Default: what changed

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Title: Default

New		Line
1	+	`Default` is
2	+
3	+	- a compiler intrinsic [[Function\|function]] returning a zeroed out value, or
4	+	- a [[Keyword\|keyword]] marking a [[Class\|class]]’s [[Array\|array]] [[Property\|property]] as implicitly accessible.
5	+
6	+	This article describes the compiler intrinsic. See other articles for the notion in OOP.
7	+
8	+	## zero-idiom
9	+
10	+	[[FPC New Features 3.0.0\|Since FPC 3.0.0]] `default(dataType)` returns a zero value for the specified [[Data type\|dataType]].
11	+
12	+	```delphi
13	+	program defaults(input, output, stdErr);
14	+	var
15	+	i: integer;
16	+	s: string;
17	+	r: tOpaqueData;
18	+	begin
19	+	i := default(integer); { assigns `0` }
20	+	s := default(string); { assigns `nil` or `''` (empty string) }
21	+	r := default(tOpaqueData); { assigns `tOpaqueData[]` }
22	+	end.
23	+	```
24	+
25	+	### advantages
26	+
27	+	- The most important “advantage” is that `default` can be used in `generic` definitions with template parameters.
28	+
29	+	{\$mode objFPC} { --- generic thing -------------------------------------- } type
30	+
31	+	` generic thing<storageType> = object`\
32	+	` data: storageType;`\
33	+	` constructor init;`\
34	+	` end;`
35	+
36	+	constructor thing.init; begin
37	+
38	+	` data := default(storageType);`
39	+
40	+	end;
41	+
42	+	{ === MAIN =============================================== } type
43	+
44	+	` arr = array[1..10] of integer;`
45	+
46	+	var
47	+
48	+	` x: specialize thing<tBoundArray>;`\
49	+	` y: specialize thing<arr>;`
50	+
51	+	begin end.
52	+
53	+	</syntaxhighlight>
54	+
55	+	In the context of the definition of the `generic` data type `thing` it is not yet known what `storageType` will be. Therefore, we could not possibly write a literal value such as `0` or [[Nil\|nil]]. However, we can use `default` to overcome this hurdle. Upon specialization the correct zero value will be inserted.
56	+
57	+	- Furthermore, the compiler can choose a faster implementation than, for instance, a respective `fillChar(myVariable, sizeOf(myVariable), chr(0))`.
58	+
59	+	### caveats
60	+
61	+	- Despite its name, `default` is really just a synonym for zero. It can be used to [[Becomes\|assign]] values out of range:
62	+
63	+	{\$rangeChecks on} type
64	+
65	+	` typeWithoutZeroValue = -1337..-42;`
66	+
67	+	var
68	+
69	+	` i: typeWithoutZeroValue;`
70	+
71	+	begin
72	+
73	+	` i := -1024; { ✔ OK }`\
74	+	` {i := 0; ✘ not OK }`\
75	+	` i := default(typeWithoutZeroValue); { OK again }`\
76	+	` writeLn(i);`
77	+
78	+	end.
79	+
80	+	</syntaxhighlight>
81	+
82	+	See [FPC issue 34972](https://gitlab.com/freepascal.org/fpc/source/-/issues/34972).
83	+
84	+	- `Default` cannot be applied on [[File\|file]] or [[Text\|text]] data types and structured data types containing such.
85	+
86	+	## see also
87	+
88	+	- [[management operators]]
89	+
90	+	---
91	+	From the Free Pascal wiki, page "Default", as it stood on 2022-04-01 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).
92	+	Category: Code
93	+	Category: FPC

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