From: Robert A Duff <bobduff@shell01.TheWorld.com>
Subject: Re: if Restrict_Func /= null confusion
Date: Sun, 30 Mar 2008 17:37:26 -0400
Date: 2008-03-30T17:37:26-04:00 [thread overview]
Message-ID: <wccabkfuc4p.fsf@shell01.TheWorld.com> (raw)
In-Reply-To: 95f36a62-b94c-4211-896a-300fa98932b7@x41g2000hsb.googlegroups.com
ma740988@gmail.com writes:
> Well folks its been at least 4 years since I've perused and/or written
> Ada source. That aside I'm perusing source code written in Ada. So
> consider:
>
> -- used to restrict a variable within a range
> -- for example: -PI to +PI
> type Restrict_Function is access function (X: Real4) return Real4;
>
> type Filt_Data is
> record
> Restrict_Func : Restrict_Function ;
>
> end record ;
>
>
> -- within a procedure we have
> Fdata : Filt_Data;
> Restrict_Func : Restrict_Function := FData.Restrict_Func ;
>
> if Restrict_Func /= null then
> -- stuff
> endif
>
> At issue: I'm not following the conditional logic 'if (Restrict_Func /
> = null)'. Not understanding the impetus behind the check for null.
Restrict_Func is a pointer to a function. To call the function,
you say something like "Restrict_Func.all(X)". You can leave
the ".all" implicit: "Restrict_Func(X)", but the ".all" still
happens. ".all" on a null pointer raises an exception.
In your example, presumably "stuff" calls the function -- so it needs to
be protected by the 'if'.
It's usually better to declare:
type Restrict_Function is not null access function ...;
^^^^^^^^
so you don't need to worry about null, which is just a tripping hazard.
Or use primitive operations of tagged types and dispatching calls.
- Bob
next prev parent reply other threads:[~2008-03-30 21:37 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-03-30 20:58 if Restrict_Func /= null confusion ma740988
2008-03-30 21:34 ` jimmaureenrogers
2008-03-30 21:36 ` Ludovic Brenta
2008-03-30 21:37 ` Robert A Duff [this message]
2008-03-30 21:44 ` Georg Bauhaus
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