So I have standard C string:
char* name = "Jakub";
And I want to convert it to UTF-16. I figured out, that UTF-16 will be twice as long - one character takes two chars.
So I create another string:
char name_utf_16[10]; //"Jakub" is 5 characters
Now, I believe with ASCII characters I will only use lower bytes, so for all of them it will be like 74 00 for J and so on. With that belief, I can make such code:
void charToUtf16(char* input, char* output, int length) {
/*Todo: how to check if output is long enough?*/
for(int i=0; i<length; i+=2) //Step over 2 bytes
{
//Lets use little-endian - smallest bytes first
output[i] = input[i];
output[i+1] = 0; //We will never have any data for this field
}
}
But, with this process, I ended with "Jkb". I know no way to test this properly - I've just sent the string to Minecraft Bukkit Server. And this is what it said upon disconnecting:
13:34:19 [INFO] Disconnecting jkb?? [/127.0.0.1:53215]: Outdated server!
Note: I'm aware that Minecraft uses big-endian. Code above is just an example, in fact, I have my conversion implemented in class.
Before I answer your question, consider this:
This area of programming is full of man traps. It makes a lot of sense to understand the differences between ASCII, UTF7/8 and ANSI/'MultiByte Character Strings (MBCS)', all of which to an english speaking programmer will look and feel identical, but need very different handling if they are introduced to a european or asian user.
ASCII: Characters are in range 32-127. only ever one byte. The clue is in the name, they are great for Americans, but not fit for purpose in the rest of the world.
ANSI/MBCS: This is the reason for 'code pages'. Characters 32-127 are the same as ASCII, but it is possible to have characters in the range of 128-255 as well for additional characters, and some of the 128-255 range can be used as a flag to mark that the character continues into a second, third or even fourth byte. To process the string correctly, you need both the string bytes and the correct code page. If you try processing the string using the wrong code page you will not have the right characters, and misinterpret whether a character is a one, two or even 4 byte character.
UTF7/8: These are 8-bit wide formatting of 21-bit unicode character points. in UTF-7 and UTF-8 unicode characters can be between one and four bytes long. The advantage that UTF encodings have over ANSI/MBCS is that there is no ambiguity caused by code pages. Each glyph in every script has a unique unicode code point, which means it is not possible to mangle the character sets by interpreting the data on a different computer with different regional settings.
So to to start to answer your question:
Whilst you are making the assumption that your char* will only point to an ASCII string, that is a really dangerous choice to make, users are in control of data that is typed in, not the programmer. Windows programs will be storing this as MBCS by default.
You are making the second assumption is that a UTF-16 encoding will be twice the size of an 8 bit encoding. That is not generally a safe assumption. depending on the source encoding the UTF-16 encoding may be twice the size, may be less than twice the size, and in an extreme example may actually be shorter in length.
So, what is the safe solution?
The safe option is to implement your application internally as unicode. On windows, this is a compiler option, and then means your windows controls all use wchar_t* strings for their data type. On linux I'm less sure that you can always use unicide graphics and OS libraries. You must also use the wcslen() functions to get the length of strings etc. When you interact with the outside world, be precise in the character encodings used.
To answer to your question then becomes changing the question to, what do i do when I receive non UTF-16 data?
Firstly, be very clear about what assumptions on its formatting are you making? and secondly, accept the fact that sometimes the conversion to UTF-16 may fail.
If you are clear on the source formatting, you can then choose the appropriate win32 or the stl converter to convert the format, and you should then look for evidence the conversion failed before using the result. e.g. mbstowcs in or MultiByteToWideChar() on windows. However the use of both of these approaches safely means you need to understand ALL of the above answer.
All other options introduce risk. Use mbcs strings and you will have data strings mangled by being entered using one code page, and processed using a different code page. Assume ASCII data, and when you encounter a non ascii character your code will break, and you will 'blame' the user for your short comings.
Why do you want to make your own Unicode conversion functionality when theres's existing C/C++ functions for this, like mbstowcs() which is included in <cstdlib>.
