Rust 學習日記 #UnsafeCell

大家好，今天分享一個我自己實現的一個UnsafeCell，我也是最近剛開始認真學Rust，如果有什麼說錯了，請各位大佬指教。

use std::{fmt, mem};
use std::alloc::{alloc, handle_alloc_error, Layout};

struct SimpleUnsafeCell<T: fmt::Display> {
    value: *mut T,
}

impl<T: fmt::Display> SimpleUnsafeCell<T> {
    fn new(v: T) -> Self {
        unsafe {
            let layout = Layout::new::<T>();

            let ptr = alloc(layout) as *mut T; // 申請T Size的內存

            if ptr.is_null() { // 如果內存申請出現了有什麼問題，導致null pointer，交給handle_alloc_error處理
                handle_alloc_error(layout);
            }

            ptr.write(v); // 把v的值寫入內存中。

            Self { value: ptr } // 把value指針指向ptr所指的內存。
        }
    }

    unsafe fn get_mut(&self) -> &mut T {
        &mut *self.value
    }

    unsafe fn get_ref(&self) -> &T {
        &*self.value
    }
}

impl<T: fmt::Display> Drop for SimpleUnsafeCell<T> {
    fn drop(&mut self) {
        unsafe {
            // 顯式調用T的釋構函數
            std::ptr::drop_in_place(self.value);

            // 然後再把申請來的內存釋放掉
            std::alloc::dealloc(
                self.value as *mut u8,
                Layout::new::<T>()
            );
        }
    }
}

impl<T: fmt::Display> fmt::Display for SimpleUnsafeCell<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", unsafe { self.get_mut() })
    }
}

struct Point<T: fmt::Display> {
    x: SimpleUnsafeCell<T>,
    y: SimpleUnsafeCell<T>,
}

impl<T: fmt::Display> fmt::Display for Point<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{} {}", self.x, self.y)
    }
}
fn main() {
    let mut suc = SimpleUnsafeCell::new(String::from("Hello"));

    unsafe {
        let x = suc.get_ref();

        println!("{}", *x);

        let y = suc.get_mut();

        y.push_str(", world!");

        println!("{}", *x);
    }

    let point = Point { x: SimpleUnsafeCell::new(1), y: SimpleUnsafeCell::new(1) };

    println!("{}", point);
}