Re: A question and a request

[Preben Randhol <randhol+abuse@pvv.org>]

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
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