泛型：什么是“CONSTRUCTOR约束”？ [英] Generics: What's a "CONSTRUCTOR constraint"?

问题描述

我制作了一个自定义的TObjectList子类，用于保存基础对象类的子类。它看起来像这样：

I made a custom TObjectList descendant designed to hold subclasses of a base object class. It looks something like this:

interface
   TMyDataList<T: TBaseDatafile> = class(TObjectList<TBaseDatafile>)
   public
      constructor Create;
      procedure upload(db: TDataSet);
   end;

implementation

constructor TMyDataList<T>.Create;
begin
   inherited Create(true);
   self.Add(T.Create);
end;

我想让每个新列表从一个空白对象开始。这很简单，对吧？但编译器不喜欢它。它说：

I want each new list to start out with one blank object in it. It's pretty simple, right? But the compiler doesn't like it. It says:

在类型参数声明中无法创建没有CONSTRUCTOR约束的新实例
我只能假设这是泛型相关的。任何人都知道发生了什么，以及我如何使这个构造函数工作。

"Can't create new instance without CONSTRUCTOR constraint in type parameter declaration" I can only assume this is something generics-related. Anyone have any idea what's going on and how I can make this constructor work?

推荐答案

您正在尝试创建一个实例 T 通过 T.Create 。这不工作，因为编译器不知道你的泛型类型有一个无参数的构造函数（记住：这不是必需的）。要纠正这个问题，你必须创建一个构造函数约束，如下所示：

You're trying to create an instance of T via T.Create. This doesn't work because the compiler doesn't know that your generic type has a parameterless constructor (remember: this is no requirement). To rectify this, you've got to create a constructor constraint, which looks like this:

<T: constructor>

或在您的特定情况下：

<T: TBaseDatafile, constructor>

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