FreeAndNil: what changed

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Title: FreeAndNil

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1	+	FreeAndNil is a procedure defined in the [[SysUtils unit]] of the Free Pascal [[RTL\|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]].
2	+
3	+	``` pascal
4	+	procedure FreeAndNil( var obj );
5	+	```
6	+
7	+	Example:
8	+
9	+	``` pascal
10	+	{$mode ObjFPC}
11	+	uses SysUtils;
12	+
13	+	type
14	+	SomeClass = class(TObject)
15	+	destructor Destroy; override;
16	+	end;
17	+
18	+	destructor SomeClass.Destroy;
19	+	begin
20	+	WriteLn('SomeClass Destructor called');
21	+	inherited;
22	+	end;
23	+
24	+	var
25	+	myClass : SomeClass;
26	+	myClass2: SomeClass;
27	+	begin
28	+	myClass := SomeClass.Create;
29	+	WriteLn('myClass is assigned? ', Assigned(myClass));
30	+	myClass2 := SomeClass.Create;
31	+	WriteLn('myClass2 is assigned? ', Assigned(myClass2));
32	+	myClass.Destroy;
33	+	WriteLn('myClass is assigned? ', Assigned(myClass));
34	+	FreeAndNil(myClass2);
35	+	WriteLn('myClass2 is assigned? ', Assigned(myClass2));
36	+	// assigning Nil after destructor is called is the same as
37	+	// FreeAndNil
38	+	myClass := Nil;
39	+	WriteLn('myClass is assigned? ', Assigned(myClass));
40	+	end.
41	+	```
42	+
43	+	Output:
44	+
45	+	` myClass is assigned? TRUE `\
46	+	` myClass2 is assigned? TRUE`\
47	+	` SomeClass Destructor called`\
48	+	` myClass is assigned? TRUE`\
49	+	` SomeClass Destructor called`\
50	+	` myClass2 is assigned? FALSE`\
51	+	` myClass is assigned? FALSE`
52	+
53	+	---
54	+	From 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](/wiki-contact).
55	+	Category: FPC

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