Re: Language lawyer question: Equality on 'Access attributes

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

Adam Beneschan wrote:
> Given this code:
> 
> package Pak1 is
>     type Ptr1 is access all Integer;
>     X : aliased Integer;
>     Y : aliased Integer;
>     B : Boolean := X'Access = Y'Access;
> end Pak1;
> 
> GNAT gives me this error: "two access attributes cannot be compared
> directly / they must be converted to an explicit type for comparison".

GNAT is right.  This is a new instance of an issue that has been around 
since Ada 83.  In this case, the "general" access type returned by 
'Access may have no relation to type Ptr1.  If you want to imagine that 
the implementation has other access all Integer types around, fine.  But 
technically, the Access attribute returns a type for which no comparison 
operations are defined.  You can convert this type implicitly or 
explicitly to Ptr1, and the comparison will be okay:

   B : Boolean := Ptr1(X'Access) = Ptr1(Y'Access);

or:

   Temp: Ptr1 := X'Access;
   B: Boolean := Temp = Y'Access;

I haven't tested this code, it is very late at night.  But GNAT is being 
helpful in their error message.

Why not "fix" the language so this problem doesn't occur?  It works 
correctly now, thank you very much!  You may think you "know" what the 
values in this case look like, but in the general case, the compiler 
needs to know what kind of pointer to generate for the 'Access 
attribute.  Go find any discussion of "fat pointers" vs. thin pointers, 
and for Ada a discussion of how some compilers will "pack" some access 
types to require less than the space for a full address.

In this case, the two 'Access attributes are probably going to be of the 
same type.  But if you are really using such a comparison in a real 
program, you don't want the Boolean value to depend on which version of 
which compiler you use.  So you have to tell the compiler which type, 
and implicitly which "=" operation to use for the comparison.

-- 
                                           Robert I. Eachus

"The war on terror is a different kind of war, waged capture by capture, 
cell by cell, and victory by victory. Our security is assured by our 
perseverance and by our sure belief in the success of liberty." -- 
George W. Bush