c++ - char vs wchar_t vs char16_t vs char32_t (c++11)

Question

From what I understand, a char is safe to house ASCII characters whereas char16_t and char32_t are safe to house characters from unicode, one for the 16-bit variety and another for the 32-bit variety (Should I have said "a" instead of "the"?). But I'm then left wondering what the purpose behind the wchar_t is. Should I ever use that type in new code, or is it simply there to support old code? What was the purpose of wchar_t in old code if, from what I understand, its size had no guarantee to be bigger than a char? Clarification would be nice!

Answer 1

char is for 8-bit code units, char16_t is for 16-bit code units, and char32_t is for 32-bit code units. Any of these can be used for 'Unicode'; UTF-8 uses 8-bit code units, UTF-16 uses 16-bit code units, and UTF-32 uses 32-bit code units.

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The guarantee made for wchar_t was that any character supported in a locale could be converted from char to wchar_t, and whatever representation was used for char, be it multiple bytes, shift codes, what have you, the wchar_t would be a single, distinct value. The purpose of this was that then you could manipulate wchar_t strings just like the simple algorithms used with ASCII.

For example, converting ascii to upper case goes like:

auto loc = std::locale("");

char s[] = "hello";
for (char &c : s) {
  c = toupper(c, loc);
}

But this won't handle converting all characters in UTF-8 to uppercase, or all characters in some other encoding like Shift-JIS. People wanted to be able to internationalize this code like so:

auto loc = std::locale("");

wchar_t s[] = L"hello";
for (wchar_t &c : s) {
  c = toupper(c, loc);
}

So every wchar_t is a 'character' and if it has an uppercase version then it can be directly converted. Unfortunately this doesn't really work all the time; For example there exist oddities in some languages such as the German letter ? where the uppercase version is actually the two characters SS instead of a single character.

So internationalized text handling is intrinsically harder than ASCII and cannot really be simplified in the way the designers of wchar_t intended. As such wchar_t and wide characters in general provide little value.

The only reason to use them is that they've been baked into some APIs and platforms. However, I prefer to stick to UTF-8 in my own code even when developing on such platforms, and to just convert at the API boundaries to whatever encoding is required.