c++ - What exactly can wchar_t represent?

Question

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?

Answer 1

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.