If you still want to make your own stuff, then have a look at Unicode Consortium's open source code which can be found here:
Convert UTF-16 to UTF-8 under Windows and Linux, in C
output[i] = input[i];
This will assign every other byte of the input, because you increment i by 2. So no wonder that you obtain "Jkb".
You probably wanted to write:
output[i] = input[i / 2];
Related
According to cppreference.com's doc on wchar_t:
wchar_t - type for wide character representation (see wide strings). Required to be large enough to represent any supported character code point (32 bits on systems that support Unicode. A notable exception is Windows, where wchar_t is 16 bits and holds UTF-16 code units) It has the same size, signedness, and alignment as one of the integer types, but is a distinct type.
The Standard says in [basic.fundamental]/5:
Type wchar_t is a distinct type whose values can represent distinct codes for all members of the largest extended character set specified among the supported locales. Type wchar_t shall have the same size, signedness, and alignment requirements as one of the other integral types, called its underlying type. Types char16_t and char32_t denote distinct types with the same size, signedness, and alignment as uint_least16_t and uint_least32_t, respectively, in <cstdint>, called the underlying types.
So, if I want to deal with unicode characters, should I use wchar_t?
Equivalently, how do I know if a specific unicode character is "supported" by wchar_t?
So, if I want to deal with unicode characters, should I use
wchar_t?
First of all, note that the encoding does not force you to use any particular type to represent a certain character. You may use char to represent Unicode characters just as wchar_t can - you only have to remember that up to 4 chars together will form a valid code point depending on UTF-8, UTF-16, or UTF-32 encoding, while wchar_t can use 1 (UTF-32 on Linux, etc) or up to 2 working together (UTF-16 on Windows).
Next, there is no definite Unicode encoding. Some Unicode encodings use a fixed width for representing codepoints (like UTF-32), others (such as UTF-8 and UTF-16) have variable lengths (the letter 'a' for instance surely will just use up 1 byte, but apart from the English alphabet, other characters surely will use up more bytes for representation).
So you have to decide what kind of characters you want to represent and then choose your encoding accordingly. Depending on the kind of characters you want to represent, this will affect the amount of bytes your data will take. E.g. using UTF-32 to represent mostly English characters will lead to many 0-bytes. UTF-8 is a better choice for many Latin based languages, while UTF-16 is usually a better choice for Eastern Asian languages.
Once you have decided on this, you should minimize the amount of conversions and stay consistent with your decision.
In the next step, you may decide what data type is appropriate to represent the data (or what kind of conversions you may need).
If you would like to do text-manipulation/interpretation on a code-point basis, char certainly is not the way to go if you have e.g. Japanese kanji. But if you just want to communicate your data and regard it no more as a quantitative sequence of bytes, you may just go with char.
The link to UTF-8 everywhere was already posted as a comment, and I suggest you having a look there as well. Another good read is What every programmer should know about encodings.
As by now, there is only rudimentary language support in C++ for Unicode (like the char16_t and char32_t data types, and u8/u/U literal prefixes). So chosing a library for manging encodings (especially conversions) certainly is a good advice.
wchar_t is used in Windows which uses UTF16-LE format. wchar_t requires wide char functions. For example wcslen(const wchar_t*) instead of strlen(const char*) and std::wstring instead of std::string
Unix based machines (Linux, Mac, etc.) use UTF8. This uses char for storage, and the same C and C++ functions for ASCII, such as strlen(const char*) and std::string (see comments below about std::find_first_of)
wchar_t is 2 bytes (UTF16) in Windows. But in other machines it is 4 bytes (UTF32). This makes things more confusing.
For UTF32, you can use std::u32string which is the same on different systems.
You might consider converting UTF8 to UTF32, because that way each character is always 4 bytes, and you might think string operations will be easier. But that's rarely necessary.
UTF8 is designed so that ASCII characters between 0 and 128 are not used to represent other Unicode code points. That includes escape sequence '\', printf format specifiers, and common parsing characters like ,
Consider the following UTF8 string. Lets say you want to find the comma
std::string str = u8"汉,🙂"; //3 code points represented by 8 bytes
The ASCII value for comma is 44, and str is guaranteed to contain only one byte whose value is 44. To find the comma, you can simply use any standard function in C or C++ to look for ','
To find 汉, you can search for the string u8"汉" since this code point cannot be represented as a single character.
