Re: Constant array declarations are not causing a compilation error when not fully initialized.

[Tucker Taft <stt@avercom.net>]

Mark Doherty wrote:
> 
> Why does the Ada95 LRM not gaurantee a compilation error for the
> following, albeit does produce a CONTRAINT_ERROR.

This is similar to: "X : Positive := -1;"
This is legal at compile-time, but will result in
a constraint error at run-time.  Most compilers
are "friendly" enough to report cases where code
will unevitably raise a Constraint_Error, but by
the rules of the language, they are *not* allowed
to reject such code as being illegal.  

The reason the rules don't require such rejection is
that it is generally difficult to specify exactly
how sophisticated such inevitable-run-time-error detection
must be.  In fact, Ada 95 goes further than Ada 83 in
terms of specifying cases where inevitable-run-time-errors
must be detected, but it clearly can't go all the way,
as that would require the solution to the "halting" problem ;-).


> 
> procedure Test is
> 
>   type Enum is (A, B, C);
> 
>   -- the following don't produce compilation errors (just warning on
> different
>   -- versions of the compiler
>   Lookup_1 : constant array (Enum) of Integer := (A => 1);
>   Lookup_2 : constant array (Enum) of Integer := (A => 1, B => 2);
>   Lookup_3 : constant array (Enum) of Integer := (B => 1, C => 3);
>   Lookup_4 : constant array (Enum) of Integer := (10, 12);
> 
>   -- the following does produce compilation error (as expected)
>   Lookup_5 : constant array (Enum) of Integer := (A => 1,C => 2);
> 
> begin
>     null;
> end Test;
> 
> p.s. Different versions of the Rational compiler (but not all) issue a
> warning that ... CONSTRAINT_ERROR will be raised...

-- 
-Tucker Taft   stt@avercom.net   http://www.avercom.net
Chief Technology Officer, AverCom Corporation (A Titan Company) 
Bedford, MA  USA (AverCom was formerly the Commercial Division of AverStar:
http://www.averstar.com/~stt)