How do I properly use std::string on UTF-8 in C++?

My platform is a Mac. I'm a C++ beginner and working on a personal project which processes Chinese and English. UTF-8 is the preferred encoding for this project.

I read some posts on Stack Overflow, and many of them suggest using std::string when dealing with UTF-8 and avoid wchar_t as there's no char8_t right now for UTF-8.

However, none of them talk about how to properly deal with functions like str[i], std::string::size(), std::string::find_first_of() or std::regex as these function usually returns unexpected results when facing UTF-8.

Should I go ahead with std::string or switch to std::wstring? If I should stay with std::string, what's the best practice for one to handle the above problems?

Solution 1:

Unicode Glossary

Unicode is a vast and complex topic. I do not wish to wade too deep there, however a quick glossary is necessary:

1. Code Points: Code Points are the basic building blocks of Unicode, a code point is just an integer mapped to a meaning. The integer portion fits into 32 bits (well, 24 bits really), and the meaning can be a letter, a diacritic, a white space, a sign, a smiley, half a flag, ... and it can even be "the next portion reads right to left".
2. Grapheme Clusters: Grapheme Clusters are groups of semantically related Code Points, for example a flag in unicode is represented by associating two Code Points; each of those two, in isolation, has no meaning, but associated together in a Grapheme Cluster they represent a flag. Grapheme Clusters are also used to pair a letter with a diacritic in some scripts.

This is the basic of Unicode. The distinction between Code Point and Grapheme Cluster can be mostly glossed over because for most modern languages each "character" is mapped to a single Code Point (there are dedicated accented forms for commonly used letter+diacritic combinations). Still, if you venture in smileys, flags, etc... then you may have to pay attention to the distinction.

UTF Primer

Then, a serie of Unicode Code Points has to be encoded; the common encodings are UTF-8, UTF-16 and UTF-32, the latter two existing in both Little-Endian and Big-Endian forms, for a total of 5 common encodings.

In UTF-X, X is the size in bits of the Code Unit, each Code Point is represented as one or several Code Units, depending on its magnitude:

• UTF-8: 1 to 4 Code Units,
• UTF-16: 1 or 2 Code Units,
• UTF-32: 1 Code Unit.

std::string and std::wstring.

1. Do not use std::wstring if you care about portability (wchar_t is only 16 bits on Windows); use std::u32string instead (aka std::basic_string<char32_t>).
2. The in-memory representation (std::string or std::wstring) is independent of the on-disk representation (UTF-8, UTF-16 or UTF-32), so prepare yourself for having to convert at the boundary (reading and writing).
3. While a 32-bits wchar_t ensures that a Code Unit represents a full Code Point, it still does not represent a complete Grapheme Cluster.

If you are only reading or composing strings, you should have no to little issues with std::string or std::wstring.

Troubles start when you start slicing and dicing, then you have to pay attention to (1) Code Point boundaries (in UTF-8 or UTF-16) and (2) Grapheme Clusters boundaries. The former can be handled easily enough on your own, the latter requires using a Unicode aware library.

Picking std::string or std::u32string?

If performance is a concern, it is likely that std::string will perform better due to its smaller memory footprint; though heavy use of Chinese may change the deal. As always, profile.

If Grapheme Clusters are not a problem, then std::u32string has the advantage of simplifying things: 1 Code Unit -> 1 Code Point means that you cannot accidentally split Code Points, and all the functions of std::basic_string work out of the box.

If you interface with software taking std::string or char*/char const*, then stick to std::string to avoid back-and-forth conversions. It'll be a pain otherwise.

UTF-8 in std::string.

UTF-8 actually works quite well in std::string.

Most operations work out of the box because the UTF-8 encoding is self-synchronizing and backward compatible with ASCII.

Due the way Code Points are encoded, looking for a Code Point cannot accidentally match the middle of another Code Point:

• str.find('\n') works,
• str.find("...") works for matching byte by byte¹,
• str.find_first_of("\r\n") works if searching for ASCII characters.

Similarly, regex should mostly works out of the box. As a sequence of characters ("haha") is just a sequence of bytes ("哈"), basic search patterns should work out of the box.

Be wary, however, of character classes (such as [:alphanum:]), as depending on the regex flavor and implementation it may or may not match Unicode characters.

Similarly, be wary of applying repeaters to non-ASCII "characters", "哈?" may only consider the last byte to be optional; use parentheses to clearly delineate the repeated sequence of bytes in such cases: "(哈)?".

¹The key concepts to look-up are normalization and collation; this affects all comparison operations. std::string will always compare (and thus sort) byte by byte, without regard for comparison rules specific to a language or a usage. If you need to handle full normalization/collation, you need a complete Unicode library, such as ICU.

Solution 2:

std::string and friends are encoding-agnostic. The only difference between std::wstring and std::string are that std::wstring uses wchar_t as the individual element, not char. For most compilers the latter is 8-bit. The former is supposed to be large enough to hold any unicode character, but in practice on some systems it isn't (Microsoft's compiler, for example, uses a 16-bit type). You can't store UTF-8 in std::wstring; that's not what it's designed for. It's designed to be an equivalent of UTF-32 - a string where each element is a single Unicode codepoint.

If you want to index UTF-8 strings by Unicode codepoint or composed unicode glyph (or some other thing), count the length of a UTF-8 string in Unicode codepoints or some other unicode object, or find by Unicode codepoint, you're going to need to use something other than the standard library. ICU is one of the libraries in the field; there may be others.

Something that's probably worth noting is that if you're searching for ASCII characters, you can mostly treat a UTF-8 bytestream as if it were byte-by-byte. Each ASCII character encodes the same in UTF-8 as it does in ASCII, and every multi-byte unit in UTF-8 is guaranteed not to include any bytes in the ASCII range.

Solution 3:

Consider upgrading to C++20 and std::u8string that is the best thing we have as of 2019 for holding UTF-8. There are no standard library facilities to access individual code points or grapheme clusters but at least your type is strong enough to at least say it is true UTF-8.

Solution 4:

Both std::string and std::wstring must use UTF encoding to represent Unicode. On macOS specifically, std::string is UTF-8 (8-bit code units), and std::wstring is UTF-32 (32-bit code units); note that the size of wchar_t is platform-dependent.

For both, size tracks the number of code units instead of the number of code points, or grapheme clusters. (A code point is one named Unicode entity, one or more of which form a grapheme cluster. Grapheme clusters are the visible characters that users interact with, like letters or emojis.)

Although I'm not familiar with the Unicode representation of Chinese, it's very possible that when you use UTF-32, the number of code units is often very close to the number of grapheme clusters. Obviously, however, this comes at the cost of using up to 4x more memory.

The most accurate solution would be to use a Unicode library, such as ICU, to calculate the Unicode properties that you are after.

Finally, UTF strings in human languages that don't use combining characters usually do pretty well with find/regex. I'm not sure about Chinese, but English is one of them.