028 Rust死靈書之Vec的push和pop

本系列錄製的影片主要放在B站上Rust死靈書學習影片

Rust 死靈書相關的原始碼資料在github.com/anonymousGiga/Rustonomi...

本節給MyVec實現push和pop方法，完整程式碼如下：

#![feature(ptr_internals)]
use std::mem;
use std::alloc::{alloc, realloc, Layout, handle_alloc_error};
use std::ptr::{Unique, self};

pub struct MyVec<T> {
    ptr: Unique<T>,
    cap: usize,
    len: usize,
}

impl<T> MyVec<T> {
    fn new() -> Self {
        assert!(mem::size_of::<T>() != 0, "還沒準備好處理零尺寸型別");
        MyVec { ptr: Unique::dangling(), len: 0, cap: 0 }
    }

    fn grow(&mut self) {
        unsafe {
            let align = mem::align_of::<T>();
            let elem_size = mem::size_of::<T>();
            let layout: Layout;

            let (new_cap, ptr) = if self.cap == 0 {
                layout = Layout::from_size_align_unchecked(elem_size, align);
                let ptr = alloc(layout);
                (1, ptr)
            } else {
                let new_cap = self.cap * 2;
                let old_num_bytes = self.cap * elem_size;

                assert!(old_num_bytes <= (isize::MAX as usize) / 2,
                        "capacity overflow");


                let new_num_bytes = old_num_bytes * 2;
                layout = Layout::from_size_align_unchecked(new_num_bytes, align);
                let ptr = realloc(self.ptr.as_ptr() as *mut _,
                                      layout,
                                    new_num_bytes);
                (new_cap, ptr)
            };

            if ptr.is_null() { handle_alloc_error(layout); }

            if let Some(ptr) = Unique::new(ptr as *mut _) {
                self.ptr = ptr;
            } else {
                panic!("error!");
            }
            self.cap = new_cap;
        }
    }

    fn push(&mut self, elem: T) {
        if self.len == self.cap { 
            self.grow(); 
        }

        //關鍵點在於要直接覆蓋，因為不知道記憶體之前是否有東西
        unsafe {
            ptr::write(self.ptr.as_ptr().offset(self.len as isize), elem);
        }

        self.len += 1;
    }

    pub fn pop(&mut self) -> Option<T> {
        if self.len == 0 {
            None
        } else {
            self.len -= 1;
            unsafe {                       
                Some(ptr::read(self.ptr.as_ptr().offset(self.len as isize)))
            }
        }
    }
}

fn main() {
    let mut vec: MyVec<i32> = MyVec::new();
    vec.push(8);
    if let Some(v) = vec.pop() {
        println!("v == {}", v);
    }

    println!("Hello, world!");
}

關鍵點說明：

實現pop的時候，需要對記憶體直接寫入。

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