mirror of https://github.com/rust-lang/reference
Merge pull request #1451 from udoprog/document-ptr-casts
Clarify semantics of the various pointer to pointer casts
This commit is contained in:
commit
b25c20b494
|
@ -478,15 +478,20 @@ unsafe {
|
|||
assert_eq!(values[1], 3);
|
||||
```
|
||||
|
||||
#### Slice DST pointer to pointer cast
|
||||
#### Pointer-to-pointer cast
|
||||
|
||||
For slice types like `[T]` and `[U]`, the raw pointer types `*const [T]`, `*mut [T]`,
|
||||
`*const [U]`, and `*mut [U]` encode the number of elements in this slice. Casts between
|
||||
these raw pointer types preserve the number of elements. Note that, as a consequence,
|
||||
such casts do *not* necessarily preserve the size of the pointer's referent (e.g.,
|
||||
casting `*const [u16]` to `*const [u8]` will result in a raw pointer which refers to an
|
||||
object of half the size of the original). The same holds for `str` and any compound type
|
||||
whose unsized tail is a slice type, such as struct `Foo(i32, [u8])` or `(u64, Foo)`.
|
||||
`*const T` / `*mut T` can be cast to `*const U` / `*mut U` with the following behavior:
|
||||
|
||||
- If `T` and `U` are both sized, the pointer is returned unchanged.
|
||||
- If `T` and `U` are both unsized, the pointer is also returned unchanged.
|
||||
In particular, the metadata is preserved exactly.
|
||||
|
||||
For instance, a cast from `*const [T]` to `*const [U]` preserves the number of elements.
|
||||
Note that, as a consequence, such casts do not necessarily preserve the size of the pointer's referent
|
||||
(e.g., casting `*const [u16]` to `*const [u8]` will result in a raw pointer which refers to an object of half the size of the original).
|
||||
The same holds for `str` and any compound type whose unsized tail is a slice type,
|
||||
such as `struct Foo(i32, [u8])` or `(u64, Foo)`.
|
||||
- If `T` is unsized and `U` is sized, the cast discards all metadata that completes the wide pointer `T` and produces a thin pointer `U` consisting of the data part of the unsized pointer.
|
||||
|
||||
## Assignment expressions
|
||||
|
||||
|
@ -687,3 +692,17 @@ See [this test] for an example of using this dependency.
|
|||
[_TypeNoBounds_]: ../types.md#type-expressions
|
||||
[_RangeExpression_]: ./range-expr.md
|
||||
[_UnderscoreExpression_]: ./underscore-expr.md
|
||||
|
||||
<script>
|
||||
(function() {
|
||||
var fragments = {
|
||||
"#slice-dst-pointer-to-pointer-cast": "operator-expr.html#pointer-to-pointer-cast",
|
||||
};
|
||||
var target = fragments[window.location.hash];
|
||||
if (target) {
|
||||
var url = window.location.toString();
|
||||
var base = url.substring(0, url.lastIndexOf('/'));
|
||||
window.location.replace(base + "/" + target);
|
||||
}
|
||||
})();
|
||||
</script>
|
||||
|
|
Loading…
Reference in New Issue