rust - Why does an enum require extra memory size?

Question

My understanding is that enum is like union in C and the system will allocate the largest of the data types in the enum.

enum E1 {
    DblVal1(f64),
}

enum E2 {
    DblVal1(f64),
    DblVal2(f64),
    DblVal3(f64),
    DblVal4(f64),
}

fn main() {
    println!("Size is {}", std::mem::size_of::<E1>());
    println!("Size is {}", std::mem::size_of::<E2>());
}

Why does E1 takes up 8 bytes as expected, but E2 takes up 16 bytes?

Answer 1

In Rust, unlike in C, enums are tagged unions. That is, the enum knows which value it holds. So 8 bytes wouldn't be enough because there would be no room for the tag.