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

Answer 1

As @MSalters pointed out, some platforms work more efficiently with larger data items.

Many "RISC" CPUs (e.g., MIPS, PowerPC, early versions of the Alpha) have/had a considerably more difficult time working with data smaller than one word, so they do the same. IIRC, with at least some compilers on the Alpha a bool actually occupied 64 bits.

gcc for PowerPC Macs defaulted to using 4 bytes for a bool, but had a switch to change that to one byte if you wanted to.

Even for the x86, there's some advantage to using a 32-bit data item. gcc for the x86 has (or at least used to have -- I haven't looked recently at all) a define in one of its configuration files for BOOL_TYPE_SIZE (going from memory, so I could have that name a little wrong) that you could set to 1 or 4, and then re-compile the compiler to get a bool of that size.

Edit: As for the reason behind this, I'd say it's a simple reflection of a basic philosophy of C and C++: leave as much room for the implementation to optimize/customize its behavior as reasonable. Require specific behavior only when/if there's an obvious, tangible benefit, and unlikely to be any major liability, especially if the change would make it substantially more difficult to support C++ on some particular platform (though, of course, if the platform is sufficiently obscure, it might get ignored).

Answer 2

The operation resulted in 'sizeof' is MADUs (minimal addresible unit), not bytes. So family processors C54 *. C55 * Texas Instuments, the expression 1 MADU = 2 bytes.

For this platform sizeof (bool) = sizeof (char) = 1 MADUs = 2 bytes. This does not violate the C + + standard, but clarifies the situation.

Answer 3

Many platforms cannot effectively load values smaller than 32 bits. They have to load 32 bits, and use a shift-and-mask operation to extract 8 bits. You wouldn't want this for single bools, but it's OK for strings.

Answer 4

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.