Mastering Rust - Second Edition

Unions

For interoperability with C code, Rust also supports the union type, which maps directly to a C union. Unions are unsafe to read from. Let's see an example of how to create and interact with them:

// unions.rs#[repr(C)]union Metric {    rounded: u32,    precise: f32,}fn main() {    let mut a = Metric { rounded: 323 };    unsafe {        println!("{}", a.rounded);    }    unsafe {        println!("{}", a.precise);    }    a.precise = 33.3;    unsafe {        println!("{}", a.precise);    }}

We created a union type, Metric, that has two fields rounded and precise, and represents some measurement. In main, we initialize an instance of it in the a variable.

We can only initialize one of the variables, otherwise the compiler complains with the following message:

error: union expressions ...