fix: make Variant conversion operator explicit (#428)

Having explicit conversion operator is a good practice according to the C++ core guidelines, as it makes the code safer and better follows the principle of least astonishment. Also, it is consistent with the standard library style, where wrappers like std::variant, std::any, std::optional... also do not provide an implicit conversion to the underlying type. Last but not least, it paves the way for the upcoming std::variant <-> sdbus::Variant implicit conversions without surprising behavior in some edge cases.

docs/using-sdbus-c++.md

@@ -1765,6 +1765,7 @@ sdbus-c++ v2 is a major release that comes with a number of breaking API/ABI/beh
 * `PollData::getRelativeTimeout()` return type was changed to `std::chrono::microseconds`.
 * `IConnection::processPendingRequest()` was renamed to `IConnection::processPendingEvent()`.
 * `Variant` constructor is now explicit.
+* `Variant`'s conversion operator to the underlying type is now explicit.
 * `IProxy::getCurrentlyProcessedMessage()` now returns `Message` by value instead of a raw pointer to it. The caller assumes ownership of the message.
 * Object D-Bus API registration is now done through `IObject::addVTable()` method. The vtable gets active immediately. No `finishRegistration()` call is needed anymore. vtables can be added and removed dynamically at run time. In addition to API simplification this brings consistency with sd-bus API and increases flexibility.
 * Subscription to signals has been simplified. The subscription is active right after the `registerSignalHandler`/`uponSignal()` call. No need for the final call to `finishRegistration()`.

examples/org.freedesktop.DBus.ObjectManager/examplemanager-planet1-client-glue.h

@@ -47,7 +47,7 @@ public:
 public:
     std::string Name()
     {
-        return proxy_->getProperty("Name").onInterface(INTERFACE_NAME);
+        return proxy_->getProperty("Name").onInterface(INTERFACE_NAME).get<std::string>();
     }
 
 private:

include/sdbus-c++/Types.h

@@ -87,7 +87,7 @@ namespace sdbus {
 
         // Only allow conversion operator for true D-Bus type representations in C++
         template <typename _ValueType, typename = std::enable_if_t<signature_of<_ValueType>::is_valid>>
-        operator _ValueType() const
+        explicit operator _ValueType() const
         {
             return get<_ValueType>();
         }

tests/integrationtests/DBusMethodsTests.cpp

@@ -107,9 +107,9 @@ TYPED_TEST(SdbusTestObject, CallsMethodWithStructVariantsAndGetMapSuccesfully)
     std::vector<int32_t> x{-2, 0, 2};
     sdbus::Struct<sdbus::Variant, sdbus::Variant> y{false, true};
     std::map<int32_t, sdbus::Variant> mapOfVariants = this->m_proxy->getMapOfVariants(x, y);
-    decltype(mapOfVariants) res{ {sdbus::Variant{-2}, sdbus::Variant{false}}
-                               , {sdbus::Variant{0}, sdbus::Variant{false}}
-                               , {sdbus::Variant{2}, sdbus::Variant{true}}};
+    decltype(mapOfVariants) res{ {-2, sdbus::Variant{false}}
+                               , {0, sdbus::Variant{false}}
+                               , {2, sdbus::Variant{true}}};
 
     ASSERT_THAT(mapOfVariants[-2].get<bool>(), Eq(res[-2].get<bool>()));
     ASSERT_THAT(mapOfVariants[0].get<bool>(), Eq(res[0].get<bool>()));

tests/integrationtests/integrationtests-proxy.h

@@ -198,7 +198,7 @@ public:
 public:
     uint32_t action()
     {
-        return proxy_->getProperty("action").onInterface(INTERFACE_NAME);
+        return proxy_->getProperty("action").onInterface(INTERFACE_NAME).get<uint32_t>();
     }
 
     void action(const uint32_t& value)
@@ -208,7 +208,7 @@ public:
 
     bool blocking()
     {
-        return proxy_->getProperty("blocking").onInterface(INTERFACE_NAME);
+        return proxy_->getProperty("blocking").onInterface(INTERFACE_NAME).get<bool>();
     }
 
     void blocking(const bool& value)
@@ -218,7 +218,7 @@ public:
 
     std::string state()
     {
-        return proxy_->getProperty("state").onInterface(INTERFACE_NAME);
+        return proxy_->getProperty("state").onInterface(INTERFACE_NAME).get<std::string>();
     }
 
 private: