phantom-data.md

Table of contents

• Table of contents
• PhantomData
• Example

PhantomData

struct Tagged<T>(usize);

Output:

error[E0392]: parameter `T` is never used
 --> src/main.rs:1:15
  |
1 | struct Tagged<T>(usize);
  |               ^ unused type parameter
  |
  = help: consider removing `T` or using a marker such as `std::marker::PhantomData`

That’s right, we’re not allowed to have a type parameter that goes unused.

If we want to have an unused type parameter we have to add a PhantomData to it like so:

use std::marker::PhantomData;
struct Tagged<T>(usize, PhantomData<T>);

Declaration of PhantomData in core:

pub struct PhantomData<T: ?Sized>;

PhantomData is marker type. It is ZST.
Adding a PhantomData<T> to your type tells the compiler that your type acts like it stores a value of type T, even though it doesn’t really.

Example

use std::marker::PhantomData;

struct author_id;
struct post_id;

struct ID<P>(i64, PhantomData<P>);

fn get_post(aid: ID<author_id>, pid: ID<post_id>) -> String {
    format!("{aid}, {pid}", aid=aid.0, pid=pid.0)
}

fn main() {
    let id1: ID<author_id> = ID(5, PhantomData);
    let id2: ID<post_id> = ID(77, PhantomData);
    println!("{}", get_post(id1, id2));
    
    // println!("{}", get_post(id2, id1));  // ERROR
}