阿达：如何解决“循环单位依赖”？

发布时间：2020-12-14 04:48:42 所属栏目：百科来源：网络整理

导读：假设我有两个记录：人和动物.每条记录都在一个单独的包中. 套餐人员： with animals;use animals;package persons is type person is record ... animalref: animalPOINTER; ... end record; type personPOINTER is access person;end persons; 包装动物： w

假设我有两个记录：人和动物.每条记录都在一个单独的包中.

套餐人员：

with animals;
use animals;

package persons is 

    type person is record
     ...
     animalref: animalPOINTER;
     ...
    end record;

    type personPOINTER is access person;

end persons;

包装动物：

with persons;
use persons;

package animals is 
    type animal is record
     ...
     ownerref:  personPOINTER;
     ...
    end record;

    type animalPOINTER is access animal;

end animals;

我在这里有循环单元依赖,编译器会产生致命错误.

有没有人有解决这个问题的模式？

谢谢！

解决方法

你需要限制,这是为了解决这个问题而引入的.见 Rationale for Ada 2005,section 4.2.

动物和人是对称的(我的编辑器调整了布局和外壳;我为每个添加了一个记录组件,因此下面的演示程序可以打印一些东西)：

limited with Animals;
package Persons is

   --  One of the few things you can do with an incomplete type,which
   --  is what Animals.Animal is in the limited view of Animals,is to
   --  declare an access to it.
   type AnimalPOINTER is access Animals.Animal;

   type Person is record
      Name : Character;
      Animalref : AnimalPOINTER;
   end record;

end Persons;

limited with Persons;
package Animals is

   type PersonPOINTER is access Persons.Person;

   type Animal is record
      Name : Character;
      Ownerref : PersonPOINTER;
   end record;

end Animals;

演示程序可以全面了解动物和人物.这个例子非常笨拙;您可以通过向动物和人物添加子程序来更好地组织事物.请注意,动物的身体可以(并且必须)与人类;如果需要在人物中使用任何东西.

with Ada.Text_IO; use Ada.Text_IO;
with Animals;
with Persons;
procedure Animals_And_Persons is
   A : Persons.animalPOINTER := new Animals.Animal;
   P : Animals.PersonPOINTER := new Persons.Person;
begin
   A.all := (Name => 'a',Ownerref => P);
   P.all := (Name => 'p',Animalref => A);
   Put_Line (P.Name & " owns " & P.Animalref.Name);
   Put_Line (A.Name & " is owned by " & A.Ownerref.Name);
end Animals_And_Persons;

在编译和运行时给出

$./animals_and_persons 
p owns a
a is owned by p

（编辑：李大同）

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