pitfalls of enum

[Ali Çehreli]

On 11/02/2013 02:14 PM, JR wrote:

 > But in Andrei's thread on tristates[2] he lists this code excerpt:
 >> struct Tristate
 >> {
 >>     private static Tristate make(ubyte b)
 >>     {
 >>         Tristate r = void;
 >>         r.value = b;
 >>         return r;
 >>     }
 >>
 >>     enum no = make(0), yes = make(1), unknown = make(4);  // <---
 >
 > Is it okay to use enum there? If so, is it because (whatwith being a
 > struct) it's not on the heap?

I think it is more like because its value is known at compile time. If 
Tristate were a class, then the class variable r would not have any 
class object that it pointed to.

 > What if it were a class?

I tried changing struct to class:

class Tristate
{
     ubyte value;

     private static Tristate make(ubyte b)
     {
         Tristate r = new Tristate();
         r.value = b;
         return r;
     }

     enum no = make(0), yes = make(1), unknown = make(4);
}

Error: variable deneme.Tristate.no : Unable to initialize enum with 
class or pointer to struct. Use static const variable instead.

So, class can't work.

 > A string?

That would be ok. A compile-time string is known to the compiler and can 
appear in the program binary to be referenced from multiple places.

 > A
 > string instantiated with the value of another string, which would
 > normally simply create an alias?

Would work.

 > When is an enum *better* than a normal (static const/immutable) constant?

Good question. :)

Ali