Unions

Unions are like enums, but you need to track the active field yourself:


#[repr(C)]
union MyUnion {
    i: u8,
    b: bool,
}
fn main() {
    let u = MyUnion { i: 42 };
    println!("int: {}", unsafe { u.i });
    println!("bool: {}", unsafe { u.b });  // Undefined behavior!
}
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Speaker Notes

Unions are very rarely needed in Rust as you can usually use an enum. They are occasionally needed for interacting with C library APIs.

If you just want to reinterpret bytes as a different type, you probably want std::mem::transmute or a safe wrapper such as the zerocopy crate.