В Delphi вроде как нет Nullable типов, но если нет, но очень хочется, то они могут появиться ))) Читал книгу Delphi Memory Management, и там наткнулся на раздел про Nullable, автор книги ссылался на MVP Allen Bauer.
Идея проста…
To implement a nullable type, we need to create a wrapper around a value type and add an
undefined flag. That part can be easily achieved with a generic record storing our wrapped
Value and HasValue flag.
А вот и первоисточник…
Класс Nullable
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unit uNullable; interface uses System.SysUtils, System.Classes, System.Generics.Defaults; type TNullable<T> = record strict private FValue: T; FHasValue: IInterface; function GetValue: T; function GetHasValue: boolean; public constructor Create(AValue: T); function ValueOrDefault: T; overload; function ValueOrDefault(Default: T): T; overload; property HasValue: boolean read GetHasValue; property Value: T read GetValue; class operator Implicit(Value: TNullable<T>): T; class operator Implicit(Value: T): TNullable<T>; class operator Explicit(Value: TNullable<T>): T; class operator NotEqual(const Left, Right: TNullable<T>): boolean; class operator Equal(const Left, Right: TNullable<T>): boolean; end; procedure SetFakeInterface(var Intf: IInterface); implementation function NopAddRef(inst: Pointer): integer; stdcall; begin Result := -1; end; function NopRelease(inst: Pointer): integer; stdcall; begin Result := -1; end; function NopQueryInterface(inst: Pointer; const IID: TGUID; out Obj): HResult; stdcall; begin Result := E_NOINTERFACE; end; const FakeInterfaceVTable: array[0..2] of Pointer = (@NopQueryInterface, @NopAddRef, @NopRelease); FakeInterfaceInstance: Pointer = @FakeInterfaceVTable; procedure SetFakeInterface(var Intf: IInterface); begin Intf := IInterface(@FakeInterfaceInstance); end; constructor TNullable<T>.Create(AValue: T); begin FValue := AValue; SetFakeInterface(FHasValue); end; function TNullable<T>.GetHasValue: boolean; begin Result := FHasValue <> nil; end; function TNullable<T>.GetValue: T; begin if not HasValue then raise Exception.Create('Invalid operation, Nullable type has no value'); Result := FValue; end; function TNullable<T>.ValueOrDefault: T; begin if HasValue then Result := fValue else Result := default(T); end; function TNullable<T>.ValueOrDefault(Default: T): T; begin if not HasValue then Result := Default else Result := fValue; end; class operator TNullable<T>.Explicit(Value: TNullable<T>): T; begin Result := Value.Value; end; class operator TNullable<T>.Implicit(Value: TNullable<T>): T; begin Result := Value.Value; end; class operator TNullable<T>.Implicit(Value: T): TNullable<T>; begin Result := TNullable<T>.Create(Value); end; class operator TNullable<T>.Equal(const Left, Right: TNullable<T>): boolean; var Comparer: IEqualityComparer<T>; begin if Left.HasValue and Right.HasValue then begin Comparer := TEqualityComparer<T>.Default; Result := Comparer.Equals(Left.Value, Right.Value); end else Result := Left.HasValue = Right.HasValue; end; class operator TNullable<T>.NotEqual(const Left, Right: TNullable<T>): boolean; var Comparer: IEqualityComparer<T>; begin if Left.HasValue and Right.HasValue then begin Comparer := TEqualityComparer<T>.Default; Result := not Comparer.Equals(Left.Value, Right.Value); end else Result := Left.HasValue <> Right.HasValue; end; end. |
Пример использования
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var n: ISmartPointer<TNullableClassExample>; begin ReportMemoryLeaksOnShutdown := true; n := TSmartPointer<TNullableClassExample>.Create(); // n.SomeNullableValue:='testString'; // if uncomment will not be nullable if n.SomeNullableValue.HasValue then ShowMessage('has value') else ShowMessage('no value'); end; |