TITLE: Definition for valid enum ranges

PROBLEM: mcr@holly.demon.co.uk (Mark Rogers)

How about a new syntax 

        enum small_int { MaxInt = -128, ... , MaxInt = 128 };
where ... says that all values between -128 and 128 are valid small_int's, but 
don't have names?


RESPONSE: steve@taumet.com (Steve Clamage), 31 May 93

This situation has already been addressed by the C++ Committee via
a semantic (not syntax) extension of enums.

The current rule about enums says assigning a value other than an
enumerator of the same type to an enum variable has undefined results.
(Few, if any, compilers warn about this, or give unexpected results.)

The new rule states (loosely worded) that if N bits are required to
hold the range of the enum values, then all values expressible in
N bits are valid for objects of that enum type.  (The obvious things
about sign and 1's complement versus 2's complement have to be taken
into account.)

Example:
	enum Colors { red = -5, green = 0, blue = 13 } color;
	enum Tastes { sweet = 1, salty = 3, sour = 5, bitter = 7 } taste;

It takes 5 bits, including the sign bit, to hold a value of type Colors.
It takes 3 bits for Tastes, since no negative values are involved.
Under the new rule, the following statements are all legal and
well-defined:
	color = Colors(-15); // always in range
	color = Colors(blue+2);
	taste = Tastes(4);
	taste = Tastes(0);
But not necessarily:
	color = Colors(-16);	// out of range if 1's complement
	taste = Tastes(sweet-2); // out of range (no negative Tastes)
	taste = Taste(8);	// out of range (too many bits)

[ If you have ever wondered, as I have, why you cannot have a forward
  declaration for an enum (especially useful for enums within a class
  scope), here's your answer. -adc ]