让我们来看看来源 https://doc.rust-lang.org/stable/src/collections/vec.rs.html#1715-1734的实施情况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
迭代器会产生多次额外的分配,但不会针对向量的元素进行分配;相反,向量的底层内存详细信息被注册。怎么样the code https://doc.rust-lang.org/stable/src/core/iter/iterator.rs.html#479-483 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。最后一块拼图是collect() https://doc.rust-lang.org/stable/src/core/iter/iterator.rs.html#1255-1257:
fn collect<B: FromIterator<Self::Item>>(self) -> B where Self: Sized {
FromIterator::from_iter(self)
}
这里没有答案;关于什么实施 https://doc.rust-lang.org/stable/src/collections/vec.rs.html#1689-1694 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())
}
}
这开始看起来像魔法,但也许相关规范扩展代码 https://doc.rust-lang.org/stable/src/collections/vec.rs.html?search=#1773-1798将揭示我们正在寻找的内容:
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
and Vec::with_capacity
调用的方法为结果向量分配新的空间。
TL;DR: 不,不能移动and修改向量而不需要额外的分配。