Inc and Dec: what changed

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Title: Inc and Dec

New		Line
1	+	The [[Procedure\|procedures]] `inc` and `dec` increment or decrement a given variable by default by one.
2	+
3	+	## usage
4	+
5	+	The first parameter specifies an ordinal value variable (e. g. an [[Integer\|integer]] or enumeration type) and the second optional parameter may specify a different addend/subtrahend.
6	+
7	+	``` pascal
8	+	program incDecDemo(input, output, stderr);
9	+
10	+	type
11	+	primaryColor = (red, green, blue);
12	+
13	+	var
14	+	phase: primaryColor;
15	+	x: longint;
16	+
17	+	begin
18	+	// enumeration type
19	+	phase := red;
20	+	inc(phase); // phase becomes green
21	+	writeLn(phase);
22	+
23	+	// integer
24	+	x := 1;
25	+	inc(x, -1); // x becomes zero
26	+	writeLn(x);
27	+
28	+	x := 1;
29	+	dec(x); // same as above: x becomes zero
30	+	writeLn(x);
31	+	end.
32	+	```
33	+
34	+	## background
35	+
36	+	In [[FPC]]’s [[System unit\|system unit]] the [`inc`](https://www.freepascal.org/docs-html/rtl/system/inc.html) and [`dec`](https://www.freepascal.org/docs-html/rtl/system/dec.html) procedures are compiler procedures. They exist in order to optimize for certain architectures where dedicated `inc` and `dec` [[Assembly language\|assembler]] instructions are available.
37	+
38	+	## special behaviors
39	+
40	+	If `{$rangeChecks}` are turned on, those procedures may generate a run-time error (RTE 201). Also `{$overflowChecks}` may be generated (with a small difference in [[Mode TP\|TP mode]]).
41	+
42	+	If [[Pointer\|pointer]] arithmetics are allowed by the `{$pointerMath}` compiler switch, `inc` and `dec` work on pointers, too.
43	+
44	+	In the case of typed pointers, e. g. a pointer to a [[Record\|record]], the target’s type size is considered automatically. For instance:
45	+
46	+	``` pascal
47	+	program pointerIncDemo(input, output, stderr);
48	+
49	+	{$pointerMath on}
50	+
51	+	var
52	+	p: PQWord;
53	+
54	+	begin
55	+	inc(p);
56	+	inc(p, 3);
57	+	end.
58	+	```
59	+
60	+	will generate (excerpt):
61	+
62	+	``` asm
63	+	; [pointerIncDemo.pas]
64	+	; [8] begin
65	+	leaq -8(%rsp),%rsp
66	+	.Lc3:
67	+	; Var p located in register rax
68	+	call FPC_INITIALIZEUNITS
69	+	movq $0,%rax
70	+	; [9] inc(p);
71	+	addq $8,%rax
72	+	; [10] inc(p, 3);
73	+	addq $24,%rax
74	+	; [11] end.
75	+	call FPC_DO_EXIT
76	+	leaq 8(%rsp),%rsp
77	+	ret
78	+	```
79	+
80	+	## see also
81	+
82	+	- [`succ`](https://www.freepascal.org/docs-html/rtl/system/succ.html) and [`pred`](https://www.freepascal.org/docs-html/rtl/system/succ.html)
83	+	- [[For\|for-loops]]
84	+
85	+	---
86	+	From the Free Pascal wiki, page "Inc and Dec", as it stood on 2022-08-10 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).
87	+	Category: Pascal

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