结构中的字符串，生命周期

Question

I know what the difference between these 3 struct is - a lifetime of a

struct S1 {
    a: &'static str,
    b: int
}

struct S2<'aa> {
    a: &'aa str,
    b: int
}

struct S3 {
    a: String,
    b: int
}

fn main() {
    let s1 = S1 {a: "123", b: 123};
    let s2 = S2 {a: "123", b: 123};
    let s3 = S2 {a: "123".into_owned(), b: 123};
}

Could you show me a use case of the 1st, 2nd and 3rd, in other words, when is it better to use the 1st over 2nd and 3rd, when - 2nd over 1st and 3rd, etc? In the documentation there's no explanation.

Answer 1

S1 : This only allows you to use string literals, or other strings with a static (ie they can never be deallocated) lifetime.

S2 : This lets you use arbitrary string slices, provided they have an expressible lifetime. For example, you cannot return dynamic instances of S2 from an Iterator , because there's no way to express the lifetime involved. That said, this allows you to avoid unnecessary heap allocations.

S3 : The most general, since it owns its contents, but requires heap allocation to use.