[rustc.git] / src / doc / book / nostarch / chapter19-new-function-pointer-text.md

Please add this text at the end of the Function Pointers section, just before
the Returning Closures section starts, so at the end of page 447 and before
page 448.

---

We have another useful pattern that exploits an implementation detail of tuple
structs and tuple-struct enum variants. These types use `()` as initializer
syntax, which looks like a function call. The initializers are actually
implemented as functions returning an instance that’s constructed from their
arguments. We can use these initializer functions as function pointers that
implement the closure traits, which means we can specify the initializer
functions as arguments for methods that take closures, like so:

```
enum Status {
    Value(u32),
    Stop,
}

let list_of_statuses: Vec<Status> =
    (0u32..20)
    .map(Status::Value)
    .collect();
```

Here we create `Status::Value` instances using each `u32` value in the range
that `map` is called on by using the initializer function of `Status::Value`.
Some people prefer this style, and some people prefer to use closures. They
compile to the same code, so use whichever style is clearer to you.
Commit	Line	Data
532ac7d7 XL	1	Please add this text at the end of the Function Pointers section, just before
	2	the Returning Closures section starts, so at the end of page 447 and before
	3	page 448.
	4
	5	---
	6
	7	We have another useful pattern that exploits an implementation detail of tuple
	8	structs and tuple-struct enum variants. These types use `()` as initializer
	9	syntax, which looks like a function call. The initializers are actually
	10	implemented as functions returning an instance that’s constructed from their
	11	arguments. We can use these initializer functions as function pointers that
	12	implement the closure traits, which means we can specify the initializer
	13	functions as arguments for methods that take closures, like so:
	14
	15	```
	16	enum Status {
	17	Value(u32),
	18	Stop,
	19	}
	20
	21	let list_of_statuses: Vec<Status> =
	22	(0u32..20)
	23	.map(Status::Value)
	24	.collect();
	25	```
	26
	27	Here we create `Status::Value` instances using each `u32` value in the range
	28	that `map` is called on by using the initializer function of `Status::Value`.
	29	Some people prefer this style, and some people prefer to use closures. They
	30	compile to the same code, so use whichever style is clearer to you.
	31