幻像类型参数

幻像类型参数是在运行时不显示，但在编译时（并且仅在编译时）进行静态检查的参数。

数据类型可以使用额外的泛型类型参数作为标记或在编译时执行类型检查。这些额外的参数不保存存储值，并且没有运行时行为。

在以下示例中，我们将 std::marker::PhantomData 与幻像类型参数概念相结合，以创建包含不同数据类型的元组。

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 main() {
    // 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);
}

另请参阅

派生、结构体 和 元组结构体