实现 Size 特征

Question

Created a simple struct that implements Sized trait.

struct FixedIndividual<T: Sized,A: cmp::Ord, >{
    chromosome: T,
    score: Option<A>,
}
impl<T: Sized, A: cmp::Ord> FixedIndividual<T,A>{
    fn new(chromosome: T) -> Self{
        FixedIndividual { chromosome , score: None}
    }
}

However, I've managed to create an instance that includes Vec(implement only?Size),

 #[test]
    fn init_vector(){
        let chromosome: Vec<i32> = vec![1,2,3,4,5];
        let chromosome_cpy = chromosome.clone();
        let indv:FixedIndividual<Vec<i32>, OrderedFloat<f64>> = FixedIndividual::new(chromosome);
        assert_eq!(indv.score, None);
        assert_eq!( indv.chromosome
                    .iter()
                    .zip(chromosome_cpy.iter())
                    .all(|(a,b)| a == b ), true);
    } 

Answer 1

Created a simple struct that implements Sized trait.

Your bounds are useless, generic bounds are Sized by default, you have to opt out of it.

However, I've managed to create an instance that includes Vec(implement only?Size),

Not sure where you got that idea, and ?Sized is not a trait, it's only a bound which means the type (or function) is sizedness-agnostic . That doesn't imply it's unsized itself. For instance Box<T> has T: ?Sized , meaning T may be sized or unsized. Box is sized either way.

A type being unsized means it implements !Sized , which rather few types do.

Answer 2

That's fine because Vec s are Sized , like almost all struct values. The actual Vec value does not contain its elements, but instead references them, so it has a defined, constant size no matter how many elements there are, similar to how a pointer is stored in an integer value. You may also have confused vecs with slices, such as [i32] , which are ?Sized .