C++11 中字符串文字的 Unicode 编码

问题描述

按照相关问题，我想询问 C++11 中新的字符和字符串文字类型.看起来我们现在有四种字符和五种字符串文字.字符类型:

Following a related question, I'd like to ask about the new character and string literal types in C++11. It seems that we now have four sorts of characters and five sorts of string literals. The character types:

char     a =  'x30';         // character, no semantics
wchar_t  b = L'xFFEF';       // wide character, no semantics
char16_t c = u'u00F6';       // 16-bit, assumed UTF16?
char32_t d = U'U0010FFFF';   // 32-bit, assumed UCS-4

和字符串文字:

char     A[] =  "Hellox0A";         // byte string, "narrow encoding"
wchar_t  B[] = L"HellxF6x0A";      // wide string, impl-def'd encoding
char16_t C[] = u"Hellu00F6";        // (1)
char32_t D[] = U"HellU000000F6U0010FFFF"; // (2)
auto     E[] = u8"u00F6U0010FFFF"; // (3)

问题是:x/u/U 字符引用是否可以与所有字符串类型自由组合?是否所有字符串类型都是固定宽度的，即数组包含与文本中出现的元素一样多的元素，或者 x/u/U 引用被扩展为可变字节数?u"" 和 u8"" 字符串是否具有编码语义，例如我可以说 char16_t x[] = u"U0010FFFF"，并且非 BMP 代码点被编码为两个单元的 UTF16 序列吗?和 u8 类似?在 (1) 中，我可以用 u 编写单独的代理吗?最后，是否有任何字符串函数编码感知(即它们感知字符并且可以检测无效字节序列)?

The question is this: Are the x/u/U character references freely combinable with all string types? Are all the string types fixed-width, i.e. the arrays contain precisely as many elements as appear in the literal, or to x/u/U references get expanded into a variable number of bytes? Do u"" and u8"" strings have encoding semantics, e.g. can I say char16_t x[] = u"U0010FFFF", and the non-BMP codepoint gets encoded into a two-unit UTF16 sequence? And similarly for u8? In (1), can I write lone surrogates with u? Finally, are any of the string functions encoding aware (i.e. they are character-aware and can detect invalid byte sequences)?

这是一个开放式问题，但我想尽可能完整地了解新 C++11 的新 UTF 编码和类型工具.

This is a bit of an open-ended question, but I'd like to get as complete a picture as possible of the new UTF-encoding and type facilities of the new C++11.

推荐答案

x/u/U 字符引用是否可以与所有字符串类型自由组合?

Are the x/u/U character references freely combinable with all string types?

没有.x 可以用在任何东西中，但 u 和 U 只能用在专门用 UTF 编码的字符串中.但是，对于任何 UTF 编码的字符串，u 和 U 可以根据需要使用.

No. x can be used in anything, but u and U can only be used in strings that are specifically UTF-encoded. However, for any UTF-encoded string, u and U can be used as you see fit.

所有字符串类型是否都是固定宽度的，即数组包含的元素数量与文本中出现的元素数量一样多，或者 x/u/U 引用被扩展为可变字节数?

Are all the string types fixed-width, i.e. the arrays contain precisely as many elements as appear in the literal, or to x/u/U references get expanded into a variable number of bytes?

不是你的意思.x、u 和 U 根据字符串编码进行转换.这些代码单元"(使用 Unicode 术语.char16_t 是 UTF-16 代码单元)值的数量取决于包含字符串的编码.文字 u8"u1024" 将创建一个包含 2 个 char 加上一个空终止符的字符串.文字 u"u1024" 将创建一个包含 1 个 char16_t 加上一个空终止符的字符串.

Not in the way you mean. x, u, and U are converted based on the string encoding. The number of those "code units" (using Unicode terms. A char16_t is a UTF-16 code unit) values depends on the encoding of the containing string. The literal u8"u1024" would create a string containing 2 chars plus a null terminator. The literal u"u1024" would create a string containing 1 char16_t plus a null terminator.

所使用的代码单元数量基于 Unicode 编码.

The number of code units used is based on the Unicode encoding.

u"" 和 u8"" 字符串是否具有编码语义，例如我可以说 char16_t x[] = u"U0010FFFF"，并且非 BMP 代码点被编码为两个单元的 UTF16 序列吗?

Do u"" and u8"" strings have encoding semantics, e.g. can I say char16_t x[] = u"U0010FFFF", and the non-BMP codepoint gets encoded into a two-unit UTF16 sequence?

u"" 创建一个 UTF-16 编码的字符串.u8"" 创建一个 UTF-8 编码的字符串.它们将按照 Unicode 规范进行编码.

u"" creates a UTF-16 encoded string. u8"" creates a UTF-8 encoded string. They will be encoded per the Unicode specification.

在 (1) 中，我可以用 u 编写单独的代理吗?

In (1), can I write lone surrogates with u?

绝对不是.该规范明确禁止使用 UTF-16 代理对 (0xD800-0xDFFF) 作为 u 或 U 的代码点.

Absolutely not. The specification expressly forbids using the UTF-16 surrogate pairs (0xD800-0xDFFF) as codepoints for u or U.

最后，是否有任何字符串函数可以识别编码(即它们可以识别字符并且可以检测无效的字节序列)?

Finally, are any of the string functions encoding aware (i.e. they are character-aware and can detect invalid byte sequences)?

绝对不是.好吧，让我重新表述一下.

Absolutely not. Well, allow me to rephrase that.

std::basic_string 不处理 Unicode 编码.它们当然可以存储 UTF 编码的字符串.但是他们只能将它们视为char、char16_t 或char32_t 的序列；他们不能将它们视为使用特定机制编码的 Unicode 代码点序列.basic_string::length() 将返回代码单元的数量，而不是代码点.显然，C 标准库字符串函数完全没用

std::basic_string doesn't deal with Unicode encodings. They certainly can store UTF-encoded strings. But they can only think of them as sequences of char, char16_t, or char32_t; they can't think of them as a sequence of Unicode codepoints that are encoded with a particular mechanism. basic_string::length() will return the number of code units, not code points. And obviously, the C standard library string functions are totally useless

但是应该注意的是，Unicode 字符串的长度"并不意味着代码点的数量.一些代码点正在组合字符"(一个不幸的名字)，它与之前的代码点组合.因此多个代码点可以映射到单个视觉字符.

It should be noted however that "length" for a Unicode string does not mean the number of codepoints. Some code points are combining "characters" (an unfortunate name), which combine with the previous codepoint. So multiple codepoints can map to a single visual character.

Iostreams 实际上可以读/写 Unicode 编码的值.为此，您必须使用语言环境来指定编码并将其正确地注入各个地方.这说起来容易做起来难，我身上没有任何代码可以向您展示如何操作.

Iostreams can in fact read/write Unicode-encoded values. To do so, you will have to use a locale to specify the encoding and properly imbue it into the various places. This is easier said than done, and I don't have any code on me to show you how.

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