I am not able to understand the differences between std::string
and std::wstring
. I know wstring
supports wide characters such as Unicode characters. I have got the following questions:
std::wstring
over std::string
?std::string
hold the entire ASCII character set, including the special characters?std::wstring
supported by all popular C++ compilers?string
? wstring
?std::string
is a basic_string
templated on a char
, and std::wstring
on a wchar_t
.
char
vs. wchar_t
char
is supposed to hold a character, usually an 8-bit character.
wchar_t
is supposed to hold a wide character, and then, things get tricky:
On Linux, a wchar_t
is 4 bytes, while on Windows, it's 2 bytes.
The problem is that neither char
nor wchar_t
is directly tied to unicode.
Let's take a Linux OS: My Ubuntu system is already unicode aware. When I work with a char string, it is natively encoded in UTF-8 (i.e. Unicode string of chars). The following code:
#include <cstring>
#include <iostream>
int main(int argc, char* argv[])
{
const char text[] = "olé" ;
std::cout << "sizeof(char) : " << sizeof(char) << std::endl ;
std::cout << "text : " << text << std::endl ;
std::cout << "sizeof(text) : " << sizeof(text) << std::endl ;
std::cout << "strlen(text) : " << strlen(text) << std::endl ;
std::cout << "text(ordinals) :" ;
for(size_t i = 0, iMax = strlen(text); i < iMax; ++i)
{
std::cout << " " << static_cast<unsigned int>(
static_cast<unsigned char>(text[i])
);
}
std::cout << std::endl << std::endl ;
// - - -
const wchar_t wtext[] = L"olé" ;
std::cout << "sizeof(wchar_t) : " << sizeof(wchar_t) << std::endl ;
//std::cout << "wtext : " << wtext << std::endl ; <- error
std::cout << "wtext : UNABLE TO CONVERT NATIVELY." << std::endl ;
std::wcout << L"wtext : " << wtext << std::endl;
std::cout << "sizeof(wtext) : " << sizeof(wtext) << std::endl ;
std::cout << "wcslen(wtext) : " << wcslen(wtext) << std::endl ;
std::cout << "wtext(ordinals) :" ;
for(size_t i = 0, iMax = wcslen(wtext); i < iMax; ++i)
{
std::cout << " " << static_cast<unsigned int>(
static_cast<unsigned short>(wtext[i])
);
}
std::cout << std::endl << std::endl ;
return 0;
}
outputs the following text:
sizeof(char) : 1
text : olé
sizeof(text) : 5
strlen(text) : 4
text(ordinals) : 111 108 195 169
sizeof(wchar_t) : 4
wtext : UNABLE TO CONVERT NATIVELY.
wtext : ol�
sizeof(wtext) : 16
wcslen(wtext) : 3
wtext(ordinals) : 111 108 233
You'll see the "olé" text in char
is really constructed by four chars: 110, 108, 195 and 169 (not counting the trailing zero). (I'll let you study the wchar_t
code as an exercise)
So, when working with a char
on Linux, you should usually end up using Unicode without even knowing it. And as std::string
works with char
, so std::string
is already unicode-ready.
Note that std::string
, like the C string API, will consider the "olé" string to have 4 characters, not three. So you should be cautious when truncating/playing with unicode chars because some combination of chars is forbidden in UTF-8.
On Windows, this is a bit different. Win32 had to support a lot of application working with char
and on different charsets/codepages produced in all the world, before the advent of Unicode.
So their solution was an interesting one: If an application works with char
, then the char strings are encoded/printed/shown on GUI labels using the local charset/codepage on the machine. For example, "olé" would be "olé" in a French-localized Windows, but would be something different on an cyrillic-localized Windows ("olй" if you use Windows-1251). Thus, "historical apps" will usually still work the same old way.
For Unicode based applications, Windows uses wchar_t
, which is 2-bytes wide, and is encoded in UTF-16, which is Unicode encoded on 2-bytes characters (or at the very least, the mostly compatible UCS-2, which is almost the same thing IIRC).
Applications using char
are said "multibyte" (because each glyph is composed of one or more char
s), while applications using wchar_t
are said "widechar" (because each glyph is composed of one or two wchar_t
. See MultiByteToWideChar and WideCharToMultiByte Win32 conversion API for more info.
Thus, if you work on Windows, you badly want to use wchar_t
(unless you use a framework hiding that, like GTK+ or QT...). The fact is that behind the scenes, Windows works with wchar_t
strings, so even historical applications will have their char
strings converted in wchar_t
when using API like SetWindowText()
(low level API function to set the label on a Win32 GUI).
UTF-32 is 4 bytes per characters, so there is no much to add, if only that a UTF-8 text and UTF-16 text will always use less or the same amount of memory than an UTF-32 text (and usually less).
If there is a memory issue, then you should know than for most western languages, UTF-8 text will use less memory than the same UTF-16 one.
Still, for other languages (chinese, japanese, etc.), the memory used will be either the same, or slightly larger for UTF-8 than for UTF-16.
All in all, UTF-16 will mostly use 2 and occassionally 4 bytes per characters (unless you're dealing with some kind of esoteric language glyphs (Klingon? Elvish?), while UTF-8 will spend from 1 to 4 bytes.
See http://en.wikipedia.org/wiki/UTF-8#Compared_to_UTF-16 for more info.
When I should use std::wstring over std::string?
On Linux? Almost never (§).
On Windows? Almost always (§).
On cross-platform code? Depends on your toolkit...
(§) : unless you use a toolkit/framework saying otherwise
Can std::string
hold all the ASCII character set including special characters?
Notice: A std::string
is suitable for holding a 'binary' buffer, where a std::wstring
is not!
On Linux? Yes.
On Windows? Only special characters available for the current locale of the Windows user.
Edit (After a comment from Johann Gerell):
a std::string
will be enough to handle all char
-based strings (each char
being a number from 0 to 255). But:
char
s are NOT ASCII.char
from 0 to 127 will be held correctlychar
from 128 to 255 will have a signification depending on your encoding (unicode, non-unicode, etc.), but it will be able to hold all Unicode glyphs as long as they are encoded in UTF-8.Is std::wstring
supported by almost all popular C++ compilers?
Mostly, with the exception of GCC based compilers that are ported to Windows.
It works on my g++ 4.3.2 (under Linux), and I used Unicode API on Win32 since Visual C++ 6.
What is exactly a wide character?
On C/C++, it's a character type written wchar_t
which is larger than the simple char
character type. It is supposed to be used to put inside characters whose indices (like Unicode glyphs) are larger than 255 (or 127, depending...).