readQByteArray(QDataStream *s)
 {
     QByteArray test(qByteArrayData(dataIndex(QTest::currentDataTag())));
@@ -1016,6 +1026,13 @@ void tst_QDataStream::readQByteArray(QDataStream *s)
     QCOMPARE(d4, test);
 }
 
+void tst_QDataStream::readnullptr_t(QDataStream *s)
+{
+    std::nullptr_t ptr;
+    *s >> ptr;
+    QCOMPARE(ptr, nullptr);
+}
+
 // ************************************
 #ifndef QT_NO_CURSOR
 static QCursor qCursorData(int index)
@@ -1263,6 +1280,17 @@ void tst_QDataStream::stream_QDateTime()
     STREAM_IMPL(QDateTime);
 }
 
+void tst_QDataStream::stream_nullptr_t_data()
+{
+    stream_data(1); // there's only one value possible
+}
+
+void tst_QDataStream::stream_nullptr_t()
+{
+    using namespace std;
+    STREAM_IMPL(nullptr_t);
+}
+
 void tst_QDataStream::writeQDateTime(QDataStream *s)
 {
     QDateTime dt(qDateTimeData(dataIndex(QTest::currentDataTag())));