我怎样才能声明一个基于泛型类型的指针？ [英] How can I declare a pointer based on a generic type?

问题描述

我有这样的一个班级：

  type A = class 
 procedure< T> DoStuff（tPtr：^ T）; 
 end; 
  

但是当我尝试编译时，Delphi给了我这个错误：

  [DCC Error] RPML.pas（57）：E2029标识符预期，但找到'^'
  

如何在Delphi过程中使用指向参数化类型的指针？我不想让整个类都成为一个模板类。

解决方案

为此，您需要声明一个指针类型为通用类中的嵌套类型：

 类型
 TMyGeneric< T> = class 
 type 
 P = ^ T; 
 public 
程序DoStuff（tPtr：P）; 
 end; 
  

如果你想要一个类方法（即不是实例方法），你可以这样做：

 类型
 TMyGeneric< T> =记录
类型
 P = ^ T; 
 public 
类程序DoStuff（tPtr：P）;静态的; 
 end; 
 
 var 
 int：Integer; 
 ... 
 TMyGeneric< Integer> .DoStuff（@int）; 
  

或者使用var参数：

 键入
 TMyGeneric< T> =记录
 public 
类程序DoStuff（var a：T）;静态的; 
 end; 
  

对于通用类型，使用记录而不是类是常见的， 。

最后，在Delphi中，不能使泛型类成为通用方法。换句话说，没有类似于以下C ++模板代码：

Thorsten的答案显示了如何在不使类通用的情况下实现泛型方法，即以下C ++模板代码的Delphi模拟：

  class C {
 public：
 template< ; typename T> 
 int SomeTemplateFunction（T * data）{
 printf（参数的地址是％p \ n，data）; 
返回0; 
} 
}; 
 
 int a; 
 char c; 
 C cinst; 
 cinst.SomeTemplateFunction< int>（& a）; 
 cinst.SomeTemplateFunction< char>（& c）; 
  

Thorsten的答案为您提供了一个类函数，但在您声明的注释中您正在寻找一个正常的成员函数

 类型
 TMyClass = class 
 public 
过程DoStuff< T>（var a ：T）; 
 end; 
 
 procedure TMyClass.DoStuff< T>（var a：T）; 
 begin 
 end; 
 
 ... 
 var 
实例：TMyClass; 
我：整数; 
 s：string; 
 ... 
 instance.DoStuff< Integer>（i）; 
 instance.DoStuff< string>（s）; 
  

然而，我正在努力的是如何做到这一点非常有用，德尔福，如果没有一个通用的解决方案，就无法做到这一点。

我很感激任何建议，并且很乐意编辑答案以适应它们。 p>

I have a class like this:

type A = class
    procedure<T> DoStuff(tPtr: ^T);
end;

But when I try to compile, Delphi gives me this error:

[DCC Error] RPML.pas(57): E2029 Identifier expected but '^' found

How can I use a pointer to a parameterized type in a Delphi procedure? I don't want to make the whole class a template class.

解决方案

To do this you need to declare a pointer type as a nested type in the generic class:

type 
  TMyGeneric<T> = class
  type
    P = ^T;
  public
    procedure DoStuff(tPtr: P);
  end;

And if you want a class method (i.e. not an instance method) you can do it this way:

type
  TMyGeneric<T> = record
  type
    P = ^T;
  public
    class procedure DoStuff(tPtr: P); static;
  end;

var
  int: Integer;
...
TMyGeneric<Integer>.DoStuff(@int);

Or using a var parameter:

type
  TMyGeneric<T> = record
  public
    class procedure DoStuff(var a: T); static;
  end;

It seems to be common to use records rather than classes for generic types that don't ever get instantiated.

Finally, you cannot have, in Delphi, a generic method without making the class generic. In other words there is no analogue of the following C++ template code:

Thorsten's answer shows how to implement a generic method without making the class generic, that is the Delphi analogue of of the following C++ template code:

class C {
public:
   template <typename T>
   int SomeTemplateFunction(T* data) {
      printf("Address of parameter is %p\n", data);
      return 0;
   }
};

int a; 
char c; 
C cinst; 
cinst.SomeTemplateFunction<int>(&a); 
cinst.SomeTemplateFunction<char>(&c);

Thorsten's answer gives you a class function but in the comments you state you are looking for a normal member function.

type
  TMyClass = class
  public
    procedure DoStuff<T>(var a: T);
  end;

procedure TMyClass.DoStuff<T>(var a: T);
begin
end;

...
var
  instance: TMyClass;
  i: Integer;
  s: string;
...
  instance.DoStuff<Integer>(i);
  instance.DoStuff<string>(s);

However, what I'm struggling with is how exactly you could do anything very useful with this, in Delphi, that could not be done just as effectively without a generic solution.

I'd appreciate any suggestions and would be happy to edit the answer to accommodate them.

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