Re: Renaming vs assignment to constant

[Shark8 <onewingedshark@gmail.com>]

On Friday, December 6, 2013 8:14:51 AM UTC-7, Simon Wright wrote:
> A poster on Stack Overflow wrote
> 
>    Now : Time renames Clock;

That was me! :)


> (using Ada.Calendar) which made me blink until I realised that it is the
> equivalent of
> 
>    Now : constant Time := Clock;

Yep, it is. I probably used it because I had recently run into a C# problem where renaming a variable [and typecast] would have been *VERY* useful.

'Renames' doesn't get enough love; its been neglected over the past [few] standard revisions. -- After using some of the Ada1012 features it'd be kinda nice to put pre/post conditions on a callback independent of those of the actual subprogram referenced; perhaps:

type callback_type is not null access procedure;
Procedure Some_Procedure;
--...
 callback : callback_type renames procedure'access
               with pre => [...], post => [...];

Generics are another feature that seems to have been neglected (for 2005 and 2012), even though they're really pretty good in Ada they could be better still... perhaps a bit of polishing such as allowing Instantiation.Input_variable on formal parameter Input_Variable (conceptually IN would map to constant, IN OUT to a variable, and OUT should likely be handled as an out parameter in a subprogram): this would allow for clients of an instantiation to depend on it w/o having to be generics themselves.

> GNAT generates the same code for these two forms. Is there a difference?
> Which form is preferred?

I think the big difference happens when the result has subcomponents:
    ch   : constant Character:= 'X';             -- Character X.
    temp : String  renames Character'Image(ch);  -- Its image: 'X'.
    ch2  : Character renames temp(2);            -- The unquoted character.