Some C and C++ functions don't work smoothly with UTF8. These include
strtok
strspn
std::find_first_of
The argument for above functions is a set of characters, not an actual string.
So str.find_first_of(u8"汉") does not work. Because u8"汉" is 3 bytes, and find_first_of will look for any of those bytes. There is a chance that one of those bytes are used to represent a different code point.
On the other hand, str.find_first_of(u8",;abcd") is safe, because all the characters in the search argument are ASCII (str itself can contain any Unicode character)
In rare cases UTF32 might be required (although I can't imagine where!) You can use std::codecvt to convert UTF8 to UTF32 to run the following operations:
std::u32string u32 = U"012汉"; //4 code points, represented by 4 elements
cout << u32.find_first_of(U"汉") << endl; //outputs 3
cout << u32.find_first_of(U'汉') << endl; //outputs 3
Side note:
You should use "Unicode everywhere", not "UTF8 everywhere".
In Linux, Mac, etc. use UTF8 for Unicode.
In Windows, use UTF16 for Unicode. Windows programmers use UTF16, they don't make pointless conversions back and forth to UTF8. But there are legitimate cases for using UTF8 in Windows.
Windows programmer tend to use UTF8 for saving files, web pages, etc. So that's less worry for non-Windows programmers in terms of compatibility.
The language itself doesn't care which Unicode format you want to use, but in terms of practicality use a format that matches the system you are working on.
So, if I want to deal with unicode characters, should I use wchar_t?
That depends on what encoding you're dealing with. In case of UTF-8 you're just fine with char and std::string.
UTF-8 means the least encoding unit is 8 bits: all Unicode code points from U+0000 to U+007F are encoded by only 1 byte.
Beginning with code point U+0080 UTF-8 uses 2 bytes for encoding, starting from U+0800 it uses 3 bytes and from U+10000 4 bytes.
To handle this variable width (1 byte - 2 byte - 3 byte - 4 byte) char fits best.
Be aware that C-functions like strlen will provide byte-based results: "öö" in fact is a 2-character text but strlen will return 4 because 'ö' is encoded to 0xC3B6.
UTF-16 means the least encoding unit is 16 bits: all code points from U+0000 to U+FFFF are encoded by 2 bytes; starting from U+100000 4 bytes are used.
In case of UTF-16 you should use wchar_t and std::wstring because most of the characters you'll ever encounter will be 2-byte encoded.
When using wchar_t you can't use C-functions like strlen any more; you have to use the wide char equivalents like wcslen.
When using Visual Studio and building with configuration "Unicode" you'll get UTF-16: TCHAR and CString will be based on wchar_t instead of char.
It all depends what you mean by 'deal with', but one thing is for sure: where Unicode is concerned std::basic_string doesn't provide any real functionality at all.
In any particular program, you will need to perform X number of Unicode-aware operations, e.g. intelligent string matching, case folding, regex, locating word breaks, using a Unicode string as a path name maybe, and so on.
Supporting these operations there will almost always be some kind of library and / or native API provided by the platform, and the goal for me would be to store and manipulate my strings in such a way that these operations can be carried out without scattering knowledge of the underlying library and native API support throughout the code any more than necessary. I'd also want to future-proof myself as to the width of the characters I store in my strings in case I change my mind.
Suppose, for example, you decide to use ICU to do the heavy lifting. Immediately there is an obvious problem: an icu::UnicodeString is not related in any way to std::basic_string. What to do? Work exclusively with icu::UnicodeString throughout the code? Probably not.
Or maybe the focus of the application switches from European languages to Asian ones, so that UTF-16 becomes (perhaps) a better choice than UTF-8.
So, my choice would be to use a custom string class derived from std::basic_string, something like this:
typedef wchar_t mychar_t; // say
class MyString : public std::basic_string <mychar_t>
{
...
