11

addr_of_mut

 3 years ago
source link: https://doc.rust-lang.org/stable/std/ptr/macro.addr_of_mut.html
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Macro std::ptr::addr_of_mut1.51.0[−][src]

macro_rules! addr_of_mut {
    ($place:expr) => { ... };
}

Create a mut raw pointer to a place, without creating an intermediate reference.

Creating a reference with &/&mut is only allowed if the pointer is properly aligned and points to initialized data. For cases where those requirements do not hold, raw pointers should be used instead. However, &mut expr as *mut _ creates a reference before casting it to a raw pointer, and that reference is subject to the same rules as all other references. This macro can create a raw pointer without creating a reference first.

Example

use std::ptr;

#[repr(packed)]
struct Packed {
    f1: u8,
    f2: u16,
}

let mut packed = Packed { f1: 1, f2: 2 };
// `&mut packed.f2` would create an unaligned reference, and thus be Undefined Behavior!
let raw_f2 = ptr::addr_of_mut!(packed.f2);
unsafe { raw_f2.write_unaligned(42); }
assert_eq!({packed.f2}, 42); // `{...}` forces copying the field instead of creating a reference.
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