是否可以在Delphi中为通用类创建一个类别别名 [英] Is it possible to create a type alias to a generic class in Delphi

问题描述

我想为泛型类定义一个类类型（类型别名）。我想这样做，所以单位b的用户可以访问TMyType而不使用单位a。
我有这样的单位：

I would like to define a class type (type alias) for a generic class. I would like to do this so users of unit b can have access to TMyType without using unit a. I have units like this:

unit a;
interface
type
  TMyNormalObject = class
    FData: Integer;
  end;
  TMyType<T> = class
    FData: <T>;
  end;
implementation
end.

unit b;
interface
type
  TMyNormalObject = a.TMyNormalObject;  // works
  TMyType<T> = a.TMyType<T>; // E2508 type parameters not allowed on this type
implementation
end.

我已经找到了一个我不喜欢的可能的解决方法，因为它可以很容易地发现错误：

I already found a possible workaround which I don't like because it can introduce hard to find bugs:

TMyType<T> = class(a.TMyType<T>); 

这种方法的问题是它引入了一个新的类类型，而a.TMyType实例不是一个b.TMyType（而a.TMyNormallClass是一个b.TMyNormalClass，反之亦然 - 它们指的是同一个类）。

The problem with this approach is that it introduces a new class type and an a.TMyType instance is not a b.TMyType (while a.TMyNormallClass is a b.TMyNormalClass and vice versa - they are referring to the same class).

推荐答案

p>

示例：

TMyClass<T> = class
end;
TMyClassClass<T> = class of TMyClass<T>; //E2508 type parameters not allowed on this type

呈现的解决方法如下所示： p>

The workaround that is presented looks like this :

TMyIntClass = TMyType<Integer>;
TMyIntClassClass = Class of TMyIntClass;

但是如同评论的那样，这将会击败泛型的整个想法，因为该类必须被子类化对于每个通用实例化，

But as commented, that would defeat the whole idea of generics, since the class would have to be subclassed for every generic instantiation.

这里也是一个类似的解决方法的链接，用于生成泛型类型的专门子类：派生自专门的通用类型。在这种情况下，它将如下所示：

Here is also a link to a similar workaround on generating a specialized subclass of a generic type: derive-from-specialized-generic-types. In this case it would look like this :

TMySpecialClass = Class(TMyType<Integer>);

更新：

RM提出的解决方法：

The workaround proposed by RM:

TMyType<T> = class(a.TMyType<T>);

可以使用以下方案实现类型安全：

can be implemented with type safety using following scheme:

unit Unita;
interface
type
  TMyType<T> = class
    Constructor Create;
  end;

implementation

uses
  Unitb;

constructor TMyType<T>.Create;
begin
  Inherited Create;
  //WriteLn( Self.QualifiedClassName,' ',Unitb.TMyType<T>.QualifiedClassName);
  Assert(Self.QualifiedClassName = Unitb.TMyType<T>.QualifiedClassName);
end;

end.

---

---

unit Unitb;

interface

uses Unita;

type
  TMyType<T> = class(Unita.TMyType<T>);
implementation
end.

---

---

Project Test;
{$APPTYPE CONSOLE}    
uses
  System.SysUtils,
  Unita in 'Unita.pas',
  Unitb in 'Unitb.pas';

var
  t1 : Unita.TMyType<Integer>;
  t2 : Unitb.TMyType<Integer>;
  t3 : TMyType<Integer>;    
begin
  try
    //t1 := Unita.TMyType<Integer>.Create;  //Exception EAssertionFailed !!
    t2 := Unitb.TMyType<Integer>.Create;
    t3 := TMyType<Integer>.Create;
    ReadLn;
  finally
    //t1.Free;
    t2.Free;
    t3.Free;
  end;
end.

创建通用类时，会进行一次测试，以检查创建的类是否从类型派生在单位b中宣布。因此，检测到从单位a创建此类的所有尝试。

When creating the generic class, a test is made to check that the created class is derived from the type declared in unit b. Thereby all attempts to create this class from unit a is detected.

更新2：

为了清楚起见，引用通用类类型为< T> 是不可能的，但泛型类的副本是正常的。

Just to be clear, a reference to a generic class, "class of type<T>" is not possible, but a copy of a generic class is fine.

这篇关于是否可以在Delphi中为通用类创建一个类别别名的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！