035 Rust死靈書之Vec處理零尺寸型別

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...

在我們之前的MyVec的實現中，一直沒有考慮零尺寸型別，本節我們來完善。

在處理零尺寸型別時，我們需要注意以下幾點：

• 當分配器API傳遞分配尺寸為0時，會導致未定義行為；
• 對零尺寸型別的裸指標做offset是一個no-op，這會破壞我們的C-style指標迭代器。

支援零尺寸型別的程式碼如下：

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

#[derive(Debug)]
struct RawVec<T> {
    ptr: Unique<T>,
    cap: usize,
}

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

    fn grow(&mut self) {
        unsafe {
            let align = mem::align_of::<T>();
            let elem_size = mem::size_of::<T>();
            assert!(elem_size != 0, "capacity overflow");
            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;
        }
    }
}

impl<T> Drop for RawVec<T> {
    fn drop(&mut self) {
        let elem_size = mem::size_of::<T>();

        if self.cap != 0 && elem_size != 0 {
            let align = mem::align_of::<T>();
            let elem_size = mem::size_of::<T>();
            let num_bytes = elem_size * self.cap;

            unsafe {
                let layout: Layout = Layout::from_size_align_unchecked(num_bytes, align);
                dealloc(self.ptr.as_ptr() as *mut _, layout)
            }

            println!("release memory in drop function!");
        }
    }
}

struct IntoIter<T> {
     _buf: RawVec<T>, 
    iter: RawValIter<T>,
}

impl<T> Drop for IntoIter<T> {
    fn drop(&mut self) {
        for _ in &mut *self {}
    }
}

#[derive(Debug)]
pub struct MyVec<T> {
    buf: RawVec<T>,
    len: usize,
}

impl<T> MyVec<T> {
    fn ptr(&self) -> *mut T { 
        self.buf.ptr.as_ptr() 
    }

    fn cap(&self) -> usize { 
        self.buf.cap 
    }

    pub fn new() -> Self {
        MyVec { buf: RawVec::new(), len: 0 }
    }

    fn push(&mut self, elem: T) {
        if self.len == self.cap() { 
            self.buf.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().offset(self.len as isize), elem);
        }

        self.len += 1;
    }

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

    fn insert(&mut self, index: usize, elem: T) {
        assert!(index <= self.len, "越界");
        if self.cap() == self.len { 
            self.buf.grow(); 
        }

        unsafe {
            if index < self.len {
                ptr::copy(self.ptr().offset(index as isize),
                        self.ptr().offset((index as isize) + 1),
                        self.len - index);
            }
            ptr::write(self.ptr().offset(index as isize), elem);
            self.len += 1;
        }
    }

    fn remove(&mut self, index: usize) -> T {
        assert!(index < self.len, "越界");
        unsafe {
            self.len -= 1;
            let result = ptr::read(self.ptr().offset(index as isize));
            ptr::copy(self.ptr().offset(index as isize + 1), 
                    self.ptr().offset(index as isize),
                    self.len - index);
            result
        }
    }

    fn into_iter(self) -> IntoIter<T> {
        unsafe {
            let iter = RawValIter::new(&self);
            let buf = ptr::read(&self.buf);
            mem::forget(self);

            IntoIter {
                iter: iter,
                _buf: buf,
            }
        }
    }
}

impl<T> Drop for MyVec<T> {
    fn drop(&mut self) {
        while let Some(_) = self.pop() {}
    }
}

impl<T> Deref for MyVec<T> {
    type Target = [T];
    fn deref(&self) -> &[T] {
        unsafe {
            slice::from_raw_parts(self.buf.ptr.as_ptr(), self.len)
        }
    }
}

impl<T> DerefMut for MyVec<T> {
    fn deref_mut(&mut self) -> &mut [T] {
        unsafe {
            slice::from_raw_parts_mut(self.buf.ptr.as_ptr(), self.len)
        }
    }
}

