Re: Discriminated types with default discriminants

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"Jeffrey R. Carter" <spam@spam.com> writes:

> There are 2 ways the compiler may implement this. Many compilers,
> including GNAT, allocate enough storage for the largest value, and then
> use parts of this storage depending on the current value of the
> discriminant. If the largest value is too big, such a declaration
> results in Storage_Error being raised.
> 
> A few compilers will add a level of indirection, and store the
> variable-sized stuff elsewhere, reallocating storage when the amount
> available is not big enough.

It is quite difficult to get the indirect approach to work right.
Think about renaming a subcomponent of the thing, or taking
'Access of a subcomponent.  Then the thing moves elsewhere in
the heap when it grows.  Bad news!  I've seen compiler bugs
in this area.

> As with most implementation decisions, it's a trade off among
> complexity, space, and time.

Indeed.

- Bob