Re: Logical constants

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 2017-09-29 20:12, Victor Porton wrote:
> I pass a private type T (whose full type is a record) containing a Handle to
> a subprogram.
> 
> (In fact, a handle is a pointer to a C struct.)
> 
> All such subprograms can be written as subprograms with "in" mode for T,
> because they do not modify the handle.
> 
> But some of these programs logically modify the corresponding C object
> (without modifying the handle itself, they may modify the struct the handle
> points to).
> 
> Should I pass T arguments to such subprograms in "in out" mode? To use "in"
> only is allowed by the language rules, but logically them are "in out".
> 
> What are the arguments for using either "in" or "in out" in this case?

It is undecided.

1. A "transitive" approach is that the mode of a proxy object must 
reflect the mode of the target.

2. A "non-transitive" one is that modes of two objects are separate.

E.g. Ada's access types take the second approach. A pointer can be 
constant and target mutable and conversely.

Arguably the first one should be chosen when the proxy hides/represents 
the target and the second when both types are independently visible.

But Ada.Text_IO still takes the second approach. Ada.Text_IO.Create has 
in out File_Type. Ada.Text_IO.Put has in File_Type, though the target is 
clearly invisible.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de