Macro std::ptr::addr_of

pub macro addr_of($place : expr) {
    ...
}

Create a const 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, &expr as *const _ 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.

Note, however, that the expr in addr_of!(expr) is still subject to all the usual rules. In particular, addr_of!(*ptr::null()) is Undefined Behavior because it dereferences a null pointer.

Example

use std::ptr;

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

let packed = Packed { f1: 1, f2: 2 };
// `&packed.f2` would create an unaligned reference, and thus be Undefined Behavior!
let raw_f2 = ptr::addr_of!(packed.f2);
assert_eq!(unsafe { raw_f2.read_unaligned() }, 2);

See addr_of_mut for how to create a pointer to unininitialized data. Doing that with addr_of would not make much sense since one could only read the data, and that would be Undefined Behavior.