关于C ++中unsigned char类型的大小

Question

The standard C++ [3.9.1-1] says that

For unsigned character types, all possible bit patterns of the value representation represent numbers.

The standard [18.3.2.4-(comment 197)] also says that the maximum value of unsigned char are equivalent to UCHAR_MAX in C, which is defined as 255 in C standard [5.2.4.2.1]. So does this mean the size of unsigned char type in C++ is exactly 8 bits?

Answer 1

From C11 5.2.4.2.1

The values given below shall be replaced by constant expressions suitable for use in #if preprocessing directives. Moreover, except for CHAR_BIT and MB_LEN_MAX, the following shall be replaced by expressions that have the same type as would an expression that is an object of the corresponding type converted according to the integer promotions. Their implementation-defined values shall be equal or greater in magnitude (absolute value) to those shown, with the same sign.

^{(emphasis mine)}

So the standard defines that at a minimum UCHAR_MAX needs to be 255 but it can be greater than that.

The guarantees that we have on size are:

sizeof(char) = 1 and sizeof(char) <= sizeof(short) <= sizeof(int) <= sizeof(long) <= sizeof(long long)

And at a minimum the signed versions of the data types must be able to hold:

• char [-127, 127]
• short [-32767, 32767]
• int [-32767, 32767]
• long [-2147483647, 2147483647]
• long long [-9223372036854775807, 9223372036854775807]