Nullable types
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Nullable types are types which can have no value (can be unassigned). One such type in Pascal is Pointer type which can have nil value which means that it isn't assigned to any specific address. Same behavior can be implemented using generic types and advanced records with operator overloading.
unit Nullable;
{$mode delphi}{$H+}
interface
uses
Classes, SysUtils;
type
TNullable<T> = record
private
FHasValue: Boolean;
FValue: T;
function GetValue: T;
procedure SetValue(AValue: T);
public
procedure Clear;
property HasValue: Boolean read FHasValue;
property Value: T read GetValue write SetValue;
class operator Implicit(A: T): TNullable<T>;
class operator Implicit(A: Pointer): TNullable<T>;
end;
implementation
{ TNullable }
function TNullable<T>.GetValue: T;
begin
if FHasValue then
Result := FValue
else
raise Exception.Create('Variable has no value');
end;
procedure TNullable<T>.SetValue(AValue: T);
begin
FValue := AValue;
FHasValue := True;
end;
procedure TNullable<T>.Clear;
begin
FHasValue := False;
end;
class operator TNullable<T>.Implicit(A: T): TNullable<T>;
begin
Result.Value := A;
end;
class operator TNullable<T>.Implicit(A: Pointer): TNullable<T>;
begin
if A = nil then Result.Clear
else raise Exception.Create('Pointer value not allowed');
end;
end.
Then you can define nullable types like:
NullableChar = TNullable<Char>;
NullableInteger = TNullable<Integer>;
NullableString = TNullable<string>;