//為IntoIter實現迭代器
impl<T> Iterator for IntoIter<T> {
    type Item = T;
    fn next(&mut self) -> Option<T> {
        self.iter.next()
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
}

impl<T> DoubleEndedIterator for IntoIter<T> {
    fn next_back(&mut self) -> Option<T> {
        self.iter.next_back()
    }
}

struct RawValIter<T> {
    start: *const T,
    end: *const T,
}

impl<T> RawValIter<T> {
    unsafe fn new(slice: &[T]) -> Self {
        RawValIter {
            start: slice.as_ptr(),
            end: if mem::size_of::<T>() == 0 {
                ((slice.as_ptr() as usize) + slice.len()) as *const _
            } else if slice.len() == 0 {
                slice.as_ptr()
            } else {
                slice.as_ptr().offset(slice.len() as isize)
            }
        }
    }
}

impl<T> Iterator for RawValIter<T> {
    type Item = T;
    fn next(&mut self) -> Option<T> {
        if self.start == self.end {
            None
        } else {
            unsafe {
                let result = ptr::read(self.start);
                self.start = if mem::size_of::<T>() == 0 {
                    (self.start as usize + 1) as *const _
                } else {
                    self.start.offset(1)
                };
                Some(result)
            }
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let elem_size = mem::size_of::<T>();
        let len = (self.end as usize - self.start as usize) 
                / if elem_size == 0 { 0 } else { elem_size };
        (len, Some(len))
    }
}


impl<T> DoubleEndedIterator for RawValIter<T> {
    fn next_back(&mut self) -> Option<T> {
        if self.start == self.end {
            None
        } else {
            unsafe {
                self.end = if mem::size_of::<T>() == 0 {
                    (self.end as usize - 1) as *const _
                } else {
                    self.end.offset(-1)
                };
                Some(ptr::read(self.end))
            }
        }
    }
}

use std::marker::PhantomData;
//Drain是一個API集合，將容器內資料的所有權移出，卻不佔有容器本身
struct Drain<'a, T: 'a> {
    vec: PhantomData<&'a mut MyVec<T>>,
    iter: RawValIter<T>,
}

impl<'a, T> Iterator for Drain<'a, T> {
    type Item = T;
    fn next(&mut self) -> Option<T> { 
        self.iter.next() 
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
}

impl<'a, T> DoubleEndedIterator for Drain<'a, T> {
    fn next_back(&mut self) -> Option<T> { self.iter.next_back() }
}

impl<'a, T> Drop for Drain<'a, T> {
    fn drop(&mut self) {
        for _ in &mut self.iter {}
    }
}

impl<T> MyVec<T> {
    fn drain(&mut self) -> Drain<T> {
        unsafe {
            let iter = RawValIter::new(&self);
            self.len = 0;
            Drain {
                iter: iter,
                vec: PhantomData,
            }
        }
    }
}


fn main() {
    {
        let mut vec: MyVec<i32> = MyVec::new();
        vec.push(8);
        vec.push(7);
        vec.push(6);

        while let Some(v) = vec.pop() {
            println!("v == {}", v);
        }

        vec.push(8);
        vec.push(7);
        vec.push(6);

        let s = &vec[1..];
        println!("s[0] == {}", s[0]);

        let s = &mut vec[1..];
        s[0] = 10;
        println!("s[0] == {}", s[0]);

        println!("-------------------------------");
        let mut vec2: MyVec<i32> = MyVec::new();
        vec2.push(1);
        vec2.push(2);
        vec2.push(3);
        //列印
        //while let Some(v) = vec2.pop() {
        //    println!("v == {}", v);
        //}

        vec2.insert(1, 11);
        let ret = vec2.remove(2);
        println!("remove elem: {}", ret);
        //列印
        while let Some(v) = vec2.pop() {
            println!("v == {}", v);
        }


        println!("-------------------------------");
        let mut vec3: MyVec<i32> = MyVec::new();
        vec3.push(1);
        vec3.push(2);
        vec3.push(3);
        vec3.push(4);
        let iter = vec3.iter();

        for val in iter {
            println!("Got: {}", val);
        }

        println!("-------------------------------");
        let iter3: IntoIter<i32> = vec3.into_iter();
        for mut val in iter3 {
            println!("Got: {}", val);
            val = 111;
            println!("Got: {}", val);
        }

        println!("-------------------------------");
        let mut vec4: MyVec<i32> = MyVec::new();
        vec4.push(1);
        vec4.push(2);
        let mut iter4: IntoIter<i32> = vec4.into_iter();
        while let Some(val) = iter4.next_back() {
            println!("Got: {}", val);
        }


        println!("-------------------------------");
        let mut vec5: MyVec<i32> = MyVec::new();
        vec5.push(11);
        vec5.push(12);
        vec5.push(13);
        vec5.push(14);
        let mut drain = vec5.drain();
        let a = drain.next().unwrap();
        println!("drain: {}", a);
        //從下面的列印可以看出已經借用了第一個元素
        while let Some(val) = drain.next_back() {
            println!("Got: {}", val);
        }

    }

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

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