Use so_default_state as a parent state for an another state

[zamazan4ik]

Hi!

Is it possible with SObjectizer to use default agent state (which is returned with so_default_state) as a parent state for another state? I am trying to do something like this:

state_t NotStarted{initial_substate_of(so_default_state()), "NotStarted"};

Unfortunately initial_substate_of takes a non-const reference but so_default_state returns const reference, so it doesn't compile.

Is there any workaround (except introducing own "default" state and making it as a parent for another events)? Is it a limitation by design?

Thank you.

[eao197]

Good question! :)

I think it's some heritage from ancient times. I don't think there are some reasons that prevent making agent_t::st_default non-const member of agent_t. I'll try to look deeper tomorrow (or Monday) and if there is no any other issues I'll change the st_default and add a new overload for so_default_state.

[eao197]

There is a hidden underwater rock: a call to on_enter handlers for child states. For example:

class demo final : public so_5::agent_t {
   state_t st_parent{this, "parent"};
   state_t st_child_1{initial_substate_of{st_parent}, "child_1"};
   state_t st_child_2{substate_of{st_parent}, "child_2"};
   ...
   void so_define_agent() override {
      st_child.on_enter([]{ std::cout << "Hello, World!" << std::endl; });
      ...
      this >>= st_parent;
   }
};

In that case "Hello, World" would be printed at the start of demo agent.

But in that case:

class demo final : public so_5::agent_t {
   state_t st_child_1{initial_substate_of{so_default_state()}, "child_1"};
   state_t st_child_2{substate_of{so_default_state()}, "child_2"};
   ...
   void so_define_agent() override {
      st_child.on_enter([]{ std::cout << "Hello, World!" << std::endl; });
      ...
   }
};

The enter handler for st_child won't be called at the start of demo agent. It's because st_default is already the current state for the agent and SObjectizer doesn't know about changes that should be taken into the account.

[zamazan4ik]

Hm... interesting case. In my opinion that's just "nice to know" thing, which shall be documented, if you want to allow so_default_state as a parent state in SObjectizer. In my case the example above doesn't matter at all.

For now locally I just created my own default state :)