CODEWITHSUNDEEP
delphigenerics
Raffaele Rossi
Raffaele Rossi

Reputation: 3127

Delphi assign generic value in function

Since Delphi (sadly) doesn't support nullable types I wanted to try to make my own implementation of them. This is what I've written so far:

unit Nullable;

interface

uses
 System.SysUtils, Generics.Collections;

type
 Nullable<T> = class
  private
   FValue: T;
   FHasValue: boolean;
   function getValue: T;
   procedure setValue(const val: T);
  public
   constructor Create(value: T);
   procedure setNull;
   property value: T read getValue write setValue;
   property hasValue: boolean read FHasValue;
 end;

implementation

{ Nullable<T> }

constructor Nullable<T>.Create(value: T);
begin
 Fvalue := value;
 FHasValue := true;
end;

procedure Nullable<T>.setNull;
begin
 FHasValue := false;
end;

procedure Nullable<T>.setValue(const val: T);
begin
 FHasValue := true;
 FValue := T; //COMPILER ERROR HERE
end;

function Nullable<T>.getValue: T;
begin

 if (FHasValue = false) then
  raise Exception.Create('There is not a value!');

 Result := T;

end;

end.

It seems that I cannot assing the FValue with a generic value taken from the function. Is there a way to do this?

I wanted do an easy implementation of nullables. I need a setValue function because I need to assign the FHasValue to true or false (so I know if the value is "nullable" or not). In the main form I'd call the code like this:

var a: Nullable<integer>;
begin

 a := Nullable<integer>.Create(5);
 try

  a.setNull;

  if (not a.hasValue) then
   memo1.lines.add('nullo')
  else
   memo1.lines.add('valore = ' + a.value.toString);

 finally
  a.Free;
 end;

Upvotes: 2

Views: 605

Answers (1)

David Heffernan
David Heffernan

Reputation: 612794

Instead of 

FValue := T;

you mean

FValue := val;

You make the same mistake in the setter method, which is fixed in analagous fashion be replacing

Result := T;

with 

Result := FValue;

Remember that T is a type.

There are a number of good implementations of nullable types in existence already, for instance Spring has one. You might draw inspiration from those, or even use one as is.

Upvotes: 5

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