[rust-dev] On Copy = POD

Cameron Zwarich zwarich at mozilla.com
Sat Jun 21 16:05:12 PDT 2014


Another big problem with implicit copy constructors is that they make it very difficult to write correct unsafe code. When each use of a variable can call arbitrary code, each use of a variable can trigger unwinding. You then basically require people to write the equivalent of exception-safe C++ in unsafe code to preserve memory safety guarantees, and it’s notoriously difficult to do that.

Cameron

On Jun 20, 2014, at 8:49 PM, Nick Cameron <lists at ncameron.org> wrote:

> I think having copy constructors is the only way to get rid of `.clone()` all over the place when using` Rc`. That, to me, seems very important (in making smart pointers first class citizens of Rust, without this, I would rather go back to having @-pointers). The trouble is, I see incrementing a ref count as the upper bound on the work that should be done in a copy constructor and I see no way to enforce that.
> 
> So, I guess +1 to spirit of the OP, but no solid proposal for how to do it.
> 
> 
> On Sat, Jun 21, 2014 at 8:00 AM, Benjamin Striegel <ben.striegel at gmail.com> wrote:
> I'm not a fan of the idea of blessing certain types with a compiler-defined whitelist. And if the choice is then between ugly code and copy constructors, I'll take ugly code over surprising code.
> 
> 
> On Fri, Jun 20, 2014 at 3:10 PM, Patrick Walton <pcwalton at mozilla.com> wrote:
> On 6/20/14 12:07 PM, Paulo Sérgio Almeida wrote:]
> 
> Currently being Copy equates with being Pod. The more time passes and
> the more code examples I see, it is amazing the amount of ugliness that
> it causes. I wonder if there is a way out.
> 
> Part of the problem is that a lot of library code assumes that Copy types can be copied by just moving bytes around. Having copy constructors would mean that this simplifying assumption would have to change. It's doable, I suppose, but having copy constructors would have a significant downside.
> 
> Patrick
> 
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