Re: How to define an array of a variant record type?

[Craig Carey <research@ijs.co.nz>]

On 17 Nov 2003 12:35:34 -0500, Stephen Leake wrote:
...
>change this to
>
>type Name_Value(Value_Type : Name_Value_Type := String_Type) is
>
>
>This seems very odd, but it works. It says to the compiler "let the
>discriminant change at runtime". Without the default value for the
>discriminant, the discriminant is _not_ allowed to change at runtime;
>it must be known at compile time.


http://www.adaic.org/standards/95aarm/html/AA-3-7.html

AARM 3.7 Discriminants
...
NOTES
28  50  If a discriminated type has default_expressions for its
   discriminants, then unconstrained variables of the type are
   permitted, and the values of the discriminants can be changed by an
   assignment to such a variable. If defaults are not provided for the
   discriminants, then all variables of the type are constrained,
   either by explicit constraint or by their initial value; the values
   of the discriminants of such a variable cannot be changed after
   initialization. 

28.a  Discussion: This connection between discriminant defaults and
   unconstrained variables can be a source of confusion. For Ada 95,
   we considered various ways to break the connection between defaults
   and unconstrainedness, but ultimately gave up for lack of a
   sufficiently simple and intuitive alternative.
...


Presumably pesons could use Ada for 10 years without figuring that out,
except for slow Leake of information at comp.lang.ada.

----

Also there is the pragma Volatile trick (that won't run in GNAT 3.15p
due to the caching not going away): the discriminant is altered by
using X'Address or similar, and pragma Volatile suppresses the caching.