A better way to denote noncopyable types

[bstrie]

In the past the way to tell the compiler that a struct wasn't copyable was to stick an empty drop {} block at the end of it. This was hacky and non-obvious, but at least it was concise. But with the Drop trait this now expands to impl Foo : Drop { fn finalize(&self) {} }, which is rather unfortunate. A #[noncopyable] attribute that expanded to the above would be clearer and less verbose.

[brson]

There's also util::NonCopyable which is a type that can be put inside a struct to make it noncopyable.

There may also be other solutions to this like:

trait Drop {
    fn finalize() { /* default impl */ }
}
trait NonCopyable: Drop;

// Here I'm using imaginary syntax to indicate that MyType impls both NonCopyable and all the supertraits of NonCopyable
impl MyType: NonCopyable(*);

[brson]

Actually, with a default impl of finalize you could just write impl MyType: Drop, though the NonCopyable trait would be clearer.

[bstrie]

Changed issue title to be more general

[bblum]

incidentally, it makes me queasy that util::NonCopyable takes a byte of memory, because of the unit(/nil) inside.

[brson]

This could also work, depending on what generic impls are legal

trait Drop {
    fn finalize(&self);
}

trait NonCopyable;

impl <T: NonCopyable> T: Drop {
    fn finalize(&self) { }
}

[bstrie]

Now that we have #[unsafe_destructor], it seems like there's a precedent for making this an attribute rather than an impl. Perhaps we could also consider #[deriving(Noncopyable)].

[bstrie]

Now that util::Noncopyable no longer takes up space, it's the preferred way to denote these types. Closing.

[mzabaluev]

How to initialize a structure that contains an util::NonCopyable member?
This is the last wart for me in what otherwise would be a good solution.

[thestinger]

struct { non_copyable: std::util::NonCopyable, y: 5 }

Although right now, unit structs are buggy and can't be constructed cross-crate.

[mzabaluev]

2013/7/5 Daniel Micay [email protected]

Although right now, unit structs are buggy and can't be constructed
cross-crate.

Yeah, that's what I apparently ran into.

Thank you,
Mikhail