C++ : why bool is 8 bits long?


Question

In C++, I'm wondering why the bool type is 8 bits long (on my system), where only one bit is enough to hold the boolean value ?

I used to believe it was for performance reasons, but then on a 32 bits or 64 bits machine, where registers are 32 or 64 bits wide, what's the performance advantage ?

Or is it just one of these 'historical' reasons ?

1
125
11/16/2015 8:17:57 AM

Accepted Answer

Because every C++ data type must be addressable.

How would you create a pointer to a single bit? You can't. But you can create a pointer to a byte. So a boolean in C++ is typically byte-sized. (It may be larger as well. That's up to the implementation. The main thing is that it must be addressable, so no C++ datatype can be smaller than a byte)

214
1/14/2010 2:06:32 PM

Memory is byte addressable. You cannot address a single bit, without shifting or masking the byte read from memory. I would imagine this is a very large reason.


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