Inc and Dec
│ English (en) │
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.
program incDecDemo(input, output, stderr);
type
primaryColor = (red, green, blue);
var
phase: primaryColor;
x: longint;
begin
// enumeration type
phase := red;
inc(phase); // phase becomes green
writeLn(phase);
// integer
x := 1;
inc(x, -1); // x becomes zero
writeLn(x);
x := 1;
dec(x); // same as above: x becomes zero
writeLn(x);
end.
background
In FPC’s system unit the inc and dec procedures are compiler procedures.
They exist in order to optimize for certain architectures where dedicated 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 {$pointerMath} compiler switch, inc and dec work on pointers, too.
In the case of typed pointers, e. g. a pointer to a record, the target’s type size is considered automatically.
For instance:
program pointerIncDemo(input, output, stderr);
{$pointerMath on}
var
p: PQWord;
begin
inc(p);
inc(p, 3);
end.
will generate (excerpt):
; [pointerIncDemo.pas]
; [8] begin
leaq -8(%rsp),%rsp
.Lc3:
; Var p located in register rax
call FPC_INITIALIZEUNITS
movq $0,%rax
; [9] inc(p);
addq $8,%rax
; [10] inc(p, 3);
addq $24,%rax
; [11] end.
call FPC_DO_EXIT
leaq 8(%rsp),%rsp
ret
