Why can't we define re-assignable const reference variable?

[Christopher Wright]

Janice Caron wrote:
> On 16/02/2008, none <z at gg.com> wrote:
>>  So why can't we have both (just as in C++):
> 
> This has been covered so many times before, but in summary, allowing
> that would completely break the type system.

What's the difference (save polymorphism) between a pointer to a struct 
and a reference to an object?