为什么枚举需要额外的内存大小？(Why does an enum require extra memory size?)

为什么枚举需要额外的内存大小？(Why does an enum require extra memory size?)

我的理解是enum就像C中的union一样，系统将分配enum中最大的数据类型。

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>()); }

为什么E1按预期占用8个字节，但E2占用16个字节？

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?

最满意答案

在Rust中，与C不同， enum被标记为联合 。 也就是说， enum知道它拥有哪个值。 所以8个字节是不够的，因为标签没有空间。

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.