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From: Georg Bauhaus <sb463ba@l1-hrz.uni-duisburg.de>
Newsgroups: comp.lang.ada
Subject: Re: handling null pointers/records to functions
Date: Sat, 21 Aug 2004 17:11:13 +0000 (UTC)
Organization: GMUGHDU
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Date: 2004-08-21T17:11:13+00:00
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Jim Rogers <jimmaureenrogers@worldnet.att.net> wrote:
:>quasar <quasar@nospam.com> wrote:
 
: You ask about returning some kind of null object. I suggest what 
: you might consider here is defining some value of the Data member
: to indicate no data. This approach avoids exception handling but
: is easier to get wrong at the calling context. The calling 
: context must compare all return values with the "no data" value
: before proceeding. Failure to do so will propogate bad data
: throughout the rest of the program.

Maybe you can also add a default value parameter to the function
that is returned when the Node_Ptr_Type value is null.

function Data_Part
   (Node_Ptr : Node_Ptr_Type; default_value: Data_Type) return Data_Type is
begin
    if node_ptr /= null then
        return Node_Ptr.Data;
    else
        Put_Line("Node passed to FUNCTION DATA_PART is null");
        return default_value;
    end if;
end Data_Part;

: The use of exceptions is really much more reliable in this case.

Have you, quasar, wanted a precondition of node_ptr /= null?
In this case the precondition will not be satisfied by a null
access. Therefore raising an exception seems an adequate solution.

-- Georg