Property: what changed

Revision 0 to 1. Removed lines have a minus sign; added lines have a plus sign.

Title: Property

New		Line
1	+	Back to [[Reserved words]].
2	+
3	+	The [[Reserved words\|reserved word]] `property` is part of [[object-oriented programming]]. It can allow various levels (read, read/write etc) outside access to [[Variable\|variables]] inside a [[Class\|class]].
4	+
5	+	## Example
6	+
7	+	``` pascal
8	+	type
9	+	TCar = class
10	+	private
11	+	FColor: string;
12	+	FBuildYear: integer;
13	+	procedure SetColor(CarColor: string);
14	+	public
15	+	property Color: string read FColor write SetColor; //Reads directly from the FColor variable;
16	+	// directs writes to the SetColor procedure that then changes the FColor variable.
17	+	// Another option could be to just do "write FColor" to directly write to the FColor variable..
18	+	end;
19	+
20	+	procedure TCar.SetColor(CarColor: string);
21	+	begin
22	+	//we can do other things here, too, such as housekeeping, notifying others of changes,
23	+	//validating against other variables, logging, etc -e.g.
24	+	if CarColor='Pink' then
25	+	ShowMessage('Note: pink is not a very common color for a car.');
26	+	FColor:=CarColor;
27	+	end;
28	+
29	+	procedure PropertyExample(); //shows how to set properties and read them
30	+	var
31	+	MyCar: TCar; // Will hold a TCar instance; used in the example subAuto
32	+	begin
33	+	MyCar := TCar.Create; // Create the object
34	+	try
35	+	MyCar.Color:='Green'; // Sets the color property... which calls the setcolor function... which sets the object's FColor variable
36	+	showmessage(MyCar.Color); // Now read the object property again... should show Green
37	+	finally
38	+	MyCar.Free; // Free the memory for the object again, regardless of what errors occurred in the rest of the code
39	+	end;
40	+	end;
41	+	```
42	+
43	+	## Objects as properties
44	+
45	+	You can assign objects as properties, e.g.:
46	+
47	+	``` pascal
48	+	type
49	+	TEngine = class
50	+	...
51	+	TCar = class
52	+	private
53	+	FMyEngine: TEngine;
54	+	public
55	+	property Engine: TEngine read FMyEngine;
56	+	implementation
57	+	// ...
58	+	// Code in TCar that creates and frees FMyEngine as needed
59	+	// (e.g. in the constructor and destructor)
60	+	// ...
61	+	```
62	+
63	+	Read access here means something that may not be obvious at first glance. Outside code has read only access to the pointer/reference to the TEngine object in question, which means it cannot create or destroy the object.
64	+
65	+	However, it can use this read access to access the properties of the FMyEngine object and change those.
66	+
67	+	If you wish to give calling code the responsibility of creating and destroying the object, you can also assign write access, e.g.
68	+
69	+	``` pascal
70	+	...
71	+	public
72	+	property Engine: TEngine read FMyEngine write FMyEngine;
73	+	...
74	+	```
75	+
76	+	## See also
77	+
78	+	- [property in the Official FPC documentation](https://www.freepascal.org/docs-html/ref/refse27.html)
79	+
80	+	---
81	+	From the Free Pascal wiki, page "Property", as it stood on 2022-08-06 after a change by Kai Burghardt, 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).

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