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Re-word explanation on closures in intro

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Fixes #23571
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Steve Klabnik 2015-03-22 15:36:24 -04:00
parent ecf8c64e1b
commit 81801f2171
1 changed files with 10 additions and 10 deletions
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20
src/doc/intro.md
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@ -446,16 +446,16 @@ It gives us this error:
error: aborting due to previous error
```
It mentions that "captured outer variable in an `FnMut` closure".
Because we declared the closure as a moving closure, and it referred
to `numbers`, the closure will try to take ownership of the
vector. But the closure itself is created in a loop, and hence we will
actually create three closures, one for every iteration of the
loop. This means that all three of those closures would try to own
`numbers`, which is impossible -- `numbers` must have just one
owner. Rust detects this and gives us the error: we claim that
`numbers` has ownership, but our code tries to make three owners. This
may cause a safety problem, so Rust disallows it.
This is a little confusing because there are two closures here: the one passed
to `map`, and the one passed to `thread::scoped`. In this case, the closure for
`thread::scoped` is attempting to reference `numbers`, a `Vec<i32>`. This
closure is a `FnOnce` closure, as that’s what `thread::scoped` takes as an
argument. `FnOnce` closures take ownership of their environment. That’s fine,
but there’s one detail: because of `map`, we’re going to make three of these
closures. And since all three try to take ownership of `numbers`, that would be
a problem. That’s what it means by ‘cannot move out of captured outer
variable’: our `thread::scoped` closure wants to take ownership, and it can’t,
because the closure for `map` won’t let it.
What to do here? Rust has two types that helps us: `Arc<T>` and `Mutex<T>`.
*Arc* stands for "atomically reference counted". In other words, an Arc will