Rust, PhantomData

PhantomData is a marker type, it is zero-sized type, we can use it to indicate that a type parameter is used without actually carry a value of that type.

use std::marker::PhantomData;


fn main() { 
    let ms = MyStruct {f1: 10, f2: PhantomData};
    println!("{:?}", ms)
}

#[derive(Debug)]
struct MyStruct<T> {
    f1: T,
    f2: PhantomData<T>
}

We create a struct MyStruct and add a generic type parameter T, we add 2 fields ,f1 and f2, we specify f1 field’s type is T, f2 filed’s type is PhantomData<T>. We add attribute derive(Debug), so we can print the instance of the struct .

In main function we create an instance of MyStruct ,f1 field we give it i32 value 10 , f2 filed we can give it a value PhantomData which is zero-sized value.

Last we print out this instance.