Question

How to find the range of enums ?
I have got two versions which state it differently when the lowest value is negative.Stroustrup says that for

 enum e3 { min = -10 , max = 1000000 }; // range -1048576:1048575

but
C++ Primer Plus 5th edition says

for eg if the smallest enumerator is -6, the next power of two [times a minus sign] is -8, and thus the lower limit is -7

I am confused which is correct ?

Answer 1

I believe both are correct (see below for Primer’s definition, though), depending on how compatible you want to are. The formal definition is

For an enumeration where e min is the smallest enumerator and e max is the largest, the values of the enumeration are the values of the underlying type in the range b min to b max , where b min and b max are, respectively, the smallest and largest values of the smallest bit-field that can store e min and e max .

For negative numbers, the question is what representation we use. The footnote to it says

On a two’s-complement machine, b
max is the smallest value greater than or equal to max (abs(e min ) − 1 ,abs(e max ) ) of the form
2 M − 1; b is zero if e is non-negative and − (b + 1 ) otherwise.

If you assume sign magnitude or one’s complement then the example enumeration’s range is -1048575:1048575. For two’s complement you get one more in the negative range. Primer’s definiton lacks the maximum enumerator value, so I’m not sure how it comes to lower limit -7. If you want to be maximum compatible with other implementations, I would go with -1048575:1048575.