Re: A question and a request

[Eric Merritt <cyberlync@yahoo.com>]

I knew I was missing something. That actaully make
sense, I guess I was getting this typeing behavior
mixed up with the OO inheritence model. 

Thank You,
Eric Merritt

--- Preben Randhol <randhol+abuse@pvv.org> wrote:
> On Tue, 6 Nov 2001 20:44:19 -0800 (PST), Eric
> Merritt wrote:
> > As some of you may know I am fairly new to Ada and
> I
> > just found out something that surprised me. I
> declared
> > two subtypes of an integer. I was then able to add
> > these two types together and assign the result to
> an
> > instance of one of the above subtypes. or
> > 
> >  subtype type_1 is Integer;
> >  subtype type_2 is Integer;
> >   
> >  type_1_instance   : type_1;
> >  type_2_instance   : type_2;
> >  type_1_instance_2 : type_1;
> > 
> >  type_1_instance   := 1;
> >  type_2_instance   := 2;
> >  type_1_instance_2 := type_1_instance +
> > type_2_instance;
> > 
> > The addition operator is taking two disparate but
> > related types and adding them. This is fine,
> > up-casting is a normal thing. However, when the
> result
> > is returned it is not (should not be?) an instance
> of
> > the subtype. It seems that the compiler is
> > down-casting automatically. This bothers me and it
> > seams that is violates Ada's strong typing. What
> am I
> > missing here?
> 
> Use type not subtype if you do not want the above
> behavior. Read
> below (taken from:
>
http://goanna.cs.rmit.edu.au/~dale/ada/aln/4_basic_types.htmli
> )
> 
> Subtypes
> 
> We can restrict the range of values a variable can
> take by declaring a
> subtype with a restricted range of values (this
> corresponds to Pascal's
> user defined types). Any attempt to place an
> out-of-range value into a
> variable of a subtype results in an exception
> (program error). In this
> way program errors can be discovered. The syntax for
> a subtype
> declaration is
> 
> subtype Name is Base_Type;
> subtype Name is Base_Type range lowerbound . .
> upperbound;
> 
> Examples of declaring subtypes are given below.
> 
> type Processors is (M68000,  i8086, i80386, M68030,
> Pentium, PowerPC);
> subtype Old_Processors is Processors range
> M68000..i8086;
> subtype New_Processors is Processors range
> Pentium..PowerPC;
> 
> subtype Data is Integer;
> subtype Age is Data range 0 . . 140;
> subtype Temperatures is Float range -50.0 .. 200.0;
> subtype Upper_Chars is Character range 'A' .. 'Z';
> 
> Subtypes are compatable with their base types . They
> can be placed in
> the same place as any variable of the base type can.
> Also variables of
> different subtypes that are derived from the same
> base type are
> compatable.
> 
> My_Age	: Age;
> Height	: Integer;
> 
> Height := My_Age;	-- silly, but never causes a
> problem.
> 
> My_Age := Height;	-- will cause a problem if
> height's
> 			-- value is outside the range of 
> 			-- my_age (0..140), but still 
> 			-- compilable.
> 
> 
> Preben
> -- 
> Please, stop bombing civilians in Afghanistan. One
> cannot write off
> killing innocent children and other civilians as
> "collateral damage".
> A civilian is a civilian whether he or she is
> American or from another
> country in the world.          
> http://web.amnesty.org/11september.htm
> _______________________________________________
> comp.lang.ada mailing list
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