1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
|
# As input parameters
While Rust chooses how to capture variables on the fly mostly without type
annotation, this ambiguity is not allowed when writing functions. When
taking a closure as an input parameter, the closure's complete type must be
annotated using one of a few `traits`, and they're determined by what the
closure does with captured value. In order of decreasing restriction,
they are:
* `Fn`: the closure uses the captured value by reference (`&T`)
* `FnMut`: the closure uses the captured value by mutable reference (`&mut T`)
* `FnOnce`: the closure uses the captured value by value (`T`)
On a variable-by-variable basis, the compiler will capture variables in the
least restrictive manner possible.
For instance, consider a parameter annotated as `FnOnce`. This specifies
that the closure *may* capture by `&T`, `&mut T`, or `T`, but the compiler
will ultimately choose based on how the captured variables are used in the
closure.
This is because if a move is possible, then any type of borrow should also
be possible. Note that the reverse is not true. If the parameter is
annotated as `Fn`, then capturing variables by `&mut T` or `T` are not
allowed. However, `&T` is allowed.
In the following example, try swapping the usage of `Fn`, `FnMut`, and
`FnOnce` to see what happens:
```rust,editable
// A function which takes a closure as an argument and calls it.
// <F> denotes that F is a "Generic type parameter"
fn apply<F>(f: F) where
// The closure takes no input and returns nothing.
F: FnOnce() {
// ^ TODO: Try changing this to `Fn` or `FnMut`.
f();
}
// A function which takes a closure and returns an `i32`.
fn apply_to_3<F>(f: F) -> i32 where
// The closure takes an `i32` and returns an `i32`.
F: Fn(i32) -> i32 {
f(3)
}
fn main() {
use std::mem;
let greeting = "hello";
// A non-copy type.
// `to_owned` creates owned data from borrowed one
let mut farewell = "goodbye".to_owned();
// Capture 2 variables: `greeting` by reference and
// `farewell` by value.
let diary = || {
// `greeting` is by reference: requires `Fn`.
println!("I said {}.", greeting);
// Mutation forces `farewell` to be captured by
// mutable reference. Now requires `FnMut`.
farewell.push_str("!!!");
println!("Then I screamed {}.", farewell);
println!("Now I can sleep. zzzzz");
// Manually calling drop forces `farewell` to
// be captured by value. Now requires `FnOnce`.
mem::drop(farewell);
};
// Call the function which applies the closure.
apply(diary);
// `double` satisfies `apply_to_3`'s trait bound
let double = |x| 2 * x;
println!("3 doubled: {}", apply_to_3(double));
}
```
### See also:
[`std::mem::drop`][drop], [`Fn`][fn], [`FnMut`][fnmut], [Generics][generics], [where][where] and [`FnOnce`][fnonce]
[drop]: https://doc.rust-lang.org/std/mem/fn.drop.html
[fn]: https://doc.rust-lang.org/std/ops/trait.Fn.html
[fnmut]: https://doc.rust-lang.org/std/ops/trait.FnMut.html
[fnonce]: https://doc.rust-lang.org/std/ops/trait.FnOnce.html
[generics]: ../../generics.md
[where]: ../../generics/where.md
|
