Default: what changed

Revision 1 to 2. Removed lines have a minus sign; added lines have a plus sign.

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

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