rust中的ref

理解Rust的引用與借用（好文鏈接）

#![feature(core_intrinsics)]
fn print_type_of<T>(_: T) {
    println!("{}", unsafe { std::intrinsics::type_name::<T>() });
}

fn main() {
    let ref a: i32;
    a = &1;
    let ref a: i32 = 1;
    print_type_of(a);

    let c: &u32 = &&&&&2;
    print_type_of(c);
}

上面2個a的類型都是&i32

enum帶參數時使用match會move走enum的參數，如下這樣寫會報錯

#![feature(core_intrinsics)]
fn print_type_of<T>(_: T) {
    println!("{}", unsafe { std::intrinsics::type_name::<T>() });
}

fn main() {
    let os = Some(String::from("s"));
    match os {
      Some(s) => {
        print_type_of(s);
      },
      _ => ()
    };
    println!("{:?}", os);
}

改下match的參數匹配模式，用ref來匹配就不會出錯了

#![feature(core_intrinsics)]
fn print_type_of<T>(_: T) {
    println!("{}", unsafe { std::intrinsics::type_name::<T>() });
}

fn main() {
    let os = Some(String::from("s"));
    match os {
      Some(ref s) => {
        print_type_of(s); // s此時是&alloc::string::String類型
      },
      _ => ()
    };
    println!("{:?}", os);
}

如果match的對象是一個引用，會發現參數是引用類型，比如

#![feature(core_intrinsics)]
fn print_type_of<T>(_: T) {
    println!("{}", unsafe { std::intrinsics::type_name::<T>() });
}

fn main() {
    let os = &Some(String::from("s"));
    match os {
      Some(s) => {
        print_type_of(s);
      },
      _ => ()
    };
}

打印的是&alloc::string::String

其實和下面的代碼一樣

#![feature(core_intrinsics)]
fn print_type_of<T>(_: T) {
    println!("{}", unsafe { std::intrinsics::type_name::<T>() });
}

fn main() {
    let os = &Some(String::from("s"));
    match os {
      &Some(ref s) => {
        print_type_of(s);
      },
      _ => ()
    };
}