更新时间:2023-11-22 23:19:28
Let's see the source of the implementation of into_iter()
for Vec<T>
:
fn into_iter(mut self) -> IntoIter<T> {
unsafe {
let begin = self.as_mut_ptr();
assume(!begin.is_null());
let end = if mem::size_of::<T>() == 0 {
arith_offset(begin as *const i8, self.len() as isize) as *const T
} else {
begin.offset(self.len() as isize) as *const T
};
let cap = self.buf.cap();
mem::forget(self);
IntoIter {
buf: Shared::new(begin),
cap: cap,
ptr: begin,
end: end,
}
}
}
创建IntoIter
迭代器会产生一些额外的分配,但不会分配给向量的元素;相反,将注册向量的基础内存详细信息. 代码如何在map()
后面?
Creating the IntoIter
iterator incurs several extra allocations, but not for the elements of the vector; instead, the vector's underlying memory details are registered. How about the code behind map()
?
fn map<B, F>(self, f: F) -> Map<Self, F> where
Self: Sized, F: FnMut(Self::Item) -> B,
{
Map{iter: self, f: f}
}
这里也没有分配额外的向量.最后一个难题是 collect()
:
No extra vectors allocated here either. The last piece of the puzzle is collect()
:
fn collect<B: FromIterator<Self::Item>>(self) -> B where Self: Sized {
FromIterator::from_iter(self)
}
这里没有答案; 的实现 c14>代表Vec<T>
?
No answers here; what about the implementation of from_iter()
for Vec<T>
?
impl<T> FromIterator<T> for Vec<T> {
#[inline]
fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Vec<T> {
<Self as SpecExtend<T, I::IntoIter>>::from_iter(iter.into_iter())
}
}
This is beginning to look like magic, but perhaps the related SpecExtend code will reveal what we're looking for:
impl<T, I> SpecExtend<T, I> for Vec<T>
where I: Iterator<Item=T>,
{
default fn from_iter(mut iterator: I) -> Self {
// Unroll the first iteration, as the vector is going to be
// expanded on this iteration in every case when the iterable is not
// empty, but the loop in extend_desugared() is not going to see the
// vector being full in the few subsequent loop iterations.
// So we get better branch prediction.
let mut vector = match iterator.next() {
None => return Vec::new(),
Some(element) => {
let (lower, _) = iterator.size_hint();
let mut vector = Vec::with_capacity(lower.saturating_add(1));
unsafe {
ptr::write(vector.get_unchecked_mut(0), element);
vector.set_len(1);
}
vector
}
};
<Vec<T> as SpecExtend<T, I>>::spec_extend(&mut vector, iterator);
vector
}
default fn spec_extend(&mut self, iter: I) {
self.extend_desugared(iter)
}
}
在这段代码中,我们终于可以看到Vec::new
和Vec::with_capacity
方法,它们为所得的向量分配新的空间.
In this code we can finally see the Vec::new
and Vec::with_capacity
methods called which allocate fresh space for the resulting vector.
TL; DR :不,如果没有额外的分配,不可能移动和修改向量.
TL;DR: no, it is not possible to move and modify a vector without an extra allocation.