我想使用Allen Bauer的Nullable记录在一种自制的ORM中使用“Plain Old Delphi Objects”来映射数据库行：

type
  Nullable<T> = record
    ...
  end;

  TMyTableObject = class(TDbOject)
  private
    FId: Integer;
    FOptionalField: Nullable<string>;
  protected
    procedure InternalSave; override;
  public
    property Id: Integer read FId write SetId;
    property OptionalField: Nullable<string> read FOptionalField write SetOptionalField;
  end;

  ...

  implementation

  procedure TMyTableObject.InternalSave;
  begin
    { FDbController is declared and managed in TDbObject,it contains fonction to run queries
      on the database }

    FDbController.Execute(
      'update or insert into MY_TABLE(TABLE_ID,OPTIONAL_FIELD) ' +
      'values (?,?) ' + 
      'matching(TABLE_ID) returning TABLE_ID',[FId,FOptionalField],procedure (Fields: TsqlResult)
      begin
        FId := Fields.AsInteger[0];
      end;
    end;      
  end;

上面的代码导致错误：“E2250：没有覆盖版本的”执行“函数可以用这些参数调用”(翻译自法语：E2250 Aucuneversionsurchargéede’Execute’nepeutêtreaposléeavecces arguments)

我可以显式地将FOptionalField转换为字符串或其他任何东西,因为Nullable会覆盖ad-hoc运算符,但我真的必须知道Nullable是否有值,直到我将const数组映射到数据库查询对象的Params字段：

procedure TDbContext.MapParams(Q: TUIBQuery; Params: TConstArray);
const
  BOOL_STR: array[Boolean] of string = ('F','V');
var
  i: Integer;
begin
  for i := 0 to High(Params) do
    case Params[i].VType of
      vtInteger :      Q.Params.AsInteger[i]       := Params[i].VInteger;
      vtInt64   :      Q.Params.AsInt64[i]         := Params[i].VInt64^;
      vtBoolean :      Q.Params.AsString[i]        := BOOL_STR[Params[i].VBoolean];
      vtChar    :      Q.Params.AsAnsiString[i]    := Params[i].VChar;
      vtWideChar:      Q.Params.AsString[i]        := Params[i].VWideChar;
      vtExtended:      Q.Params.AsDouble[i]        := Params[i].VExtended^;
      vtCurrency:      Q.Params.AsCurrency[i]      := Params[i].VCurrency^;
      vtString  :      Q.Params.AsString[i]        := string(Params[i].VString^);
      vtPChar   :      Q.Params.AsAnsiString[i]    := Params[i].VPChar^;
      vtAnsiString:    Q.Params.AsAnsiString[i]    := AnsiString(Params[i].VAnsiString);
      vtWideString:    Q.Params.AsUnicodeString[i] := PWideChar(Params[i].VWideString);
      vtVariant   :    Q.Params.AsVariant[i]       := Params[i].VVariant^;
      vtUnicodeString: Q.Params.AsUnicodeString[i] := string(Params[i].VUnicodeString);
      vtPointer :
      begin
        if Params[i].VPointer = nil then
          Q.Params.IsNull[i] := True
        else
          Assert(False,'not nil pointer is not supported');
      end
    else
      Assert(False,'not supported');
    end;
end;

你对如何使这种结构成为可能有任何想法吗？我找到了一种方法,通过使用RTTI在Nullable中向Variant添加显式的转换操作符覆盖,但这有点棘手,因为它需要在const调用数组中显式转换为Variant：

class operator Nullable<T>.Explicit(Value: Nullable<T>): Variant;
begin
  if Value.HasValue then
    Result := TValue.From<T>(Value.Value).AsVariant
  else
    Result := Null;
end;

...

    FDbController.Execute(
      'update or insert into MY_TABLE(TABLE_ID,Variant(FOptionalField)],procedure (Fields: TsqlResult)
      begin
        FId := Fields.AsInteger[0];
      end;
    end;