Are boolean variables typically implemented as single bits?

Question

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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.

Use `std::bitset` if you care about size. – Pubby Jan 30 '13 at 23:46 — Pubby, Jan 30 '13 at 23:46

score 3 · Accepted Answer · answered Jan 30 '13 at 23:48

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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.

answered Jan 30 '13 at 23:48

Mark B

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score 2 · Answer 2 · answered Jan 30 '13 at 23:43

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No, I don't know of any compilers which optimize a bool down to a bit.

You can force this behavior via:

unsigned int m_firstBit : 1;
unsigned int m_secondBit : 1;
unsigned int m_thirdBit : 1;

As for reasons why not, it would likely violate some language guarantees. For instance, you couldn't pass &myBool to a function which takes a bool* if it doesn't have its own reserved byte.

answered Jan 30 '13 at 23:43

StilesCrisis

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Of course, `: 1` is only going to work in bitfields. – Pubby Jan 30 '13 at 23:46
Well, he said it was already in a class, so he ought to be able to use the bitfield trick. – StilesCrisis Jan 30 '13 at 23:48

score 0 · Answer 3 · edited Jan 30 '13 at 23:49

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Compilers typically do not do that, but you could use std::bitset<2> to pack two bools into one byte.

edited Jan 30 '13 at 23:49

StilesCrisis

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answered Jan 30 '13 at 23:47

David

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Are boolean variables typically implemented as single bits?

3 Answers3