QMetaType: Support custom unary converters with optional<To> return type

To indicate success of a conversion, the public API has previously only
supported registering member functions of the form To (From::*)(bool *).
When adding custom converters for types that cannot be modified, this is
usually not a possibility.
As an alternative, this patch adds support for std::optional in the
UnaryFunction overload of QMetaType::registerConverter. If the returned
optional has no value, the conversion is considered failed.

Task-number: QTBUG-92902
Change-Id: Ibac52d2cb9b5a2457081b4bebb0def1f03e3c55d
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

1 files changed, 8 insertions, 0 deletions

diff --git a/src/corelib/doc/snippets/qmetatype/registerConverters.cpp b/src/corelib/doc/snippets/qmetatype/registerConverters.cpp
index 5c5e76c7b0..f53d04b7a6 100644
--- a/src/corelib/doc/snippets/qmetatype/registerConverters.cpp
+++ b/src/corelib/doc/snippets/qmetatype/registerConverters.cpp
@@ -1,6 +1,7 @@
 // Copyright (C) 2021 The Qt Company Ltd.
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
 
+#include <QJsonObject>
 #include <QMetaType>
 #include <QString>
 
@@ -50,5 +51,12 @@ int main() {
   QMetaType::registerConverter<CustomStringType, QString>([](const CustomStringType &str) {
       return QString::fromUtf8(str.data());
   });
+  QMetaType::registerConverter<QJsonValue, QPointF>(
+            [](const QJsonValue &value) -> std::optional<QPointF> {
+      const auto object = value.toObject();
+      if (!object.contains("x") || !object.contains("y"))
+          return std::nullopt;  // The conversion fails if the required properties are missing
+      return QPointF{object["x"].toDouble(), object["y"].toDouble()};
+  });
   //! [unaryfunc]
 }