保证在char，wchar_t，char16_t，char32_t上的大小顺序 [英] Guarantee on size ordering on char, wchar_t, char16_t, char32_t

问题描述

C ++标准是否对以char，wchar_t，char16_t，char32_t为单位的字节大小排序提供任何保证? (欢迎从标准中提取任何内容)

Does the C++ standard provide any guarantee on the ordering of the size in bytes of char, wchar_t, char16_t, char32_t? (any extract from the standard is welcome)

例如，我是否保证:

sizeof(char) <= sizeof(wchar_t) <= sizeof(char16_t) <= sizeof(char32_t)

推荐答案

它是1 == sizeof(char) <= sizeof(wchar_t)和1 == sizeof(char) <= sizeof(char16_t) <= sizeof(char32_t).

5.3.3/1 Sizeof [expr.sizeof]

5.3.3/1 Sizeof [expr.sizeof]

... sizeof(char)，sizeof(signed char)和sizeof(unsigned char)为1. ... [注意:尤其是sizeof(bool)，sizeof(char16_t)， 75-实现中定义了sizeof(char32_t)和sizeof(wchar_t). 结束语].

... sizeof(char), sizeof(signed char) and sizeof(unsigned char) are 1. ... [ Note: in particular, sizeof(bool), sizeof(char16_t), sizeof(char32_t), and sizeof(wchar_t) are implementation-defined.75 — end note ].

3.9.1/5基本类型[basic.fundamental]

3.9.1/5 Fundamental types [basic.fundamental]

...类型wchar_t必须具有相同的大小，符号和对齐方式 要求(3.11)作为其他整数类型之一，称为 基础类型.类型char16_t和char32_t表示不同的类型 具有与uint_least16_t相同的大小，符号和对齐方式，并且 < cstdint>中的uint_least32_t分别称为基础 类型.

... Type wchar_t shall have the same size, signedness, and alignment requirements (3.11) 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.

更新:我没有在标准中找到它. cppreference 表示uint_leastN_t:

Update: I haven't found it in the standard. cppreference says for uint_leastN_t:

最小无符号整数类型，宽度至少为8、16、32和64 位

smallest unsigned integer type with width of at least 8, 16, 32 and 64 bits respectively

请注意，sizeof(char)== 1并不表示 表示char有8位.另请参见 C ++常见问题解答. cppreference 谈到CHAR_BIT:

Note that sizeof(char)==1 does not mean that a char has 8 bits. See also C++ FAQ. cppreference says about CHAR_BIT:

字节数

1.7/1 C ++内存模型[intro.memory] ​​

1.7/1 The C ++ memory model [intro.memory]

C ++内存模型中的基本存储单元是字节.一种 字节至少要大到足以包含基本的任何成员 执行字符集(2.3)...

The fundamental storage unit in the C ++ memory model is the byte. A byte is at least large enough to contain any member of the basic execution character set (2.3) ...

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