创建无法在其板条箱外部实例化的零大小结构的惯用方式是什么? [英] What is an idiomatic way to create a zero-sized struct that can't be instantiated outside its crate?

问题描述

我有类似的东西:

mod private {
    // My crate
    pub struct A;
    
    impl A {
        pub fn new() -> Self {
            Self
        }
        // ...
    }
}

fn main() {
    // External code
    let obj = private::A::new();
    let obj2 = private::A;
}

当前， A 不需要存储任何内部状态即可执行我想要的操作(它只是在枚举中用作占位符)，因此我将其设置为零大小结构.但是，将来可能会发生变化，因此我想防止此板条箱外的代码实例化 A 而不进行 A :: new()(即< main()中的code> obj2 应该会失败).

Currently, A doesn't need to store any internal state to do what I want it to (it's just being used as a placeholder in an enum), so I made it a zero-sized struct. However, that might change in the future, so I want to prevent code outside this crate from instantiating A without going through A::new() (i.e. the instantiation of obj2 in main() as it stands should fail).

本质上，我希望获得与向 A 添加一个私有字段一样的效果，但是我希望它保持零大小.

Essentially, I want the same effect as if I'd added a private field to A, but I want it to remain zero-sized.

目前，我正在考虑类似这样的事情:

Currently, I'm thinking about something like this:

pub struct A {
    empty: (),
}

或者这个:

pub struct A(());

但是，我不确定哪种方法最惯用.

However, I'm not sure which way is most idiomatic.

推荐答案

没有为此建立的惯用语.我会使用元组struct版本:

There is no established idiom for this. I'd use the tuple struct version:

pub struct A(());

由于该字段是私有字段，因此该模块之外的任何人都无法构造 A .

Since the field is private, no one outside that module can construct A.

这用于在标准库中:

pub struct Stdin(());
pub struct Stdout(());
pub struct Stderr(());

标准库

The standard library also uses the named field version:

pub struct Empty {
    _priv: (),
}

您可以使用任何零大小的类型(例如，零长度数组或

You could use any zero-sized type (e.g. a zero-length array or PhantomData), but () is the simplest.

另请参阅:

• 为什么用单位类型的单个私有字段定义结构?
• 定义一个指向C透明指针的字段的Rust习惯用法是什么?
• 创建零大小结构的多种方法之间有什么区别?

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