Why is the Objective-C Boolean data type defined as a signed char?

Something that has piqued my interest is Objective-C's BOOL type definition.

Why is it defined as a signed char (which could cause unexpected behaviour if a value greater than 1 byte in length is assigned to it) rather than as an int, as C does (much less margin for error: a zero value is false, a non-zero value is true)?

The only reason I can think of is the Objective-开发者_开发问答C designers micro-optimising storage because the char will use less memory than the int. Please can someone enlighten me?

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Remember that Objective-C was created back in the 1980's, when saving bytes really mattered.

As mentioned in a comment, as long as you stick with the values YES and NO, everything will be fine.