oop - 如何在 Ada 中实现接口(interface)？

Question

不知道这个 oop 模式叫什么，但我怎样才能在 Ada 中做同样的模式？
例如这段代码:

interface Vehicle{
    string function start();
}

class Tractor implements Vehicle{
    string function start(){
        return "Tractor starting";
    }
}
class Car implements Vehicle{
    string function start(){
        return "Car  starting";
    }
}

class TestVehicle{
    function TestVehicle(Vehicle vehicle){
        print( vehicle.start() );
    }
}
new TestVehicle(new Tractor);
new TestVehicle(new Car);

我在 Ada 中的失败尝试:
如何正确修复？

with Ada.Text_IO;

procedure Main is

   package packageVehicle is
      type Vehicle is interface;
      function Start(Self : Vehicle) return String is abstract;
   end packageVehicle;

   type Tractor is new packageVehicle.Vehicle with null record;
   overriding -- optional
   function Start(Self : Tractor) return string is
   begin
      return "Tractor starting!";
   end Start;
   type Car is new packageVehicle.Vehicle with null record;
   overriding -- optional
   function Start(Self : Car) return string is
   begin
      return "Car starting!";
   end Start;


   procedure TestVehicle(Vehicle : packageVehicle.Vehicle) is
   begin
      Ada.Text_IO.Put_Line( "Testing a vehicle" );
      Ada.Text_IO.Put_Line( Start(Vehicle) );
   end;

   Tractor0 : Tractor;
   Car0 : Car;

begin

   Ada.Text_IO.Put_Line( TestVehicle(Tractor0) );
   Ada.Text_IO.Put_Line( TestVehicle(Car0) );

end Main;

编译器说:
生成器结果警告:“TestVehicle”的声明为时已晚
生成器结果警告:规范应在“车辆”声明后立即出现

Answer 1

需要注意的关键是“接口(interface)类型的所有用户定义的原始子程序都应该是抽象子程序或空过程”。 ( Ref ) 即您不能定义将接口(interface)本身作为参数的子程序(是的，我知道这与 Java 不同。)这就是您在 TestVehicles 声明中收到错误的原因。

本质上，您必须定义一个实现接口(interface)的类型，然后使用该类型。

关于 Interfaces 的 Ada 基本原理一章对此进行了一些详细的讨论。

这是一个基于您的问题的工作示例-我重命名了一些内容并修复了一些错误，这些错误可能在您看到的错误消息中丢失了:-) 请注意添加了一个实例化 Vehicle 接口(interface)的类型“Concrete_Vehicles”。

with Ada.Text_IO; use Ada.Text_IO;

procedure Interface_Test is

   package Package_Vehicle is 
      type Vehicle is interface;

      function Start(Self : Vehicle) return String is abstract;
   end Package_Vehicle;


   type Concrete_Vehicles is abstract new Package_Vehicle.Vehicle with null record;


   type Tractor is new Concrete_Vehicles with null record;

   overriding -- optional
   function Start(Self : Tractor) return string is
   begin
      return "Tractor starting!";
   end Start;

   type Car is new Concrete_Vehicles with null record;
   overriding -- optional
   function Start(Self : Car) return string is
   begin
      return "Car starting!";
   end Start;


   procedure TestVehicle(Vehicle : Concrete_Vehicles'Class) is
   begin
      Ada.Text_IO.Put_Line( "Testing a vehicle" );
      Ada.Text_IO.Put_Line( Start(Vehicle) );
   end;

   Tractor0 : Tractor;
   Car0 : Car;

begin

  TestVehicle(Tractor0);
  TestVehicle(Car0);

end Interface_Test;

编译和运行:

[22] Marc say: gnatmake interface_test.adb
gcc -c interface_test.adb
gnatbind -x interface_test.ali
gnatlink interface_test.ali
[23] Marc say: ./interface_test
Testing a vehicle
Tractor starting!
Testing a vehicle
Car starting!