delphi – 如何将对象转换为泛型？

发布时间：2020-12-15 04:15:22 所属栏目：大数据来源：网络整理

导读：我正在尝试将返回的基础对象强制转换为它的特定泛型类型.我认为下面的代码应该工作,但会产生内部编译器错误,还有另一种方法吗？ type TPersistGenericT = class private type TPointer = ^T; public class function Init : T; end;class function TPersistGe

我正在尝试将返回的基础对象强制转换为它的特定泛型类型.我认为下面的代码应该工作,但会产生内部编译器错误,还有另一种方法吗？

type
  TPersistGeneric<T> = class
  private
  type
    TPointer = ^T;
  public
    class function  Init : T;
  end;

class function  TPersistGeneric<T>.Init : T;
var
  o : TXPersistent; // root class
begin
  case PTypeInfo(TypeInfo(T))^.Kind of
    tkClass : begin
                // xpcreate returns txpersistent,a root class of T
                o := XPCreate(GetTypeName(TypeInfo(T))); // has a listed of registered classes
                result := TPointer(pointer(@o))^;
              end;
    else
      result := Default(T);
  end;
end;

解决方法

我正在使用一个类型转换辅助类来执行类型转换,并检查这两个类是否兼容.

class function TPersistGeneric<T>.Init: T;
var
  o : TXPersistent; // root class
begin
  case PTypeInfo(TypeInfo(T))^.Kind of
    tkClass : begin
                // xpcreate returns txpersistent,a root class of T
                o := XPCreate(GetTypeName(TypeInfo(T))); // has a listed of registered classes
                Result := TTypeCast.DynamicCast<TXPersistent,T>(o);
              end;
    else
      result := Default(T);
  end;

这是班级：

type
  TTypeCast = class
  public
    // ReinterpretCast does a hard type cast
    class function ReinterpretCast<ReturnT>(const Value): ReturnT;
    // StaticCast does a hard type cast but requires an input type
    class function StaticCast<T,ReturnT>(const Value: T): ReturnT;
    // DynamicCast is like the as-operator. It checks if the object can be typecasted
    class function DynamicCast<T,ReturnT>(const Value: T): ReturnT;
  end;

class function TTypeCast.ReinterpretCast<ReturnT>(const Value): ReturnT;
begin
  Result := ReturnT(Value);
end;

class function TTypeCast.StaticCast<T,ReturnT>(const Value: T): ReturnT;
begin
  Result := ReinterpretCast<ReturnT>(Value);
end;

class function TTypeCast.DynamicCast<T,ReturnT>(const Value: T): ReturnT;
var
  TypeT,TypeReturnT: PTypeInfo;
  Obj: TObject;
  LClass: TClass;
  ClassNameReturnT,ClassNameT: string;
  FoundReturnT,FoundT: Boolean;
begin
  TypeT := TypeInfo(T);
  TypeReturnT := TypeInfo(ReturnT);
  if (TypeT = nil) or (TypeReturnT = nil) then
    raise Exception.Create('Missing Typeinformation');
  if TypeT.Kind <> tkClass then
    raise Exception.Create('Source type is not a class');
  if TypeReturnT.Kind <> tkClass then
    raise Exception.Create('Destination type is not a class');

  Obj := TObject(Pointer(@Value)^);
  if Obj = nil then
    Result := Default(ReturnT)
  else
  begin
    ClassNameReturnT := UTF8ToString(TypeReturnT.Name);
    ClassNameT := UTF8ToString(TypeT.Name);
    LClass := Obj.ClassType;
    FoundReturnT := False;
    FoundT := False;
    while (LClass <> nil) and not (FoundT and FoundReturnT) do
    begin
      if not FoundReturnT and (LClass.ClassName = ClassNameReturnT) then
        FoundReturnT := True;
      if not FoundT and (LClass.ClassName = ClassNameT) then
        FoundT := True;
      LClass := LClass.ClassParent;
    end;
    //if LClass <> nil then << TObject doesn't work with this line
    if FoundT and FoundReturnT then
      Result := ReinterpretCast<ReturnT>(Obj)
    else
    if not FoundReturnT then
      raise Exception.CreateFmt('Cannot cast class %s to %s',[Obj.ClassName,ClassNameReturnT])
    else
      raise Exception.CreateFmt('Object (%s) is not of class %s',ClassNameT]);
  end;
end;

（编辑：李大同）

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