};
Straightaway you have flexibility in choosing the size of the code units stored in your container. But you can do much more than that. For example, with the above declaration (and after you add in boilerplate for the various constructors that you need to provide to forward them to std::basic_string), you still cannot say:
MyString s = "abcde";
Because "abcde" is a narrow string and various the constructors for std::basic_string <wchar_t> all expect a wide string. Microsoft solve this with a macro (TEXT ("...") or __T ("...")), but that is a pain. All we need to do now is to provide a suitable constructor in MyString, with signature MyString (const char *s), and the problem is solved.
In practise, this constructor would probably expect a UTF-8 string, regardless of the underlying character width used for MyString, and convert it if necessary. Someone comments here somewhere that you should store your strings as UTF-8 so that you can construct them from UTF-8 literals in your code. Well now we have broken that constraint. The underlying character width of our strings can be anything we like.
Another thing that people have been talking about in this thread is that find_first_of may not work properly for UTF-8 strings (and indeed some UTF-16 ones also). Well, now you can provide an implementation that does the job properly. Should take about half an hour. If there are other 'broken' implementations in std::basic_string (and I'm sure there are), then most of them can probably be replaced with similar ease.
As for the rest, it mainly depends what level of abstraction you want to implement in your MyString class. If your application is happy to have a dependency on ICU, for example, then you can just provide a couple of methods to convert to and from an icu::UnicodeString. That's probably what most people would do.
Or if you need to pass UTF-16 strings to / from native Windows APIs then you can add methods to convert to and from const WCHAR * (which again you would implement in such a way that they work for all values of mychar_t). Or you could go further and abstract away some or all of the Unicode support provided by the platform and library you are using. The Mac, for example, has rich Unicode support but it's only available from Objective-C so you have to wrap it.
It depends on how portable you want your code to be.
So you can add in whatever functionality you like, probably on an on-going basis as work progresses, without losing the ability to carry your strings around as a std::basic_string. Of one sort or another. Just try not to write code that assumes it knows how wide it is, or that it contains no surrogate pairs.
First of all, you should check (as you point out in your question) if you are using Windows and Visual Studio C++ with wchar_t being 16bits, because in that case, to use full unicode support, you'll need to assume UTF-16 encoding.
The basic problem here is not the sizeof wchar_t you are using, but if the libraries you are going to use, support full unicode support.
Java has a similar problem, as its char type is 16bit wide, so it couldn't a priori support full unicode space, but it does, as it uses UTF-16 encoding and the pair surrogates to cope with full 24bit codepoints.
It's also worth to note that UNICODE uses only the high plane to encode rare codepoints, that are not normally used daily.
For unicode support anyway, you need to use wide character sets, so wchar_t is a good beginning. If you are going to work with visual studio, then you have to check how it's libraries deal with unicode characters.
Another thing to note is that standard libraries deal with character sets (and this includes unicode) only when you add locale support (this requires some library to be initialized, e.g. setlocale(3)) and so, you'll see no unicode at all (only basic ascii) in cases where you have not called setlocale(3).
There are wide char functions for almost any str*(3) function, as well as for any stdio.h library function, to deal with wchar_ts. A little dig into the /usr/include/wchar.h file will reveal the names of the routines. Go to the manual pages for documentation on them: fgetws(3), fputwc(3), fputws(3), fwide(3), fwprintf(3), ...
Finally, consider again that, if you are dealing with Microsoft Visual C++, you have a different implementation from the beginning. Even if they cope to be completely standard compliant, you'll have to cope with some idiosyncrasies of having a different implementation. Probably you'll have different function names for some uses.
I have a large char array, which contains Czech diacritical characters (e.g. "á"), coded in UTF-8. I need to replace them to their ASCII equivalents (e.g. "a"), because program must work on Windows (Linux console accepts these chars perfectly).
I am reading array char by char and writing content into string.
Here is code I am using, this doesnt work:
int array_size = 50000; //size of file array
char * array = new char[array_size]; //array to store file contents
string ascicontent="";
if ('\u00E1'==array[zacatek]) { //check if char is "á"
ascicontent +='a'; //write ordinal "a" into string
}
I even tried replacing '\u00E1' with 'á', but it also doesnt work. Guessing there is problem that these chars are longer than ascii.
