rust-ape-example/src/bin/generics_phantom.rs

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// ./src/generics/phantom.md
use std::marker::PhantomData;
// A phantom tuple struct which is generic over `A` with hidden parameter `B`.
#[derive(PartialEq)] // Allow equality test for this type.
struct PhantomTuple<A, B>(A, PhantomData<B>);
// A phantom type struct which is generic over `A` with hidden parameter `B`.
#[derive(PartialEq)] // Allow equality test for this type.
struct PhantomStruct<A, B> { first: A, phantom: PhantomData<B> }
// Note: Storage is allocated for generic type `A`, but not for `B`.
// Therefore, `B` cannot be used in computations.
fn part0() {
// Here, `f32` and `f64` are the hidden parameters.
// PhantomTuple type specified as `<char, f32>`.
let _tuple1: PhantomTuple<char, f32> = PhantomTuple('Q', PhantomData);
// PhantomTuple type specified as `<char, f64>`.
let _tuple2: PhantomTuple<char, f64> = PhantomTuple('Q', PhantomData);
// Type specified as `<char, f32>`.
let _struct1: PhantomStruct<char, f32> = PhantomStruct {
first: 'Q',
phantom: PhantomData,
};
// Type specified as `<char, f64>`.
let _struct2: PhantomStruct<char, f64> = PhantomStruct {
first: 'Q',
phantom: PhantomData,
};
// Compile-time Error! Type mismatch so these cannot be compared:
// println!("_tuple1 == _tuple2 yields: {}",
// _tuple1 == _tuple2);
// Compile-time Error! Type mismatch so these cannot be compared:
// println!("_struct1 == _struct2 yields: {}",
// _struct1 == _struct2);
}
pub fn main() {
part0();
}