Are C++ enums signed or unsigned? And by extension is it safe to validate an input by checking that it is <= your max value, and leave out >= your min value (assuming you started at 0 and incremented by 1)?
You shouldn't rely on any specific representation. Read the following link. Also, the standard says that it is implementation-defined which integral type is used as the underlying type for an enum, except that it shall not be larger than int, unless some value cannot fit into int or an unsigned int.
In short: you cannot rely on an enum being either signed or unsigned.