Re: generic parameter Copy for primitifs types.

[Jeffrey Carter <spam@spam.com>]

nblanpain@hotmail.com wrote:
> 
> -----
> generic
> type T_Item is private;
> with procedure Copy (Left : in out T_Item; Right : in T_Item);
> package Toto is
> ...
> end Toto;

Not that both ":=" and Copy are available to Toto for type T_Item. That 
seems odd; Toto is probably incorrectly specified.

Making Left (in Copy) mode in out can cause a problem with elementary 
types. Such types are checked for subtype conformance on copy-in to 
subprograms, and again on copy-out (elementary types are passed by 
copy). Consider a type

type T is range 1 .. 10;

and a variable

V : T;

and a procedure Copy such as yours for T. If I write

Copy (Left => V, Right => T'First);

the value of V will be checked against the range of T on copy-in to the 
procedure. Objects of type T will probably take 8 bits (or more) on most 
processors; thus, it's likely that the initial value of V will not be in 
T, and the call to Copy will raise Constraint_Error.

While you cannot pass default assignment to the generic, you can create 
a helper generic to make this case easier:

generic -- Assignment
    type Item is private;
procedure Assignment (To : out Item; From : in Item);

procedure Assignment (To : out Item; From : in Item) is
    -- null;
begin -- Assignment
    To := From;
end Assignment;

Then you can write

procedure Assign is new Assignment (Item => Integer);
package Dunno is new Toto (T_Item => Integer, Copy => Assign);

If you change Copy to Assign and use a default parameter:

    with procedure Assign (To : out T_Item; From : in T_Item) is <>;

then the 2nd instantiation can become

package Dummo is new Toto (T_Item => Integer);

-- 
Jeff Carter
"Sheriff murdered, crops burned, stores looted,
people stampeded, and cattle raped."
Blazing Saddles
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