C ++中的字符串和字符编码

Question

I read a few posts about best practices for strings and character encoding in C++, but I am struggling a bit with finding a general purpose approach that seems to me reasonably simple and correct. Could I ask for comments on the following? I'm inclined to use UTF-8 and UTF-32, and to define something like:

typedef std::string string8;
typedef std::basic_string<uint32_t> string32;

The string8 class would be used for UTF-8, and having a separate type is just a reminder of the encoding. An alternative would be for string8 to be a subclass of std::string and to remove the methods that aren't quite right for UTF-8.

The string32 class would be used for UTF-32 when a fixed character size is desired.

The UTF-8 CPP functions, utf8::utf8to32() and utf8::utf32to8(), or even simpler wrapper functions, would be used to convert between the two.

Answer 1

If you plan on just passing strings around and never inspect them, you can use plain std::string though it's a poor man job.

The issue is that most frameworks, even the standard, have stupidly (I think) enforced encoding in memory. I say stupid because encoding should only matter on the interface, and those encoding are not adapted for in-memory manipulation of the data.

Furthermore, encoding is easy (it's a simple transposition CodePoint -> bytes and reversely) while the main difficulty is actually about manipulating the data.

With a 8-bits or 16-bits you run the risk of cutting a character in the middle because neither std::string nor std::wstring are aware of what a Unicode Character is. Worse, even with a 32-bits encoding, there is the risk of separating a character from the diacritics that apply to it, which is also stupid.

The support of Unicode in C++ is therefore extremely subpar, as far as the standard is concerned.

If you really wish to manipulate Unicode string, you need a Unicode aware container. The usual way is to use the ICU library, though its interface is really C-ish. However you'll get everything you need to actually work in Unicode with multiple languages.

Answer 2

The traits approach described here might be helpful. It's an old but useful technique.

Answer 3

It's not specified what character encoding must be used for string, wstring etc. The common way is to use unicode in wide strings. What types and encodings should be used depends on your requirements.

If you only need to pass data from A to B, choose std::string with UTF-8 encoding (don't introduce a new type, just use std::string). If you must work with strings (extract, concat, sort, ...) choose std::wstring and as encoding UCS2/UTF-16 (BMP only) on Windows and UCS4/UTF-32 on Linux. The benefit is the fixed size: each character has a size of 2 (or 4 for UCS4) bytes while std::string with UTF-8 returns wrong length() results.

For conversion, you can check sizeof(std::wstring::value_type) == 2 or 4 to choose UCS2 or UCS4. I'm using the ICU library, but there may be simple wrapper libs.

Deriving from std::string is not recommended because basic_string is not designed for (lacks of virtual members etc..). If you really really really need your own type like std::basic_string< my_char_type > write a custom specialization for this.

The new C++0x standard defines wstring_convert<> and wbuffer_convert<> to convert with a std::codecvt from a narrow charset to a wide charset (for example UTF-8 to UCS2). Visual Studio 2010 has already implemented this, afaik.