我正在阅读Rust文档,但似乎无法理解正在发生的事情。例如,在这里,我看到了以下示例:
// This function takes ownership of a box and destroys it
fn eat_box_i32(boxed_i32: Box<i32>) {
println!("Destroying box that contains {}", boxed_i32);
}
// This function borrows an i32
fn borrow_i32(borrowed_i32: &i32) {
println!("This int is: {}", borrowed_i32);
}
fn main() {
// Create a boxed i32, and a stacked i32
let boxed_i32 = Box::new(5_i32);
let stacked_i32 = 6_i32;
// Borrow the contents of the box. Ownership is not taken,
// so the contents can be borrowed again.
borrow_i32(&boxed_i32);
borrow_i32(&stacked_i32);
{
// Take a reference to the data contained inside the box
let _ref_to_i32: &i32 = &boxed_i32;
// Error!
// Can't destroy `boxed_i32` while the inner value is borrowed later in scope.
eat_box_i32(boxed_i32);
// FIXME ^ Comment out this line
// Attempt to borrow `_ref_to_i32` after inner value is destroyed
borrow_i32(_ref_to_i32);
// `_ref_to_i32` goes out of scope and is no longer borrowed.
}
// `boxed_i32` can now give up ownership to `eat_box` and be destroyed
eat_box_i32(boxed_i32);
}
我相信的事情:
- eat_box_i32 接受一个指向 Box 的指针
- 这行代码
let boxed_i32 = Box::new(5_i32);
使得 boxed_i32 包含了一个指针,因为 Box 不是原始类型
我不理解的事情:
- 为什么我们需要使用 & 来调用
borrow_i32(&boxed_i32);
?难道 boxed_i32 不已经是一个指针了吗? - 在这一行代码中:
let _ref_to_i32: &i32 = &boxed_i32;
为什么右边也需要加上 &?难道 boxed_i32 不已经是一个地址了吗? - 为什么 borrow_i32 可以接受指向 Box 和指向 i32 的指针?