How can I declare the non-ascii char, so it could be compared?
Each char is a single byte, however UTF-8 can use multiple bytes to encode a single character. In particular U+00E1 is encoded as two bytes: 0xC3 0xA1. So you can't do what you want with just comparing a single char.
There are multiple ways that you might be able to tackle your problem:
A) First, try googling for "windows console utf-8" and see if that gives anything which might make things just work without having to alter the characters at all. (I don't know if anything can work for you, I've never tried this.)
B) Convert the data to wide characters (wchar_t) using MultiByteToWideChar or mbstowcs and then google how to use wcout or such to output UTF-16 to the console.
C) Use MultiByteToWideChar to convert the data from UTF-8 to UTF-16. Then use WideCharToMultiByte to convert from UTF-16 to the console's code page, relying on the fact that it can automatically "best fit" common characters (such as "á" to "a").
D) If you really only care about a limited set of characters (such as only the accented characters in the Czech code page), then you could possibly write your own lookup table of UTF-8 byte sequences and your desired replacements. You just need to be doing comparisons on the UTF-8 by those multiple bytes rather than individual chars. Among various tools out there, I've found this page helpful for seeing how characters are encoded in various ways.
Which of these make the most sense for your program depends on various factors, such as how easy or hard it might be to keep the Windows-specific pieces from conflicting with the Linux-specific or cross-platform parts.
char in C is not unicode, it is really a byte; it only gets converted to a glyph by the terminal console you happen to use. On some Linux implementations (like Debian) it defaults to UTF-8, so if your program outputs a sequence of bytes encoded in UTF-8, your terminal will display the proper glyph. If you know that array is UTF-8 encoded, you must check for the proper byte sequence.
Edit: take a look at The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!)
Please take a look at this link http://en.wikipedia.org/wiki/Wide_character.
And I believe this code might help you:
std::wstring str(L"cccccááddddddd");
std::replace( str.begin(), str.end(), L'á', L'a');
Having an untyped pointer pointing to some buffer which can hold either ANSI or Unicode string, how do I tell whether the current string it holds is multibyte or not?
Unless the string itself contains information about its format (e.g. a header or a byte order mark) then there is no foolproof way to detect if a string is ANSI or Unicode. The Windows API includes a function called IsTextUnicode() that basically guesses if a string is ANSI or Unicode, but then you run into this problem because you're forced to guess.
Why do you have an untyped pointer to a string in the first place? You must know exactly what and how your data is representing information, either by using a typed pointer in the first place or provide an ANSI/Unicode flag or something. A string of bytes is meaningless unless you know exactly what it represents.
Unicode is not an encoding, it's a mapping of code points to characters. The encoding is UTF8 or UCS2, for example.
And, given that there is zero difference between ASCII and UTF8 encoding if you restrict yourself to the lower 128 characters, you can't actually tell the difference.
You'd be better off asking if there were a way to tell the difference between ASCII and a particular encoding of Unicode. And the answer to that is to use statistical analysis, with the inherent possibility of inaccuracy.
For example, if the entire string consists of bytes less than 128, it's ASCII (it could be UTF8 but there's no way to tell and no difference in that case).
If it's primarily English/Roman and consists of lots of two-byte sequences with a zero as one of the bytes, it's probably UTF16. And so on. I don't believe there's a foolproof method without actually having an indicator of some sort (e.g., BOM).
My suggestion is to not put yourself in the position where you have to guess. If the data type itself can't contain an indicator, provide different functions for ASCII and a particular encoding of Unicode. Then force the work of deciding on to your client. At some point in the calling hierarchy, someone should now the encoding.
Or, better yet, ditch ASCII altogether, embrace the new world and use Unicode exclusively. With UTF8 encoding, ASCII has exactly no advantages over Unicode :-)
In general you can't
You could check for the pattern of zeros - just one at the end probably means ansi 'c', every other byte a zero probably means ansi text as UTF16, 3zeros might be UTF32
Assuming a very simple program that:
ask a name.
store the name in a variable.
display the variable content on the screen.
It's so simple that is the first thing that one learns.
But my problem is that I don't know how to do the same thing if I enter the name using japanese characters.
