{"id":4846,"date":"2026-10-18T20:59:27","date_gmt":"2026-10-18T18:59:27","guid":{"rendered":"https:\/\/mariusbancila.ro\/blog\/?p=4846"},"modified":"2026-10-18T20:59:29","modified_gmt":"2026-10-18T18:59:29","slug":"composite-objects-and-cpp26-the-stdpolymorphic-type","status":"publish","type":"post","link":"https:\/\/mariusbancila.ro\/blog\/2026\/10\/18\/composite-objects-and-cpp26-the-stdpolymorphic-type\/","title":{"rendered":"Composite objects and C++26 the std::polymorphic type"},"content":{"rendered":"\n

In a previous article<\/a>, I discussed the C++ std::indirect<\/code> type and how it helps implementing the PImpl idiom. However, the std::indirect<\/code> type was inducted in the standard alongside another type called std::polymorphic<\/code>. This helps implementing composite classes, where composite means a class composed of member objects in an has-a relation, where the composite own the components which are part of its values. Examples include strategy objects, AST\/expression-tree nodes, shapes-style hierarchies, plugin-configured behavior – and you want the owning class to remain a regular, copyable value. In this article, we’ll look at how std::polymorphic<\/code> can be used for this purpose.<\/p>\n\n\n\n

The need for std::polymorphic<\/h2>\n\n\n\n

Let us consider the following class hierarchy:<\/p>\n\n\n\n

struct Event {\n  virtual std::string describe() const = 0;\n};\nstruct TemperatureReadingEvent : public Event {\n  std::string describe() const override { }\n};\nstruct SmokeDetectedEvent : public Event {\n  std::string describe() const override { }\n};\nstruct CarbonMonoxideDetectedEvent : public Event {\n  std::string describe() const override { }\n};\n\nclass EventLog {\n  ??? events_;\n}<\/pre>\n\n\n\n
We have various types of events from various sensors in a smart house and we want to store them in an event log. How do we do that? There are various options:<\/p>\n\n\n\n