PhantomData is a tag structure of zero size type.

Function:

  1. A type that is not used;
  2. Type variable;
  3. Mark ownership relationship;
  4. Automatic trait implementation (send/sync);

Type not used

The life cycle

struct Slice<'a, T> {
    start: *const T,
    end: *const T,
}
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However, because ‘a ‘is not used in the structure, it is unbounded. Disallow infinite lifecycles and types in structure definitions.

Modified as follows:

use std::marker;

struct Iter<'a, T: 'a> {
    ptr: *const T,
    end: *const T,
    _marker: marker::PhantomData<&'a T>,
}
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type

pub struct RetryableSendCh<T, C: Sender<T>> {
    ch: C,
    name: &'static str,
    marker: PhantomData<T>,
}
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Token ownership relationship

struct Vec<T> {
    data: *const T, // *const for variance!
    len: usize,
    cap: usize,}Copy the code

Native Pointers have no ownership semantics, and the DROP inspector will assume that Vec

does not have any values of type T. This in turn will make it unnecessary for Vec to worry about discarding any T in its destructor. But it’s possible that T will be released early. In order to drop the inspector that VEC must have the data of T, modify it as follows:

use std::marker;

struct Vec<T> {
    data: *const T, // *const for variance!
    len: usize,
    cap: usize,
    _marker: marker::PhantomData<T>,
}
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Type variable

  • The same

If you cannot replace one type with another, the type is called immutable.

  • inverter

Can be replaced by its base class.

  • covariance

Can be replaced by a derived type.

PhantomDataSchema table

Here’s a table of all the methods PhantomData can use:

Phantom type 'a T
PhantomData<T> – variant (with drop check)
PhantomData<&'a T> variant variant
PhantomData<&'a mut T> variant invariant
PhantomData<*const T> – variant
PhantomData<*mut T> – invariant
PhantomData<fn(T)> – contravariant (*)
PhantomData<fn() -> T> – variant
PhantomData<fn(T) -> T> – invariant
PhantomData<Cell<&'a ()>> invariant –

(*) This will remain constant if inversion is abolished.