So, if you know how to do this in C++, please show me an example (that I can compile and test)
Thanks.
user362981 : Thanks for your help. I compiled the code that you wrote without problem, them the console window appears and I cannot enter any Japanese characters on it (using IME). Also if
I change a word in your code ("hello") to one that contains Japanese characters, it also will not display these.
Svisstack : Also thanks for your help. But when I compile your code I get the following error:
warning: deprecated conversion from string constant to 'wchar_t*'
error: too few arguments to function 'int swprintf(wchar_t*, const wchar_t*, ...)'
error: at this point in file
warning: deprecated conversion from string constant to 'wchar_t*'
You're going to get a lot of answers about wide characters. Wide characters, specifically wchar_t do not equal Unicode. You can use them (with some pitfalls) to store Unicode, just as you can an unsigned char. wchar_t is extremely system-dependent. To quote the Unicode Standard, version 5.2, chapter 5:
With the wchar_t wide character type, ANSI/ISO C provides for
inclusion of fixed-width, wide characters. ANSI/ISO C leaves the semantics of the wide
character set to the specific implementation but requires that the characters from the portable C execution set correspond to their wide character equivalents by zero extension.
and that
The width of wchar_t is compiler-specific and can be as small as 8 bits. Consequently,
programs that need to be portable across any C or C++ compiler should not use wchar_t
for storing Unicode text. The wchar_t type is intended for storing compiler-defined wide
characters, which may be Unicode characters in some compilers.
So, it's implementation defined. Here's two implementations: On Linux, wchar_t is 4 bytes wide, and represents text in the UTF-32 encoding (regardless of the current locale). (Either BE or LE depending on your system, whichever is native.) Windows, however, has a 2 byte wide wchar_t, and represents UTF-16 code units with them. Completely different.
A better path: Learn about locales, as you'll need to know that. For example, because I have my environment setup to use UTF-8 (Unicode), the following program will use Unicode:
#include <iostream>
int main()
{
setlocale(LC_ALL, "");
std::cout << "What's your name? ";
std::string name;
std::getline(std::cin, name);
std::cout << "Hello there, " << name << "." << std::endl;
return 0;
}
...
$ ./uni_test
What's your name? 佐藤 幹夫
Hello there, 佐藤 幹夫.
$ echo $LANG
en_US.UTF-8
But there's nothing Unicode about it. It merely reads in characters, which come in as UTF-8 because I have my environment set that way. I could just as easily say "heck, I'm part Czech, let's use ISO-8859-2": Suddenly, the program is getting input in ISO-8859-2, but since it's just regurgitating it, it doesn't matter, the program will still perform correctly.
Now, if that example had read in my name, and then tried to write it out into an XML file, and stupidly wrote <?xml version="1.0" encoding="UTF-8" ?> at the top, it would be right when my terminal was in UTF-8, but wrong when my terminal was in ISO-8859-2. In the latter case, it would need to convert it before serializing it to the XML file. (Or, just write ISO-8859-2 as the encoding for the XML file.)
On many POSIX systems, the current locale is typically UTF-8, because it provides several advantages to the user, but this isn't guaranteed. Just outputting UTF-8 to stdout will usually be correct, but not always. Say I am using ISO-8859-2: if you mindlessly output an ISO-8859-1 "è" (0xE8) to my terminal, I'll see a "č" (0xE8). Likewise, if you output a UTF-8 "è" (0xC3 0xA8), I'll see (ISO-8859-2) "è" (0xC3 0xA8). This barfing of incorrect characters has been called Mojibake.
Often, you're just shuffling data around, and it doesn't matter much. This typically comes into play when you need to serialize data. (Many internet protocols use UTF-8 or UTF-16, for example: if you got data from an ISO-8859-2 terminal, or a text file encoded in Windows-1252, then you have to convert it, or you'll be sending Mojibake.)
Sadly, this is about the state of Unicode support, in both C and C++. You have to remember: these languages are really system-agnostic, and don't bind to any particular way of doing it. That includes character-sets. There are tons of libraries out there, however, for dealing with Unicode and other character sets.
