Re: Renaming of enumeration constant

["Robert I. Eachus" <rieachus@comcast.net>]

Xenos wrote:

> Just a curiosity question:
> 
> What is the difference (advantages, etc.) of defining a function rename for
> an enumeration constant over just creating a constant.  Consider:
> 
> package X is
>   type E is (A, B, C);
> end X;
> 
> with X;
> package Y is
>    function A return X.E renames X.A;
> end Y;
> 
> with X;
> package Z is
>   A : constant X.E := X.A;
> end Z;
> 
> Is the "A" in Y treated any different than the "A" in Z by the compiler, or
> will both achieve the same results.  Is one considered "better" (whatever
> that means) than the other?

The difference is that a function will overload other functions and 
procedures of the same name, while a constant will hide such names in 
the local scope, and prevent them from being use visible.

So if you have two packages that declare function A, and use clauses for 
both packages you can call either, unless the call is ambiguous.  If you 
have two packages that declare constants A, and use clauses for both, 
you will still have to use a qualified name to reference A.

A different advantage to using functions instead of constants is that 
you can do the renaming in the body of the package:

package Foo is
   function A return Bar;
...
package body Foo is
   function A renames...

I'll let you read about the joys and issues of renaming as body. But if 
the value may change during development, this is the form you want.

The advantage of the constant form is when you want it to be static.

For example:

     Pi: constant := 3.14159_26...;

(If the value of Pi changes the need to modify your program is a minor 
detail.)
-- 

                                           Robert I. Eachus

The ideology he opposed throughout his political life insisted that 
history was moved by impersonal tides and unalterable fates. Ronald 
Reagan believed instead in the courage and triumph of free men and we 
believe it all the more because we saw that courage in him.  -- George 
W. Bush June 11, 2004