Difference between revisions of "Character and string types"
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==== Reference ==== | ==== Reference ==== | ||
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== AnsiString == | == AnsiString == |
Revision as of 03:35, 22 September 2018
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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
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
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 |
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 |
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
PWideChar
A variable of type PWideChar is a pointer to a WideChar variable.
a | b | c | #0 | #0 | |||
^ |
Reference
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
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
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
UnicodeString
Like AnsiStrings, UnicodeStrings are reference counted, null-terminated arrays, but they are implemented as arrays of WideChars instead of regular Chars.
a | b | c | #0 | #0 | |||||||||||
RefCount | Length |
Reference
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
UTF16String
The type UTF16String is an alias to the type WideString. In the LCL unit lclproc it is an alias to UnicodeString.
Reference
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
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
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
PUnicodeString
Variables of type PUnicodeString are pointers to variables of type UnicodeString.
a | b | c | #0 | #0 | |||||||||||
RefCount | Length | ^ |
Reference
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
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;