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From: Mark A Biggar <mark.a.biggar@lmco.com>
Subject: Re: Enumeration Types (was: Urgent question: malloc and ada)
Date: 1998/04/13
Message-ID: <3532A14E.F0259CDA@lmco.com>#1/1
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Date: 1998-04-13T00:00:00+00:00
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Matthew Heaney wrote:
> In practice, using an integer type instead of an enumeration is only
> marginally less safe.  Consider this:
> 
> type ET is (Red, Green, Blue);
> for ET use (Red => 1, Green => 3, Blue => 7);
> for ET'Size use 8;
> 
> procedure Op (O : ET);
> 
> Now consider this:
> 
> type IT is new Interfaces.Unsigned_8;
> 
> Red : constant IT := 1;
> Green : constant IT := 3;
> Blue : constant IT := 7;
> 
> procedure Op (O : IT);
> 
> From a client's point of view, there is no difference between these types
> (though a case statement now requires an others branch).  You call
> procedure Op the same way in both cases.
> 
> You can make the argument, "But with the integer version, the client can
> pass the value 2."  But this can only happen because of deliberate misuse,
> ie
> 
> Op (2);
> 
> Here, he's being naughty, and using an integer literal.  What he should do is
> 
> Op (Red);
> 
> just like he'd do using the enumeration literal.
> 
> And no, you can't even accidently mix this type up with other integer
> types, because IT is its own type.  You'd have to deliberately convert one
> integer type into the other.

Actually you can do even better as follows:

package SomePackage is
 
 type IT is private;
 
 Red : constant IT;
 Green : constant IT;
 Blue : constant IT;
 
 procedure Op (O : IT);

private
 
 type IT is new Interfaces.Unsigned_8;
 
 Red : constant IT := 1;
 Green : constant IT := 3;
 Blue : constant IT := 7;

end SomePackage;

This eliminates the Op(2) problem while keeping all the properties of your
example.

--
Mark Biggar
mark.a.biggar@lmco.com