Rule 4-5-1 prohibits sizeof(bool_type_variable)?

[dg1980]

I think this is because sizeof(bool) is implementation-defined according to ISO C++03 chapter 5.3.3.
If you want to code a portable serialization function, you have to store it in an integer with a guaranteed fixed size across all platforms (e.g. using uint8_t from stdint.h or your own type header)