Clarified move semantics in "the details" section.
This commit is contained in:
parent
32d962d16f
commit
62b3b40ade
1 changed files with 27 additions and 7 deletions
|
|
@ -124,19 +124,39 @@ special annotation here, it’s the default thing that Rust does.
|
|||
The reason that we cannot use a binding after we’ve moved it is subtle, but
|
||||
important. When we write code like this:
|
||||
|
||||
```rust
|
||||
let x = 10;
|
||||
```
|
||||
|
||||
Rust allocates memory for an integer [i32] on the [stack][sh], copies the bit
|
||||
pattern representing the value of 10 to the allocated memory and binds the
|
||||
variable name x to this memory region for future reference.
|
||||
|
||||
Now consider the following code fragment:
|
||||
|
||||
```rust
|
||||
let v = vec![1, 2, 3];
|
||||
|
||||
let v2 = v;
|
||||
```
|
||||
|
||||
The first line allocates memory for the vector object, `v`, and for the data it
|
||||
contains. The vector object is stored on the [stack][sh] and contains a pointer
|
||||
to the content (`[1, 2, 3]`) stored on the [heap][sh]. When we move `v` to `v2`,
|
||||
it creates a copy of that pointer, for `v2`. Which means that there would be two
|
||||
pointers to the content of the vector on the heap. It would violate Rust’s
|
||||
safety guarantees by introducing a data race. Therefore, Rust forbids using `v`
|
||||
after we’ve done the move.
|
||||
The first line allocates memory for the vector object, `v`, on the stack like
|
||||
it does for `x` above. But in addition to that it also allocates some memory
|
||||
on on the [heap][sh] for the actual data `[1, 2, 3]`. Rust copies the address
|
||||
of this heap allocation to an internal pointer part of the vector object
|
||||
placed on the stack (let's call it the data pointer). It is worth pointing out
|
||||
even at the risk of being redundant that the vector object and its data live
|
||||
in separate memory regions instead of being a single contiguous memory
|
||||
allocation (due to reasons we will not go into at this point of time).
|
||||
|
||||
When we move `v` to `v2`, rust actually does a bitwise copy of the vector
|
||||
object `v` into the stack allocation represented by `v2`. This shallow copy
|
||||
does not create a copy of the heap allocation containing the actual data.
|
||||
Which means that there would be two pointers to the contents of the vector
|
||||
both pointing to the same memory allocation on the heap. It would violate
|
||||
Rust’s safety guarantees by introducing a data race if one could access both
|
||||
`v` and `v2` at the same time. Therefore, Rust forbids using `v` after we’ve
|
||||
done the move (shallow copy).
|
||||
|
||||
[sh]: the-stack-and-the-heap.html
|
||||
|
||||
|
|
|
|||
Loading…
Add table
Add a link
Reference in a new issue