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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: private types
Date: 18 Mar 2006 07:47:09 -0500
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2006-03-18T07:47:09-05:00
List-Id: <comp.lang.ada>

"Randy Brukardt" <randy@rrsoftware.com> writes:

> It's not just testing. Ada 95 is very clear that an Ada compiler cannot
> assume an object is in range unless it can prove it is initialized.

This is not quite true.  What the compiler can prove depends on the
compiler's code generation strategy.  Example:

    type Index is range 1..10;
    type A is array(Index) of Character;

    X: Index; -- not initialized here

    procedure P(Y: Index) is
    begin
        ...
    end P;

    ... -- (*) might initialize X here

    P(X);

Suppose the compiler cannot prove that the code marked "-- (*)"
will initialize X.  The compiler has a choice:  It can do
a range check at the call to P, and then assume inside the body
of P that Y is in range (even though the value _might_ have
come from an uninitialized variable).  Or, the compiler can
avoid the range check on the call to P, in which case it cannot
assume that Y is in range.

> So I recommend initalizing everything (or assigning it immediately after the
> begin) that could be significant to performance.

That also depends on the compiler.  Many compilers can prove that a
variable is initialized here:

    begin
        if ... then
            A := 3;
        else
            A := 4;
        end if;
        ... -- Here, we can presume A is in range.

Adding "A := 0;" between "begin" and "if" would be overkill
for such compilers.

- Bob