c++ - Why the sizeof(bool) is not defined to be one, by the Standard itself?

Question

Size of char, signed char and unsigned char is defined to be 1 byte, by the C++ Standard itself. I'm wondering why it didn't define the sizeof(bool) also?

C++03 Standard $5.3.3/1 says,

sizeof(char), sizeof(signed char) and sizeof(unsigned char) are 1; the result of sizeof applied to any other fundamental type (3.9.1) is implementation-defined. [Note: in particular,sizeof(bool) and sizeof(wchar_t) are implementation-defined.69)

I understand the rationale that sizeof(bool) cannot be less than one byte. But is there any rationale why it should be greater than 1 byte either? I'm not saying that implementations define it to be greater than 1, but the Standard left it to be defined by implementation as if it may be greater than 1.

If there is no reason sizeof(bool) to be greater than 1, then I don't understand why the Standard didn't define it as just 1 byte, as it has defined sizeof(char), and it's all variants.

Answer 1

The other likely size for it is that of int, being the "efficient" integer type for the platform.

On architectures where it makes any difference whether the implementation chooses 1 or sizeof(int) there could be a trade-off between size (but if you're happy to waste 7 bits per bool, why shouldn't you be happy to waste 31? Use bitfields when size matters) vs. performance (but when is storing and loading bool values going to be a genuine performance issue? Use int explicitly when speed matters). So implementation flexibility wins - if for some reason 1 would be atrocious in terms of performance or code size, it can avoid it.