Ada：具有可变大小数组属性的对象 [英] Ada: objects with variable-sized array attribute

问题描述

我想在封装内创建一个标记类型，它描述了一个二维离散空间，其大小在运行时确定。
（上下文：执行人生游戏）

我发现的第一种方式是通用性：

 通用
尺寸：自然; 
包世界是
类型World_Type被标记为private; 
类型世界正在访问World_Type'Class; 
子类型坐标为正范围1..Size; 
 private 
类型World_Array是Boolean的坐标（Coordinate，Coordinate）; 
类型World_Type被标记记录
内容：World_Array; 
结束记录; 
结束世界; 
  

但是，当为世界实现访问时，通用性成为一个大问题：

 与Worlds; 
 
包World_Visitors是
类型World_Visitor_Type被标记为private; 
类型World_Visitor访问World_Visitor_Type'Class; 
 
程序Visite（v：World_Visitor_Type; 
 w：in Worlds.World）; - 错误：选定组件中的无效前缀Worlds
 private 
类型World_Visitor_Type被标记为空记录; 
 end World_Visitors; 
  

GNAT无法编译，因为Worlds是一个通用包。
然后，因为我不想为每个可能的世界大小编写一个访问器，所以我尝试使用C ++方法：将大小声明为带标记类型的属性。

 包世界是
类型World_Type被标记为private; 
类型世界正在访问World_Type'Class; 
子类型坐标是正范围<>; 
 
函数Init（大小：自然）return World; - 将Content属性初始化为长度数组（大小*大小）
私有
类型World_Array是布尔的数组（坐标，坐标）; 
类型World_Type被标记记录
内容：World_Array; 
尺寸：自然; 
结束记录; 
结束世界; 
  

而且，预​​计这不起作用，因为World_Array需要明确的坐标范围。
实际上，我不知道如何创建一个标记类型的运行时选择大小的数组。
我从这里得到了一些建议， a href =https://stackoverflow.com/questions/22768834/ada-packing-record-with-variable-sized-array>这里，或 / 1732027 / returns-a-variable-sized-matrix-from-a-function-in-ada html＃4.3rel =nofollow noreferrer> here ，但在这种情况下没有任何意义。

大小的数组属性？

解决方案

解决此问题的正常Ada方法是使用判别式请参阅 ARM 3.7 ）。

就你的情况而言，这看起来像

 包世界是
类型World_Type（大小：Natural）被标记为private; 
类型世界正在访问World_Type'Class; - ''来修正SO颜色bug 
 private 
 type World_Array是Boolean的数组（正范围<>正范围>）; 
类型World_Type（Size：Natural）被标记为记录
 Content：World_Array（1 .. Size，1 .. Size）; 
结束记录; 
结束世界; 
  

其中 World_Array 是无约束数组类型（ ARM 3.6 ）。您可以通过代码创建一个新世界，比如：

  W：Worlds.World：= new Worlds .World_Type（Size => 100）; 
  

请注意，您无法更改大小

我省略了坐标;如果你为 Content 提供了一个初始化工具，那么你可能会在 Init> b

 内容：World_Array（1 .. Size，1 .. Size）：= 
（others =>（ others => False））; 
  

编辑过26.iii.15：代码创建一个大小<$ c $的数组c>大小+ 1 x 大小+ 1 。

I want to create a tagged type inside a package that describes a 2D discrete space, with a size determined in running time. (context : implementation of a game of life)

First way I found was genericity :

generic
    Size : Natural;
package Worlds is
    type World_Type is tagged private;
    type World is access World_Type'Class;
    subtype Coordinate is Positive range 1..Size;
private
    type World_Array is array (Coordinate, Coordinate) of Boolean;
    type World_Type is tagged record
            Content : World_Array;
    end record;
end Worlds;

But, when implementing a visitor for worlds, genericity become a big problem :

with Worlds;

package World_Visitors is
    type World_Visitor_Type is tagged private;
    type World_Visitor is access World_Visitor_Type'Class;

    procedure Visite(v : World_Visitor_Type;
                     w : in out Worlds.World); -- ERROR: invalid prefix in selected component "Worlds"
private
    type World_Visitor_Type is tagged null record;
end World_Visitors;

GNAT can't compile that, because Worlds is a generic package. Then, because I don't want to write a Visitor for each possible world size, I try the C++ way : declare the size as attribute in the tagged type.

package Worlds is
    type World_Type is tagged private;
    type World is access World_Type'Class;
    subtype Coordinate is Positive range <>;

    function Init(Size : Natural) return World; -- initialize Content attribute as an array of length (Size*Size)
private
    type World_Array is array (Coordinate, Coordinate) of Boolean;
    type World_Type is tagged record
            Content : World_Array;
            Size    : Natural;
    end record;
end Worlds;

And, expectedly, this doesn't work, because World_Array needs an explicit range for Coordinates. In fact, I have no idea how to create a running-time-choosen sized array in a tagged type. I got some ideas from here, here, here or here, but nothing seems to make sense in this case.

How does Ada implement objects with variable-sized array attribute?

解决方案

The normal Ada way to address this problem is to use a discriminant (see ARM 3.7).

In your case, this would look something like

package Worlds is
   type World_Type (Size : Natural) is tagged private;
   type World is access World_Type’Class;          -- ‘’ to fix SO colour bug
private
   type World_Array is array (Positive range <>, Positive range <>) of Boolean;
   type World_Type (Size : Natural) is tagged record
      Content : World_Array (1 .. Size, 1 .. Size);
   end record;
end Worlds;

in which World_Array is an example of an unconstrained array type (ARM 3.6). You’d create a new world by code like

W : Worlds.World := new Worlds.World_Type (Size => 100);

Note that you can’t change the Size after the object has been created.

I left out Coordinate; and you may get away without Init, especially if you provide an initializer for Content:

      Content : World_Array (1 .. Size, 1 .. Size) :=
        (others => (others => False));

Edited 26.iii.15: Code was creating an array of size Size + 1 x Size + 1.

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