Inc and Dec
Template:Translate
The procedures inc
and dec
increment or decrement a given variable by default by one.
usage
The first parameter specifies an ordinal value variable (e.g. an integer
or enumeration type) and the second optional parameter may specify a different addend/subtrahend.
1program incDecDemo(input, output, stderr);
2
3type
4 primaryColor = (red, green, blue);
5
6var
7 phase: primaryColor;
8 x: longint;
9
10begin
11 // enumeration type
12 phase := red;
13 inc(phase); // phase becomes green
14 writeLn(phase);
15
16 // integer
17 x := 1;
18 inc(x, -1); // x becomes zero
19 writeLn(x);
20
21 x := 1;
22 dec(x); // same as above: x becomes zero
23 writeLn(x);
24end.
background
In FPC's system unit the inc
and dec
procedures are compiler procedures.
They exist in order to optimize for certain architectures where faster inc
and dec
assembler instructions are available.
special behaviors
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 TP mode).
If pointer
arithmetics are allowed by the {$pointerMath}
compiler switch, inc
and dec
work on pointers, too.
In the case of typed pointers, e.g. a pointer to a record
, the target's type size is considered automatically.
For instance
1program pointerIncDemo(input, output, stderr);
2
3{$pointerMath on}
4
5var
6 p: PQWord;
7
8begin
9 inc(p);
10 inc(p, 3);
11end.
will generate (excerpt):
13; [pointerIncDemo.pas]
14; [8] begin
15 leaq -8(%rsp),%rsp
16.Lc3:
17; Var p located in register rax
18 call FPC_INITIALIZEUNITS
19 movq $0,%rax
20; [9] inc(p);
21 addq $8,%rax
22; [10] inc(p, 3);
23 addq $24,%rax
24; [11] end.
25 call FPC_DO_EXIT
26 leaq 8(%rsp),%rsp
27 ret