Character and string types/es

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Free Pascal soporta varios tipos de character and string. Estos van desde simples caracteres ANSI a cadenas UNICODE y además incluye los tipos puntero. Las diferencias también se aplican a las codificaciones y reference counting.

AnsiChar

Un tipo de variable AnsiChar, también referenciado como char, es exactamente de 1 byte de tamaño, y contiene un carácter ANSI.

a

Referencia

• Documentación FPC sobre AnsiChar
• Uso de Char

WideChar

Una variable de tipo WideChar, también referenciada como UnicodeChar, es exactamente de 2 bytes de tamaño, y contieen un (parte de) Unicode character in UTF-16 encoding. Nota: es imposible codificar todos los puntos codificables Unicode con 2 bytes. Therefore, se pueden necesitar 2 WideChars para codificar un solo code point.

a

Referencias

• Documentación FPC sobre WideChar
• Información sobre UTF-16 en la Wikipedia
• RTL Documentación UnicodeChar

Array of Char

Early Pascal implementations that were in use before 1978 did not support a string type (with the exception of string constants). The only possibility to store strings in variables was the use of arrays of char. This approach has many disadvantages and is no longer recommended. It is, however, still supported to ensure backward-compatibility with ancient code.

Matriz estática de Char

type
  TOldString4 = array[0..3] of char;
var
  aOldString4: TOldString4; 
begin
  aOldString4[0] := 'a';
  aOldString4[1] := 'b';
  aOldString4[2] := 'c';
  aOldString4[3] := 'd';
end;

La matriz estática de caracteres tiene ahora el siguiente contenido:

a

b

c

d

Nota: Los chars no asignados pueden tener cualquier contenido, dependiendo de lo que exista en ese área de memoria cuando se haga disponible la matriz.

Matriz dinámica de Char

var
  aOldString: Array of Char; 
begin
  SetLength(aOldString, 5);
  aOldString[0] := 'a';
  aOldString[1] := 'b';
  aOldString[2] := 'c';
  aOldString[3] := 'd';
end;

The dynamic array of char has now the content:

a

b

c

d

#0

PChar

Una variable de tipo PChar es básicamente un puntero a un tipo Char, pero que permite operaciones adicionales. Pchars se pueden utilizar para acceder al estilo C null-terminated strings, e.g. en interacciones con ciertas librerías del Sistema Operativo o en software de terceros.

a	b	c	#0
^

Referencia

• Documentación en FPC sobre PChar
• Funciones relacionadas con PChar

PWideChar

A variable of type PWideChar is a pointer to a WideChar variable.

	a		b		c	#0	#0
^

Texto de encabezado

Referencia

• Documentación en la RTL sobre PWideChar

String

The type String may refer to ShortString or AnsiString, depending from the {$H} switch. If the switch is off ({$H-}) then any string declaration will define a ShortString. It size will be 255 chars, if not otherwise specified. If it is on ({$H+}) string without length specifier will define an AnsiString, otherwise a ShortString with specified length. In mode delphiunicode' String is UnicodeString.

Reference

• Usage String
• String functions
• Reference for unit 'strutils': Procedures and functions

ShortString

Short strings have a maximum length of 255 characters with the implicit codepage CP_ACP. The length is stored in the character at index 0.

#3

a

b

c

Reference

• FPC AnsiString documentation

AnsiString

Ansistrings are strings that have no length limit. They are reference counted and are guaranteed to be null terminated. Internally, a variable of type AnsiString is treated as a pointer: the actual content of the string is stored on the heap, as much memory as needed to store the string content is allocated.

								a	b	c	#0
RefCount				Length

Reference

• FPC AnsiString documentation

UnicodeString

Like AnsiStrings, UnicodeStrings are reference counted, null-terminated arrays, but they are implemented as arrays of WideChars instead of regular Chars.

  Nota: The UnicodeString naming is a bit ambiguous but probably due to its use in Delphi on Windows, where the OS uses UTF16 encoding; it's not the only string type that can hold Unicode string data (see also UTF8String)...

a

b

c

#0

#0

RefCount

Length

Reference

• FPC UnicodeString documentation

UTF8String

En la versión FPC 2.6.5 e inferiores el tipo UTF8String era un alias del tipo AnsiString. En FPC 2.7.1 y posteriores se define como:

UTF8String          = type AnsiString(CP_UTF8);

Esto significa que contienen cadenas codificadas como UTF-8 (i.e. datos unicode) oscilando entre 1..4 bytes por carácter.

