Re: Renaming vs assignment to constant

[adambeneschan@gmail.com]

On Friday, December 6, 2013 7:14:51 AM UTC-8, Simon Wright wrote:
> A poster on Stack Overflow wrote
> 
>    Now : Time renames Clock;
> 
> (using Ada.Calendar) which made me blink until I realised that it is the
> equivalent of
> 
>    Now : constant Time := Clock;
> 
> GNAT generates the same code for these two forms. Is there a difference?
> 
> Which form is preferred?

I prefer the second form, mostly because in the first one, Time does not "rename" anything the same way that a variable that renames another variable, or a function that renames another function, does.  (Technically, I think Time renames an anonymous object created by the compiler to hold the function result, although it seems like incorrect English to talk about "renaming" something that didn't have a name in the first place.)

However, there *might* be a difference between the two forms if the type involved is (1) a task type or a type that contains a task as a subcomponent, (2) a controlled type or a type that contains one, (3) something with a coextension.  I'm really not sure whether there's a difference in any of these cases, and #2 may be a case where some of the differences are or were dependent on the implementation (because of build-in-place semantics).  I'd have to do some research to figure out if there are any differences.  Maybe Randy already knows the answer.  Also, there may be differences related to accessibility levels.  In this simple case that doesn't involve any of those features, I don't think there's a semantic difference.

                              -- Adam