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One-byte bool. Why?
I want to add a boolean variable to a class. However, this class is pretty size-sensitive, and as a result I'm loath to add another field. However, it is composed of a pile of members that are at least a char wide, and a single other bool.
If I were hand-writing this code, I would implement those boolean fields as bits in the last byte or so of the object. Since accesses have to be byte-aligned, this would cause no spacial overhead.
Now, do compilers typically do this trick? The only reason I can of for them not to is because it would involve an additional mask to get that bit out of there.
No, compilers can't do this trick because the address of each member has to be distinct. If you want to pack a fixed number of bits, use std::bitset. If you need a variable number of bits use boost::dynamic_bitset.