具有持续访问父类型对象的 Ada 记录

Question

我最近开始学习 Ada。我想看看是否有可能在 Ada 中创建一个 Boost::Statechart-like 框架。为此，我需要一个具有常量访问父类型组件对象的记录结构，例如静态指向另一个树节点的树节点，并且 父指针不得在所有时间。像这样：

-- Not working sample

type Node_T is record
    Parent : constant access Node_T;
    -- error: constant components are not permitted
end record;

-- I wish to create objects of this type like this

Top_Node : Node_T (null);
Child1_Node : Node_T (Top_Node'Access);
Child2_Node : Node_T (Top_Node'Access);

Ada 似乎不支持常量成员字段。所以我求助于使用访问鉴别器：

-- Not working sample

type Node_T (Parent : access Node_T) is null record;
-- error: type declaration cannot refer to itself

但是，使用命名访问类型作为判别有效

type Node_T;
type Ref_Node_T is access all Node_T;

type Node_T (Parent : Ref_Node_T) is null record;

但是，据我了解，这会导致 Node_T 对象的生命周期绑定到 Ref_Node_T 对象的生命周期，而不是另一个父 Node_T 对象的生命周期。这是真的吗？

有没有更好的方法来实现我的需要？

Answer 1

https://www.sigada.org/ada_letters/june2000/sanden.pdf 中描述了另一种创建有限状态机的方法此解决方案使用受保护对象和任务的组合来实现有限状态机。

Answer 2

FSM 的另一种替代解决方案是使用枚举和数组，如果您需要多个，generic。

Generic
   Type State is (<>); -- Any discrete type.
   Type Event is (<>);
Package Finite_State_Machine_Domain is
   Type Domain is Array(State, Event) of State;
   
   Generic
     Start,
     Error : State;
   Package Finite_State_Machine is
      Type State_Machine is private;
      Function  Create    (State_Map : Domain)            return State_Machine;
      Function  Get_State (Object : in     State_Machine) return State;
      Procedure Send_Event(Object : in out State_Machine; Transition : in Event);
   Private
      Type State_Machine is record
         Current   : State  := Start;
         State_Map : Domain := (Others => Error);
      End record;
   End Finite_State_Machine;
End Finite_State_Machine_Domain;

Package Body Finite_State_Machine_Domain is
   Package Body Finite_State_Machine is

      Function  Create (State_Map : Domain) return State_Machine is
       ( State_Machine'(State_Map => State_Map, Others => <>) );

      Function  Get_State (Object : in     State_Machine) return State is
       ( Object.Current );

      Procedure Send_Event(Object : in out State_Machine; Transition : in Event) is
      Begin
         if Object.Current /= Error then
            Object.Current:= Object.State_Map(Object.Current, Transition);
         end if;
      End Send_Event;

   End Finite_State_Machine;
End Finite_State_Machine_Domain;

具有持续访问父类型对象的 Ada 记录

Ada record with constant access-to-object-of-parent-type

ada