Marker Structs

PhantomData และ marker types

PhantomData Basics

use std::marker::PhantomData;

fn main() {
    // PhantomData is zero-sized
    println!("Size of PhantomData<i32>: {}",
        std::mem::size_of::<PhantomData<i32>>());
    println!("Size of PhantomData<String>: {}",
        std::mem::size_of::<PhantomData<String>>());

    // Usage: tell compiler about logical ownership
    struct Container<T> {
        data: *const T,  // Raw pointer - no ownership info
        _marker: PhantomData<T>,  // Adds ownership/lifetime info
    }

    impl<T> Container<T> {
        fn new(data: &T) -> Self {
            Container {
                data: data as *const T,
                _marker: PhantomData,
            }
        }
    }

    let value = 42;
    let container = Container::new(&value);
    println!("Container created");

    // Size remains same as just pointer
    println!("\nSize of Container<i32>: {}",
        std::mem::size_of::<Container<i32>>());
    println!("Size of *const i32: {}",
        std::mem::size_of::<*const i32>());
}