FreeAndNil
FreeAndNil is a procedure defined in the SysUtils unit of the Free Pascal Runtime Library. It calls an object's destructor (via TObject.Free) and also sets the object variable (which is a reference to heap memory created by a constructor call) to nil. If you just call an object's destructor, Assigned will still return True for the object variable. But after calling FreeAndNil, Assigned will return False for the object variable. While the procedure has an untyped parameter, it is only for use with object references - that is variables that are instances of a Class.
procedure FreeAndNil( var obj );Example:
{$mode ObjFPC}
uses SysUtils;
type
SomeClass = class(TObject)
destructor Destroy; override;
end;
destructor SomeClass.Destroy;
begin
WriteLn('SomeClass Destructor called');
inherited;
end;
var
myClass : SomeClass;
myClass2: SomeClass;
begin
myClass := SomeClass.Create;
WriteLn('myClass is assigned? ', Assigned(myClass));
myClass2 := SomeClass.Create;
WriteLn('myClass2 is assigned? ', Assigned(myClass2));
myClass.Destroy;
WriteLn('myClass is assigned? ', Assigned(myClass));
FreeAndNil(myClass2);
WriteLn('myClass2 is assigned? ', Assigned(myClass2));
// assigning Nil after destructor is called is the same as
// FreeAndNil
myClass := Nil;
WriteLn('myClass is assigned? ', Assigned(myClass));
end.Output:
myClass is assigned? TRUE
myClass2 is assigned? TRUE
SomeClass Destructor called
myClass is assigned? TRUE
SomeClass Destructor called
myClass2 is assigned? FALSE
myClass is assigned? FALSEFrom the Free Pascal wiki, page "FreeAndNil", as it stood on 2020-12-27 after a change by Jouborg, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's Licensing page. If you have concerns about this material, message the admin.