Difference between revisions of "^"
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<div style="float:left; margin: 0 25px 20px 0; padding:40px; font-size:500%; font-family: Georgia; background-color: #f9f9f9; border: 2px solid #777777;">^</div> | <div style="float:left; margin: 0 25px 20px 0; padding:40px; font-size:500%; font-family: Georgia; background-color: #f9f9f9; border: 2px solid #777777;">^</div> | ||
− | In [[ASCII]] the character code decimal <syntaxhighlight lang="pascal" | + | In [[ASCII]] the character code decimal <syntaxhighlight lang="pascal" inline>94</syntaxhighlight> (or [[Hexadecimal|hexadecimal]] <syntaxhighlight lang="pascal" inline>5E</syntaxhighlight>) is defined to be <syntaxhighlight lang="pascal" inline>^</syntaxhighlight> (circumflex accent). |
− | For any data type, a [[Pointer|pointer]] | + | For any data type, a [[Pointer|<syntaxhighlight lang="pascal" inline>pointer</syntaxhighlight> type]] for that data can be declared using the operator <syntaxhighlight lang="pascal" inline>^</syntaxhighlight> in front of the [[Data type|data type]]. |
− | <syntaxhighlight lang="pascal" line highlight="6-7,11 | + | <br clear="all"/> |
+ | <syntaxhighlight lang="pascal" line highlight="6-7,11"> | ||
program pointerDemo(input, output, stderr); | program pointerDemo(input, output, stderr); | ||
type | type | ||
listItem = record | listItem = record | ||
− | + | payload: integer; | |
// next _points_ _to_ a list item | // next _points_ _to_ a list item | ||
next: ^listItem; | next: ^listItem; | ||
Line 26: | Line 27: | ||
halt(1); | halt(1); | ||
end; | end; | ||
− | + | </syntaxhighlight> | |
− | // _de-reference_ the pointer, follow it | + | |
− | start^. | + | A <syntaxhighlight lang="pascal" inline>pointer</syntaxhighlight> can be followed, ''dereferenced'' by appending a <syntaxhighlight lang="pascal" inline>^</syntaxhighlight> to the [[Identifier|identifier]]. |
+ | Instead of having the memory address in your hands, you will look at the memory content ''at'' that address. | ||
+ | If it is a typed <syntaxhighlight lang="pascal" inline>pointer</syntaxhighlight>, operations and syntax for that type are valid, e.g. [[Becomes|assignment]] in the following example. | ||
+ | |||
+ | <syntaxhighlight lang="pascal" line start="21" highlight="1-2"> | ||
+ | // _de-reference_ the pointer, i.e. follow it | ||
+ | start^.payload := 7; | ||
dispose(start); | dispose(start); | ||
end.</syntaxhighlight> | end.</syntaxhighlight> | ||
+ | |||
+ | However, attempting to follow a [[Nil|<syntaxhighlight lang="pascal" inline>nil</syntaxhighlight>]] pointer will cause a [[runtime error|runtime error]] (RTE 216 “general protection fault”). | ||
+ | That is the situation the condition <syntaxhighlight lang="pascal" inline>not assigned(start)</syntaxhighlight> in line 15 is supposed to catch. | ||
+ | |||
+ | == see also == | ||
+ | * [[$pointerMath|<syntaxhighlight lang="pascal" inline>{$pointerMath off}</syntaxhighlight>]] disallows usage of pointers in arithmetic expressions | ||
+ | * [[$typeAaddress|<syntaxhighlight lang="pascal" inline>{$typedAddress on}</syntaxhighlight>]] in conjunction with the [[@|<syntaxhighlight lang="pascal" inline>@</syntaxhighlight>-address-operator]] | ||
+ | * [[sModeswitch|<syntaxhighlight lang="pascal" inline>{$modeSwitch autoDeref-}</syntaxhighlight>]] | ||
{{Symbols}} | {{Symbols}} | ||
− | |||
− |
Latest revision as of 02:19, 18 February 2023
│
English (en) │
suomi (fi) │
русский (ru) │
In ASCII the character code decimal 94
(or hexadecimal 5E
) is defined to be ^
(circumflex accent).
For any data type, a pointer
type for that data can be declared using the operator ^
in front of the data type.
1program pointerDemo(input, output, stderr);
2
3type
4 listItem = record
5 payload: integer;
6 // next _points_ _to_ a list item
7 next: ^listItem;
8 end;
9
10var
11 start: ^listItem;
12
13begin
14 new(start);
15 if not assigned(start) then
16 begin
17 writeLn(stderr, 'obtaining memory for start failed');
18 halt(1);
19 end;
A pointer
can be followed, dereferenced by appending a ^
to the identifier.
Instead of having the memory address in your hands, you will look at the memory content at that address.
If it is a typed pointer
, operations and syntax for that type are valid, e.g. assignment in the following example.
21 // _de-reference_ the pointer, i.e. follow it
22 start^.payload := 7;
23
24 dispose(start);
25end.
However, attempting to follow a nil
pointer will cause a runtime error (RTE 216 “general protection fault”).
That is the situation the condition not assigned(start)
in line 15 is supposed to catch.
see also
{$pointerMath off}
disallows usage of pointers in arithmetic expressions{$typedAddress on}
in conjunction with the@
-address-operator{$modeSwitch autoDeref-}
single characters |
|
character pairs |
|