Ada:案例子集共享共同属性的变体记录

Question

我有以下代码，其中我只希望前两种情况共享一个公共(public)属性；但是，当我尝试使用此语法时，出现错误 "id"与第 11 行的声明冲突:

   type Shape (Which : Shape_Type := SQUARE) is
      record
      case Which is
         when Square =>
            id : Natural;   -- Line 11
         when Turnout =>
            id : Natural;   -- Line that causes error to be thrown
         when Invalid =>
            null;
      end case;
      end record;

Answer 1

这个:

type Shape (Which : Shape_Type := SQUARE) is
   record
      case Which is
         when Square | Turnout =>
            id : Natural;
         when Invalid =>
            null;
      end case;
   end record;

如果您稍后希望 Turnout 案例有一个额外的属性，您可以使用嵌套的 case 来做到这一点(但您仍然必须涵盖所有备选方案):

type Shape (Which : Shape_Type := SQUARE) is
   record
      case Which is
         when Square | Turnout =>
            id : Natural;
            case Which is
               when Square =>
                  null;
               when Turnout =>
                  Deg : Natural;
               when Invalid =>
                  null;
            end case;
         when Invalid =>
            null;
      end case;
   end record;