Character and string types

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Free Pascal supports several character and string types. They range from single ANSI characters to unicode strings and also include pointer types. Differences also apply to encodings and reference counting.

AnsiChar

A variable of type AnsiChar, also referred to as char, is exactly 1 byte in size, and contains one ANSI character.

a

Reference

• FPC AnsiChar documentation
• Usage Char

WideChar

A variable of type WideChar, also referred to as UnicodeChar, is exactly 2 bytes in size, and usually contains one Unicode code point (normally a character) in UTF-16 encoding. Note: it is impossible to encode all Unicode code points in 2 bytes. Therefore, 2 WideChars may be needed to encode a single code point.

a

References

• FPC WideChar documentation
• UTF-16 information on Wikipedia
• RTL UnicodeChar documentation

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.

Static Array of 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;

The static array of char has now the content:

a

b

c

d

  Note: Unassigned chars can have any content, depending on what was just in memory when the memory for the array was made available.

Dynamic Array of 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

  Note: Unassigned chars in dynamic arrays have a content #0, cause empty positions of all dynamic arrays are initially initialised with 0 (or #0, or nil, or ...)

PChar

A variable of type PChar is basically a pointer to a Char type, but allows additional operations. PChars can be used to access C-style null-terminated strings, e.g. in interaction with certain OS libraries or third-party software.

a	b	c	#0
^

Reference

• FPC PChar documentation
• PChar related functions

PWideChar

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

	a		b		c	#0	#0
^

Reference

• RTL PWideChar documentation

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. A short string of 255 characters uses 256 bytes of memory (one byte for the length specification and 255 bytes for characters).

#3

a

b

c

Reference

• FPC Single-byte String types 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 Single-byte String types documentation

UnicodeString

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

  Note: 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 Multi-byte String types documentation

UTF8String

In FPC 2.6.5 and below the type UTF8String was an alias to the type AnsiString. In FPC 2.7.1 and above it is defined as UTF8String = type AnsiString(CP_UTF8);

It is meant to contain UTF-8 encoded strings (i.e. unicode data) ranging from 1..4 bytes per character. Note that String can also contain UTF-8 encoded characters.

Reference

• FPC UTF8String documentation

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

Reference

• FPC Multi-byte String types documentation

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

• RTL PUnicodeString documentation

PWideString

Variables of type PWideString are pointers. They point to the first char of a WideString-typed variable.

					a		b		c	#0	#0
Length				^

Reference

• RTL PWideString documentation

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