Interface function IEntityRepository::Get is returning an optional reference to query an IEntity.

In order to check whether an IEntity with a certain name/id exists, function IEntityRepository::Contains could be used to query this information. If IEntityRepository::Get is called on a non-existing name/id, the function would throw, so for defensive programming reasons a IEntityRepository::Contains should always be done before actually calling IEntityRepository::Get. The good thing about plain old C-pointers is that assignment and existence check can be done in one call. Since C++17 the same functionality is managed by std::optional.

Another disadvantage of using a plain reference to IEntity is when it is used as a class member, where it needs to be initialized when an object of that class is defined. It might be that during construction time this IEntity is not known/existing yet. Of cource, a plain old C-pointer could be used in this case, but it wouldn't be a consistant usage.

The proposal is to use std::optional<std::reference_wrapper<IEntity>> as return value for IEntityRepository::Get.

Signed-off-by: Jupp Tscheak <jupp.tscheak@daimler.com>

MantleAPI/include/MantleAPI/Traffic/i_entity_repository.h

@@ -18,6 +18,8 @@
 #include <MantleAPI/Traffic/entity_properties.h>
 #include <MantleAPI/Traffic/i_entity.h>
 
+#include <functional>
+#include <optional>
 #include <string>
 #include <vector>
 
@@ -35,8 +37,8 @@ class IEntityRepository
     virtual IStaticObject& Create(UniqueId id, const std::string& name, const StaticObjectProperties& properties) = 0;
 
     virtual IVehicle& GetHost() = 0;
-    virtual IEntity& Get(const std::string& name) = 0;
-    virtual IEntity& Get(UniqueId id) = 0;
+    virtual std::optional<std::reference_wrapper<IEntity>> Get(const std::string& name) = 0;
+    virtual std::optional<std::reference_wrapper<IEntity>> Get(UniqueId id) = 0;
     virtual bool Contains(UniqueId id) const = 0;
 
     virtual void Delete(const std::string& name) = 0;