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mentioned difference between memory allocation of arrays and vectors in book #20982

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13 changes: 11 additions & 2 deletions src/doc/trpl/arrays-vectors-and-slices.md
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Expand Up @@ -49,8 +49,7 @@ languages.

A *vector* is a dynamic or "growable" array, implemented as the standard
library type [`Vec<T>`](../std/vec/) (we'll talk about what the `<T>` means
later). Vectors are to arrays what `String` is to `&str`. You can create them
with the `vec!` macro:
later). You can create them with the `vec!` macro:

```{rust}
let v = vec![1, 2, 3]; // v: Vec<i32>
Expand All @@ -60,6 +59,16 @@ let v = vec![1, 2, 3]; // v: Vec<i32>
brackets `[]` with `vec!`. Rust allows you to use either in either situation,
this is just convention.)

Vectors are to arrays what `String` is to `&str`. While the arrays are
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This isn't correct. There's three core array-ish types:

  • [T; n]: Array
  • [T]: Slice
  • Vec<T>: Vector

[T; n] is "on the stack", because it has a compile-time-constant size.

[T] is a dynamically-sized type because it is a handle to some compile-time-unknown number of elements. In that sense, [T] can't ever exist on its own; it must exist behind a pointer as e.g. Box<[T]> or &[T]. Any such pointer becomes "fat" as it includes the number of elements in the slice as run-time information in addition to the actual address where the slice starts. A slice may just as easily refer to the contents of a Vec or Array.

Vec is a heap-allocated growable array, conceptually similar to a Box<[T]>, but without the requirement that capacity = length.

Vec == String, and &str == &[T], not [T; n]. There is no string analog to [T; n].

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Whoops, total brain fart. My brain read "slice". Sigh. You are totally correct.

allocated on the stack (unless we explicitly box them during allocation),
Vectors are always allocated on the heap.

```{rust}
let x = [0; 3]; // regular array, allocated on the stack
let y: Box<[isize; 3]> = Box::new([1, 2, 3]); // boxed array, heap allocated
let v = vec![1, 2, 3]; // vector, heap allocated
```

You can get the length of, iterate over, and subscript vectors just like
arrays. In addition, (mutable) vectors can grow automatically:

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