Type: what changed

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

Title: Type

New		Line
1	+	The [[Reserved word\|reserved word]] `type` is used to:
2	+
3	+	- start sections for user defined types, and
4	+	- identify a new type instance when referring to another [[Data type\|data type]].
5	+
6	+	## custom type definitions
7	+
8	+	`type` starts a section, where the programmer may associate [[Identifier\|identifiers]] with new data types, especially structured data types such as [[Record\|records]].
9	+
10	+	``` pascal
11	+	program typeDemo(input, output, stderr);
12	+
13	+	type
14	+	atom = record
15	+	electrons: longword;
16	+	neutrons: longword;
17	+	protons: longword;
18	+	end;
19	+
20	+	var
21	+	x: atom;
22	+
23	+	begin
24	+	x.protons := 1; // H
25	+	x.neutrons := 1; // D
26	+	x.electrons := 1; // 0
27	+	end.
28	+	```
29	+
30	+	## type aliases
31	+
32	+	In a `type` section aliases to already existing or previously defined data types can be declared. The following example utilizes [[Conditional compilation\|conditional compilation]] to alias the largest available unsigned [[Integer\|integer]] type as `wholeNumber` (note there is already [`system.nativeUInt`](https://www.freepascal.org/docs-html/rtl/system/nativeuint.html) defined).
33	+
34	+	``` pascal
35	+	program typeAliasDemo(input, output, stderr);
36	+
37	+	type
38	+	wholeNumber =
39	+	{$ifdef CPU64}
40	+	qword
41	+	{$else}
42	+	{$ifdef CPU32}
43	+	longword
44	+	{$else}
45	+	{$fatal whole number too small}
46	+	{$endif}
47	+	{$endif}
48	+	;
49	+
50	+	begin
51	+	end.
52	+	```
53	+
54	+	## type clone
55	+
56	+	In a `type` section a type identifier preceded by the word `type` actually clones the type, with its type information, but creating different types. Nontheless, these types are still assigment compatible, which is a unique feature of the FPC (e. g. Delphi does not allow this assignment).
57	+
58	+	``` pascal
59	+
60	+	program typeCloneDemo(input, output, stderr);
61	+
62	+	type
63	+	wholeNumber = type qword;
64	+
65	+	var
66	+	A: qword;
67	+	B: wholeNumber;
68	+
69	+	begin
70	+	writeLn('qword: ', sysBackTraceStr(typeInfo(qword)));
71	+	writeLn('wholeNumber: ', sysBackTraceStr(typeInfo(wholeNumber)));
72	+
73	+	A := 3;
74	+	B := A;
75	+	end.
76	+	```
77	+
78	+	You want to do this, for instance in order to define a whole new set of [[Operator\|operators]]. Otherwise the [[Operator overloading\|operator definitions]] for the type it was cloned from would still apply.
79	+
80	+	## see also
81	+
82	+	- [§ “Type Aliases” in the Reference Guide](https://www.freepascal.org/docs-html/ref/refse19.html)
83	+
84	+	---
85	+	From the Free Pascal wiki, page "Type", as it stood on 2022-07-29 after a change by BrainChemistry, 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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