[英]Can I efficiently return an object by value in Rust?
I would like to initialize a large object with a function. 我想用一个函数初始化一个大对象。 Currently I have:
目前我有:
fn initialize(mydata: &mut Vec<Vec<MyStruct>>) { /* ... */ }
I would prefer to have: 我希望拥有:
fn initialize() -> Vec<Vec<MyStruct>> { /* ... */ }
I've heard that C++ often implements return value optimization (RVO), if you are lucky and have a good compiler. 我听说C ++通常会实现返回值优化(RVO),如果您幸运的话,并且具有良好的编译器。 Can we disable copying here and have it returned by a hidden pointer that is passed into the function?
我们可以在这里禁用复制,并通过传递给函数的隐藏指针将其返回吗? Is RVO part of the language or an optional optimization?
RVO是语言的一部分还是可选的优化?
Yes, by all means, you should write 是的,一定要写
fn initialize() -> Vec<Vec<MyStruct>> { ... }
(By the way, a Vec
is not that large - it's only 3 pointer-sized integers) (顺便说一下,
Vec
并不是那么大-它只有3个指针大小的整数)
Rust has RVO, and this is advertised in guides . Rust具有RVO,这已在guide中公布 。 You can see it yourself with this code:
您可以使用以下代码自己看到它:
#[inline(never)]
fn initialize() -> Vec<i32> {
Vec::new()
}
fn main() {
let v = initialize();
}
If you compile this program in release mode on the playground , outputting assembly, among everything else you will see this: 如果您在发布模式下编译这个程序在操场上 ,输出组件,一切之中否则你会看到:
playground::initialize:
movq $4, (%rdi)
xorps %xmm0, %xmm0
movups %xmm0, 8(%rdi)
retq
Vec::new()
was inlined, but you can see the idea - the address for the fresh Vec
instance is passed into the function in %rdi
, and the function stores Vec
fields directly into this memory, avoiding unnecessary copying through the stack. 内联了
Vec::new()
,但是您可以看到这个想法-新鲜的Vec
实例的地址传递到%rdi
的函数中,并且该函数将Vec
字段直接存储到此内存中,从而避免了通过堆栈进行不必要的复制。 This is how it is called: 这就是所谓的:
playground::main:
subq $24, %rsp
movq %rsp, %rdi
callq playground::initialize
You can see that eventually the Vec
instance will be put directly into the stack memory. 您可以看到最终
Vec
实例将直接放入堆栈内存中。
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