In the end, it's not all that complicated really: Know what encoding your data is in, and know what encoding your output should be in. If they're not the same, you need to do a conversion. This applies whether you're using std::cout or std::wcout. In my examples, stdin or std::cin and stdout/std::cout were sometimes in UTF-8, sometimes ISO-8859-2.
Try replacing cout with wcout, cin with wcin, and string with wstring. Depending on your platform, this may work:
#include <iostream>
#include <string>
int main() {
std::wstring name;
std::wcout << L"Enter your name: ";
std::wcin >> name;
std::wcout << L"Hello, " << name << std::endl;
}
There are other ways, but this is sort of the "minimal change" answer.
Pre-requisite: http://www.joelonsoftware.com/articles/Unicode.html
The above article is a must read which explains what unicode is but few lingering questions remains. Yes UNICODE has a unique code point for every character in every language and furthermore they can be encoded and stored in memory potentially differently from what the actual code is. This way we can save memory by for example using UTF-8 encoding which is great if the language supported is just English and so the memory representation is essentially same as ASCII – this of course knowing the encoding itself. In theory if we know the encoding, we can store these longer UNICODE characters however we like and read it back. But real world is a little different.
How do you store a UNICODE character/string in a C++ program? Which encoding do you use? The answer is you don’t use any encoding but you directly store the UNICODE code points in a unicode character string just like you store ASCII characters in ASCII string. The question is what character size should you use since UNICODE characters has no fixed size. The simple answer is you choose character size which is wide enough to hold the highest character code point (language) that you want to support.
The theory that a UNICODE character can take 2 bytes or more still holds true and this can create some confusion. Shouldn’t we be storing code points in 3 or 4 bytes than which is really what represents all unicode characters? Why is Visual C++ storing unicode in wchar_t then which is only 2 bytes, clearly not enough to store every UNICODE code point?
The reason we store UNICODE character code point in 2 bytes in Visual C++ is actually exactly the same reason why we were storing ASCII (=English) character into one byte. At that time, we were thinking of only English so one byte was enough. Now we are thinking of most international languages out there but not all so we are using 2 bytes which is enough. Yes it’s true this representation will not allow us to represent those code points which takes 3 bytes or more but we don’t care about those yet because those folks haven’t even bought a computer yet. Yes we are not using 3 or 4 bytes because we are still stingy with memory, why store the extra 0(zero) byte with every character when we are never going to use it (that language). Again this is exactly the same reasons why ASCII was storing each character in one byte, why store a character in 2 or more bytes when English can be represented in one byte and room to spare for those extra special characters!
In theory 2 bytes are not enough to present every Unicode code point but it is enough to hold anything that we may ever care about for now. A true UNICODE string representation could store each character in 4 bytes but we just don’t care about those languages.
Imagine 1000 years from now when we find friendly aliens and in abundance and want to communicate with them incorporating their countless languages. A single unicode character size will grow further perhaps to 8 bytes to accommodate all their code points. It doesn’t mean we should start using 8 bytes for each unicode character now. Memory is limited resource, we allocate what what we need.
Can I handle UNICODE string as C Style string?
In C++ an ASCII strings could still be handled in C++ and that’s fairly common by grabbing it by its char * pointer where C functions can be applied. However applying current C style string functions on a UNICODE string will not make any sense because it could have a single NULL bytes in it which terminates a C string.
A UNICODE string is no longer a plain buffer of text, well it is but now more complicated than a stream of single byte characters terminating with a NULL byte. This buffer could be handled by its pointer even in C but it will require a UNICODE compatible calls or a C library which could than read and write those strings and perform operations.
This is made easier in C++ with a specialized class that represents a UNICODE string. This class handles complexity of the unicode string buffer and provide an easy interface. This class also decides if each character of the unicode string is 2 bytes or more – these are implementation details. Today it may use wchar_t (2 bytes) but tomorrow it may use 4 bytes for each character to support more (less known) language. This is why it is always better to use TCHAR than a fixed size which maps to the right size when implementation changes.
How do I index a UNICODE string?
