Difference between revisions of "Nullable types"

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Nullable types are types which can have no value (can be unassigned).
 
Nullable types are types which can have no value (can be unassigned).
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One such type in Pascal is [[Pointer]] type which can have [[Nil|<syntaxhighlight lang="pascal" enclose="none">nil</syntaxhighlight>]] value which means that it isn't assigned to any specific address.
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One such type in Pascal is [[Pointer]] type which can have [[Nil|<syntaxhighlight lang="pascal" inline>nil</syntaxhighlight>]] value which means that it isn't assigned to any specific address.
 
Same behavior can be implemented using [[Generics|generic types]] and advanced records with [[Operator overloading|operator overloading]].
 
Same behavior can be implemented using [[Generics|generic types]] and advanced records with [[Operator overloading|operator overloading]].
  
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<syntaxhighlight lang="pascal">
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The Nullable unit is part of FPC since 3.2.2 and its code can be seen in the GitLab repository: [https://gitlab.com/freepascal.org/fpc/source/-/blob/main/packages/rtl-objpas/src/inc/nullable.pp nullable.pp].
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unit Nullable;
 
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{$mode delphi}{$H+}
 
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interface
 
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uses
 
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  Classes, SysUtils;
 
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type
 
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  TNullable<T> = record
 
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  private
 
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    FHasValue: Boolean;
 
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    FValue: T;
 
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    function GetValue: T;
 
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    procedure SetValue(AValue: T);
 
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  public
 
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    procedure Clear;
 
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    property HasValue: Boolean read FHasValue;
 
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    property Value: T read GetValue write SetValue;
 
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    class operator Implicit(A: T): TNullable<T>;
 
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    class operator Implicit(A: Pointer): TNullable<T>;
 
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  end;
 
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implementation
 
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{ TNullable }
 
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function TNullable<T>.GetValue: T;
 
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begin
 
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  if FHasValue then
 
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    Result := FValue
 
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  else
 
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    raise Exception.Create('Variable has no value');
 
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end;
 
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procedure TNullable<T>.SetValue(AValue: T);
 
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begin
 
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  FValue := AValue;
 
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  FHasValue := True;
 
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end;
 
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procedure TNullable<T>.Clear;
 
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begin
 
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  FHasValue := False;
 
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end;
 
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class operator TNullable<T>.Implicit(A: T): TNullable<T>;
 
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begin
 
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  Result.Value := A;
 
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end;
 
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class operator TNullable<T>.Implicit(A: Pointer): TNullable<T>;
 
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begin
 
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  if A = nil then Result.Clear
 
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  else raise Exception.Create('Pointer value not allowed');
 
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end;
 
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end.
 
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</syntaxhighlight>
 
  
 
Then you can define nullable types like:
 
Then you can define nullable types like:
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== See also ==
 
== See also ==
 
* [[How to use nullable types]]
 
* [[How to use nullable types]]
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The standard nullable type in Object Pascal is Variant.
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* [[Variant]]
  
 
== External links ==
 
== External links ==
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* [https://gitlab.com/freepascal.org/fpc/source/-/blob/main/packages/rtl-objpas/src/inc/nullable.pp FPC GitLab Repository nullable.pp]
  
 
* [https://www.arbinada.com/en/node/1439 Nullables in Free Pascal and Delphi]
 
* [https://www.arbinada.com/en/node/1439 Nullables in Free Pascal and Delphi]

Latest revision as of 17:19, 6 August 2022

│ English (en) │ suomi (fi) │

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.

The Nullable unit is part of FPC since 3.2.2 and its code can be seen in the GitLab repository: nullable.pp.

Then you can define nullable types like: 

NullableChar = TNullable<Char>;
NullableInteger = TNullable<Integer>;
NullableString = TNullable<string>;

See also

The standard nullable type in Object Pascal is Variant.

External links

Retrieved from "http://wiki.freepascal.org/index.php?title=Nullable_types&oldid=153247"