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Change reference to inner struct, or is it possible to change the reference of a reference in rust(将引用更改为内部结构，或者是否可以更改Ruust中引用的引用)

转载 作者：bug小助手 更新时间：2023-10-27 20:00:42 33 4
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I'm currently trying to translate a C program to Rust.

我目前正在尝试将一个C程序翻译成Rust。


The C programm has following structures (boiled down to MRE):

C程序具有以下结构(归结为MRE)：


typedef struct {
    bool isActive;
    uint32_t value;
} tMyInnerStruct;

typedef struct {
    tMyInnerStruct inner1;
    tMyInnerStruct inner2;
} tMyStruct;

and in a function I would do the following pointer magic:

在一个函数中，我将执行以下指针魔术：


void myFunction()
{
    tMyStruct myStruct = initMyStuct();
    tMyInnerStruct *pActiveInner = &myStruct.inner1;
    // Do something with pActiveInner (pointing to inner1), like setting value...
    changeActiveStruct(&myStruct, &pActiveInner);
    // Do something with pActiveInner (pointing to inner2), like setting value...
}

void changeActiveStruct( tMyStruct *pMyStruct, tMyInnerStruct **ppActiveInner )
{
    if ( &pMyStruct->inner1 == *ppActiveInner ) {
        *ppActiveInner = &pMyStruct->inner2;
    } else {
        *ppActiveInner = &pMyStruct->inner1;
    }
}

No my question is: How would I achieve the same in Rust?

不，我的问题是：我将如何在铁锈中实现同样的目标？


What I've tried so far in Rust:

到目前为止，我在《铁锈》中尝试过的是：


#[derive(Default)]
struct MyInnerStruct {
    is_active: bool,
    value: u32,
}

#[derive(Default)]
struct MyStruct {
    inner1: MyInnerStruct,
    inner2: MyInnerStruct,
}

impl MyStruct {
    fn change_active(&mut self, active: &MyInnerStruct) -> &mut MyInnerStruct {
        if &self.inner1 as *const _ == active as *const _ {
            &mut self.inner2
        } else {
            &mut self.inner1
        }
    }
}

fn main() {
    let mut my_struct: MyStruct = Default::default();
    let mut active = &mut my_struct.inner1;
    active = my_struct.change_active(active);
    // The above complains, that my_struct cannot be borrowed mutable more than once...
}

更多回答
优秀答案推荐

You can accompilsh the same thing by using a pointer as argument instead of a reference. Because of the order that arguments and the receiver are evaluated in, you have to convert to a pointer ahead of the call and can't do it in the parameter list but this works:

您可以通过使用指针作为参数而不是引用来编译相同的内容。由于参数和接收器的求值顺序，您必须转换为调用之前的指针，并且不能在参数列表中这样做，但这是可行的：


impl MyStruct {
    fn change_active(&mut self, active: *const MyInnerStruct) -> &mut MyInnerStruct {
        if &self.inner1 as *const _ == active {
            &mut self.inner2
        } else {
            &mut self.inner1
        }
    }
}

fn main() {
    let mut my_struct: MyStruct = Default::default();
    let mut active = &mut my_struct.inner1; 

    active = { 
        let active = active as *const _;
        my_struct.change_active(active)
    };
}

Playground

操场



Minor detail, change_active does no longer seem like an apropriate name since the Rust version doesn't actually change active maybe new_active or something else still is a better name for the method

次要细节，CHANGE_ACTIVE看起来不再像是一个合适的名称，因为Rust版本实际上不会更改ACTIVE可能是NEW_ACTIVE或其他名称仍然是该方法的更好名称


更多回答

Thanks, this is what I was searching for :-) The cast during the comparison was already a hint, but I'm not so familiar with the pointers in Rust. The only drawback in my opinion, is the scoped shadowing of the active variable, as it's not a one-liner ;-)

谢谢，这就是我正在寻找的：-)对比过程中的演员阵容已经是一个提示，但我对《铁锈》中的指针不是很熟悉。在我看来，唯一的缺点是活动变量的作用域跟踪，因为它不是一行程序；-)

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