在泛型上调用方法？ [英] Invoke method on generic type?

问题描述

以下为什么在Delphi（XE）中产生错误？

Why does the following generate an error in Delphi (XE)?

unit UTest;

interface


type

TTest = class
  public
  procedure Foo<T>(A: T);
end;

implementation

{ TTest }

procedure TTest.Foo<T>(A: T);
begin
  A.Add('hej');
end;

end.

我认为Delphi中的泛型类型只是插入到泛型函数中，因此只会出错

I thought that generic types in Delphi was simply inserted into the generic function, so it would only error out if used with a type which don't have an Add(string) method.

推荐答案

您的代码会产生编译错误，因为编译器无法知道 T 有一个名为 Add 的方法，该方法接收单个字符串参数。

Your code produces a compilation error because the compiler has no way of knowing that T has a method named Add that receives a single string parameter.

我认为Delphi中的泛型类型只是简单地插入到泛型函数中，因此，如果将其与不使用

I thought that generic types in Delphi was simply inserted into the generic function, so it would only error out if used with a type which don't have an Add(string) method.

如果您使用的是Smalltalk或C ++模板，则您的假设是正确的。但是，泛型与模板不同。对于泛型，您需要将约束应用于type参数。该约束需要告诉编译器 T 必须具有什么属性。

If you were using Smalltalk or C++ templates, then your assumption would be accurate. However, generics are not the same as templates. For generics you need to apply a constraint to the type parameter. The constraint needs to tell the compiler what properties T must have.

例如，您可以约束 T 将从具有适当的 Add 方法的类派生。或者，您可以限制 T 来实现带有合适的 Add 方法的接口。

For example, you could constrain T to be derived from a class that has a suitable Add method. Or you could constrain T to implement an interface with a suitable Add method.

Delphi通用约束的文档链接： http://docwiki.embarcadero.com/RADStudio/en/Constraints_in_Generics

Documentation link for Delphi generic constraints: http://docwiki.embarcadero.com/RADStudio/en/Constraints_in_Generics

可以应用的通用约束相当有限，这实在令人遗憾。例如，我很希望能够将一个类型约束为具有某些数学运算符。例如，我希望能够将一个类型约束为具有 + 和-运算符。但是，泛型和模板都各有利弊，因此，我确实接受这些限制是Delphi语言设计师做出合理设计决定的结果。

The generic constraints that can be applied are rather limited, which is something of a shame. For example, I'd love to be able to constrain a type to have certain mathematical operators. For example, I'd like to be able to constrain a type to have + and - operators, say. However, there are pros and cons to both generics and templates, and so I do accept that these limitations are the result of a justifiable design decision by the Delphi language designers.

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