Ten en cuenta que un String puede también contener caracteres codificados enn UTF-8.

Referencia

• Documentación en RTL sobre UTF8String

UTF16String

The type UTF16String is an alias to the type WideString. In the LCL unit lclproc it is an alias to UnicodeString.

Reference

• LCL UTF16String documentation

WideString

Variables of type WideString (used to represent unicode character strings in COM applications) resemble those of type UnicodeString, but unlike them they are not reference-counted. On Windows they are allocated with a special windows function which allows them to be used for OLE automation.

WideStrings consist of COM compatible UTF16 encoded bytes on Windows machines (UCS2 on Windows 2000), and they are encoded as plain UTF16 on Linux, Mac OS X and iOS.

					a		b		c	#0	#0
Length

Referencia

• Documentación en FPC sobre WideString

PShortString

A variable of type PShortString is a pointer that points to the first byte of a ShortString-type variable (which defines the length of the ShortString).

#3	a	b	c
^

Reference

• RTL PShortString documentation

PAnsiString

Variables of type PAnsiString are pointers to AnsiString-type variables. However, unlike PShortString-type variables they don't point to the first byte of the header, but to the first char of the AnsiString.

								a	b	c	#0
RefCount				Length				^

Reference

• RTL PAnsiString documentation

PUnicodeString

Variables of type PUnicodeString are pointers to variables of type UnicodeString.

a

b

c

#0

#0

RefCount

Length

^

Reference

• Documentación en la RTL sobre PUnicodeString

PWideString

Las variables de tipo PWideString' son punteros. Apuntan al primer carácter de una variable tipo WideString.

					a		b		c	#0	#0
Length				^

Referencías

• Documentación en la RTL sobre PWideString

String constants

If you use only English constants your strings work the same with all types, on all platforms and all compiler versions. Non English strings can be loaded via resourcestrings or from files. If you want to use non English strings in code then you should read further.

There are various encodings for non English strings. By default Lazarus saves Pascal files as UTF-8 without BOM. UTF-8 supports the full Unicode range. That means all string constants are stored in UTF-8. Lazarus also supports to change the encoding of a file to other encoding, for example under Windows your local codepage. The Windows codepage is limited to your current language group.

String Type, UTF-8 Source	With or without {$codepage utf8}	FPC 2.6.5 and below	FPC 2.7.1 and above	FPC 2.7.1+ with UTF8 as default CodePage
AnAnsiString:='ãü';	Without	Needs UTF8ToAnsi in RTL/WinAPI. Ok in LCL	Needs UTF8ToAnsi in RTL/WinAPI. Ok in LCL	Ok in RTL/W-WinAPI/LCL. Needs UTF8ToWinCP in A-WinAPI.
AnAnsiString:='ãü';	With	System cp ok in RTL/WinAPI. Needs SysToUTF8 in LCL	Ok in RTL/WinAPI/LCL. Mixing with other strings converts to system cp	Ok in RTL/W-WinAPI/LCL. Needs UTF8ToWinCP in A-WinAPI
AnUnicodeString:='ãü';	Without	Wrong everywhere	Wrong everywhere	Wrong everywhere
AnUnicodeString:='ãü';	With	System cp ok in RTL/WinAPI. Needs UTF8Encode in LCL	Ok in RTL/WinAPI/LCL. Mixing with other strings converts to system cp	Ok in RTL/W-WinAPI/LCL. Needs UTF8ToWinCP in A-WinAPI
AnUTF8String:='ãü';	Without	Same as AnsiString	Wrong everywhere	Wrong everywhere
AnUTF8String:='ãü';	With	Same as AnsiString	Ok in RTL/WinAPI/LCL. Mixing with other strings converts to system cp	Ok in RTL/W-WinAPI/LCL. Needs UTF8ToWinCP in A-WinAPI.

• W-WinAPI = Windows API "W" functions, UTF-16
• A-WinAPI = Windows API non "W" functions, 8bit system code page
• System CP = The 8bit system code page of the OS. For example code page 1252.

const 
  c='ãü';
  cstring: string = 'ãü'; // see AnAnsiString:='ãü';
var
  s: string;
  u: UnicodeString;
begin
  s:=c; // same as s:='ãü';
  s:=cstring; // does not change encoding
  u:=c; // same as u:='ãü';
  u:=cstring; // fpc 2.6.1: converts from system cp to UTF-16, fpc 2.7.1+: depends on encoding of cstring
end;

See also

• FPC Unicode support
• LCL Unicode Support
• TStringList-TStrings Tutorial
• A Brief History of Strings