# Macro std::ptr::addr_of_mut

1.51.0 · source ·

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

Expand description

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.

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

## Examples

Creating a pointer to unaligned data:

    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.

Run

Creating a pointer to uninitialized data:

    use std::{ptr, mem::MaybeUninit};

    struct Demo {
        field: bool,
    }

    let mut uninit = MaybeUninit::<Demo>::uninit();
    // `&uninit.as_mut().field` would create a reference to an uninitialized `bool`,
    // and thus be Undefined Behavior!
    let f1_ptr = unsafe { ptr::addr_of_mut!((*uninit.as_mut_ptr()).field) };
    unsafe { f1_ptr.write(true); }
    let init = unsafe { uninit.assume_init() };

Run