Unique as a transitive type?

[Jason House]

Sean Kelly Wrote:

> Jason House wrote:
> > Andrei has stated previously that unique was left out of the type system because it added little value to the const system. Now that shared and multithreading are here, unique has more value.
> > 
> > I have two basic questions:
> > 
> > 1. What would make unique difficult to add?
> 
> Move semantics.  Just passing a unique value type to a function, a copy 
> is performed.

Why would copying occur? Can't a function simply pass it by reference and then zero out that register when done?

 
> > 2. What benefits do you forsee?
> 
> Message passing could be done without needless copying and with a 
> reasonable degree of safety.  I've done this before in C++ by storing 
> the reference in a modified shared_ptr (so it can live in containers), 
> then asserting is_unique() and transferring the reference to an auto_ptr 
> when passing into the message.  Kind of messy, but it works well enough.