Difference between revisions of "Type"
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{{Type}} | {{Type}} | ||
| − | The type [[ | + | The [[Reserved word|reserved word]] <syntaxhighlight lang="pascal" inline>type</syntaxhighlight> is used to: |
| + | * start sections for user defined types, and | ||
| + | * identify a new type instance when referring to another [[Data type|data type]]. | ||
| + | |||
| + | == custom type definitions == | ||
| + | <syntaxhighlight lang="pascal" inline>type</syntaxhighlight> starts a section, where the programmer may associate [[Identifier|identifiers]] with new data types, especially structured data types such as [[Record|records]]. | ||
| + | <syntaxhighlight lang="pascal" line highlight="3"> | ||
| + | program typeDemo(input, output, stderr); | ||
| + | |||
| + | type | ||
| + | atom = record | ||
| + | electrons: longword; | ||
| + | neutrons: longword; | ||
| + | protons: longword; | ||
| + | end; | ||
| + | |||
| + | var | ||
| + | x: atom; | ||
| + | |||
| + | begin | ||
| + | x.protons := 1; // H | ||
| + | x.neutrons := 1; // D | ||
| + | x.electrons := 1; // 0 | ||
| + | end. | ||
| + | </syntaxhighlight> | ||
| + | |||
| + | == type aliases == | ||
| + | In a <syntaxhighlight lang="pascal" inline>type</syntaxhighlight> 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 <syntaxhighlight lang="pascal" inline>wholeNumber</syntaxhighlight> (note there is already {{Doc|package=RTL|unit=system|identifier=nativeuint|text=<syntaxhighlight lang="pascal" inline>system.nativeUInt</syntaxhighlight>}} defined). | ||
| + | <syntaxhighlight lang="pascal" line highlight="3"> | ||
| + | program typeAliasDemo(input, output, stderr); | ||
| + | |||
| + | type | ||
| + | wholeNumber = | ||
| + | {$ifdef CPU64} | ||
| + | qword | ||
| + | {$else} | ||
| + | {$ifdef CPU32} | ||
| + | longword | ||
| + | {$else} | ||
| + | {$fatal whole number too small} | ||
| + | {$endif} | ||
| + | {$endif} | ||
| + | ; | ||
| + | |||
| + | begin | ||
| + | end. | ||
| + | </syntaxhighlight> | ||
| + | |||
| + | == type clone == | ||
| + | In a <syntaxhighlight lang="pascal" inline>type</syntaxhighlight> section a type identifier preceded by the word <syntaxhighlight lang="pascal" inline>type</syntaxhighlight> 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). | ||
| + | |||
| + | <syntaxhighlight lang="pascal" line highlight="3,4,15"> | ||
| + | |||
| + | program typeCloneDemo(input, output, stderr); | ||
| + | |||
| + | type | ||
| + | wholeNumber = type qword; | ||
| + | |||
| + | var | ||
| + | A: qword; | ||
| + | B: wholeNumber; | ||
| + | |||
| + | begin | ||
| + | writeLn('qword: ', sysBackTraceStr(typeInfo(qword))); | ||
| + | writeLn('wholeNumber: ', sysBackTraceStr(typeInfo(wholeNumber))); | ||
| + | |||
| + | A := 3; | ||
| + | B := A; | ||
| + | end. | ||
| + | </syntaxhighlight> | ||
| + | |||
| + | 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. | ||
| + | |||
| + | == see also == | ||
| + | * [https://www.freepascal.org/docs-html/ref/refse19.html § “Type Aliases” in the Reference Guide] | ||
Latest revision as of 15:24, 29 July 2022
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The reserved word type is used to:
- start sections for user defined types, and
- identify a new type instance when referring to another data type.
custom type definitions
type starts a section, where the programmer may associate identifiers with new data types, especially structured data types such as records.
1program typeDemo(input, output, stderr);
2
3type
4 atom = record
5 electrons: longword;
6 neutrons: longword;
7 protons: longword;
8 end;
9
10var
11 x: atom;
12
13begin
14 x.protons := 1; // H
15 x.neutrons := 1; // D
16 x.electrons := 1; // 0
17end.
type aliases
In a type section aliases to already existing or previously defined data types can be declared.
The following example utilizes conditional compilation to alias the largest available unsigned integer type as wholeNumber (note there is already system.nativeUInt defined).
1program typeAliasDemo(input, output, stderr);
2
3type
4 wholeNumber =
5 {$ifdef CPU64}
6 qword
7 {$else}
8 {$ifdef CPU32}
9 longword
10 {$else}
11 {$fatal whole number too small}
12 {$endif}
13 {$endif}
14 ;
15
16begin
17end.
type clone
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).
1program typeCloneDemo(input, output, stderr);
2
3type
4 wholeNumber = type qword;
5
6var
7 A: qword;
8 B: wholeNumber;
9
10begin
11 writeLn('qword: ', sysBackTraceStr(typeInfo(qword)));
12 writeLn('wholeNumber: ', sysBackTraceStr(typeInfo(wholeNumber)));
13
14 A := 3;
15 B := A;
16end.
You want to do this, for instance in order to define a whole new set of operators. Otherwise the operator definitions for the type it was cloned from would still apply.
