Re: Why default values not taken?

[Tucker Taft <stt@averstar.com>]

"M. A. Alves" wrote:
> 
> I want to create an object with only some components different from their
> default values e.g.
> 
>   Contact_Person := new Text_Item'(Remark => US("name and email"));
> 
> where Text_Item inherits Remark from Input_Item and adds Text, both
> defaulting to Null_Unbounded_String:
> 
>   type Input_Item is tagged record
>     Remark: Unbounded_String;
>   end record;
> 
>   type Text_Item is new Input_Item with record
>     Text: Unbounded_String;
>   end record;
> 
> Of course I get the compile error "no value supplied for component Text".
> 
> But do I really have to supply values for all components?  What is the
> best idiom here?

Aggregates must be complete.  Default expressions for record
components are only used for default initializations.

I would recommend you define a "constructor" function
or procedure, with an aggregate inside the subprogram.  
You can use defaulted parameters to minimize
the amount of information a given call on the
constructor needs to specify.

In general, there are various good software-engineering
reasons to minimize the use of aggregates outside the package
where a type is defined.  This pragmatic issue with defaults
just reinforces the situation.

> Thanks,
> 
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-- 
-Tucker Taft   stt@avercom.net   http://www.avercom.net
Chief Technology Officer, AverCom Corporation (A Titan Company) 
Burlington, MA  USA (AverCom was formerly the Commercial Division of AverStar:
http://www.averstar.com/~stt)