It is also worth noting and particularly in C style handling of strings that they use index to traverse or find sub string in a string. This index in ASCII string directly corresponded to the position of item in that string but it has no meaning in a UNICODE string and should be avoided.
What happens to the string terminating NULL byte?
Are UNICODE strings still terminated by NULL byte? Is a single NULL byte enough to terminate the string? This is an implementation question but a NULL byte is still one unicode code point and like every other code point, it must still be of same size as any other(specially when no encoding). So the NULL character must be two bytes as well if unicode string implementation is based on wchar_t. All UNICODE code points will be represented by same size irrespective if its a null byte or any other.
Does Visual C++ Debugger shows UNICODE text?
Yes, if text buffer is type LPWSTR or any other type that supports UNICODE, Visual Studio 2005 and up support displaying the international text in debugger watch window (provided fonts and language packs are installed of course).
Summary:
C++ doesn’t use any encoding to store unicode characters but it directly stores the UNICODE code points for each character in a string. It must pick character size large enough to hold the largest character of desirable languages (loosely speaking) and that character size will be fixed and used for all characters in the string.
Right now, 2 bytes are sufficient to represent most languages that we care about, this is why 2 bytes are used to represent code point. In future if a new friendly space colony was discovered that want to communicate with them, we will have to assign new unicode code pionts to their language and use larger character size to store those strings.
You can do simple things with the generic wide character support in your OS of choice, but generally C++ doesn't have good built-in support for unicode, so you'll be better off in the long run looking into something like ICU.
#include <stdio.h>
#include <wchar.h>
int main()
{
wchar_t name[256];
wprintf(L"Type a name: ");
wscanf(L"%s", name);
wprintf(L"Typed name is: %s\n", name);
return 0;
}
I'm using TinyXML to parse/build XML files. Now, according to the documentation this library supports multibyte character sets through UTF-8. So far so good I think. But, the only API that the library provides (for getting/setting element names, attribute names and values, ... everything where a string is used) is through std::string or const char*. This has me doubting my own understanding of multibyte character set support. How can a string that only supports 8-bit characters contain a 16 bit character (unless it uses a code page, which would negate the 'supports Unicode' claim)? I understand that you could theoretically take a 16-bit code point and split it over 2 chars in a std::string, but that wouldn't transform the std::string to a 'Unicode' string, it would make it invalid for most purposes and would maybe accidentally work when written to a file and read in by another program.
So, can somebody explain to me how a library can offer an '8-bit interface' (std::string or const char*) and still support 'Unicode' strings?
(I probably mixed up some Unicode terminology here; sorry about any confusion coming from that).
First, utf-8 is stored in const char * strings, as #quinmars said. And it's not only a superset of 7-bit ASCII (code points <= 127 always encoded in a single byte as themselves), it's furthermore careful that bytes with those values are never used as part of the encoding of the multibyte values for code points >= 128. So if you see a byte == 44, it's a '<' character, etc. All of the metachars in XML are in 7-bit ASCII. So one can just parse the XML, breaking strings where the metachars say to, sticking the fragments (possibly including non-ASCII chars) into a char * or std::string, and the returned fragments remain valid UTF-8 strings even though the parser didn't specifically know UTF-8.
Further (not specific to XML, but rather clever), even more complex things genrally just work (tm). For example, if you sort UTF-8 lexicographically by bytes, you get the same answer as sorting it lexicographically by code points, despite the variation in # of bytes used, because the prefix bytes introducing the longer (and hence higher-valued) code points are numerically greater than those for lesser values).
UTF-8 is compatible to 7-bit ASCII code. If the value of a byte is larger then 127, it means a multibyte character starts. Depending on the value of the first byte you can see how many bytes the character will take, that can be 2-4 bytes including the first byte (technical also 5 or 6 are possible, but they are not valid utf-8). Here is a good resource about UTF-8: UTF-8 and Unicode FAQ, also the wiki page for utf8 is very informative. Since UTF-8 is char based and 0-terminated, you can use the standard string functions for most things. The only important thing is that the character count can differ from the byte count. Functions like strlen() return the byte count but not necessarily the character count.
By using between 1 and 4 chars to encode one Unicode code point.