Re: if Restrict_Func /= null confusion

["jimmaureenrogers@worldnet.att.net" <jimmaureenrogers@worldnet.att.net>]

On Mar 30, 2:58 pm, ma740...@gmail.com wrote:
> 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.

Type Restrict_Function is an access type. It is possible for an
instance of Restrict_Function to have a null value. This kind of
problem can occur if you are creating an array of Restrict_Function
and only some of the elements of the array are set to non-null values.
You will encounter a run-time error if you try to dereference a null
access value.

Jim Rogers