强制 auto 成为循环范围内的引用类型 [英] Forcing auto to be a reference type in a range for loop

问题描述

假设我有 foo，它是一个填充的 std::vector.

Suppose I have foo which is a populated std::vector<double>.

我需要对这个向量的元素进行操作.我有动力写

I need to operate on the elements of this vector. I'm motivated to write

for (auto it : foo){
   /*ToDo - Operate on 'it'*/
}

但看起来这不会写回 foo，因为 it 是一个值类型:向量元素的深层副本已被获取.

But it appears that this will not write back to foo since it is a value type: a deep copy of the vector element has been taken.

我可以为 auto 提供一些指导以使 it 成为引用类型吗?然后我就可以直接对it进行操作了.

Can I give some guidance to auto to make it a reference type? Then I could operate directly on it.

我怀疑我遗漏了一些琐碎的语法.

I suspect I'm missing some trivial syntax.

推荐答案

最小的 auto 参考

A minimal auto reference

循环可以声明如下:

for (auto& it : foo) {
   //    ^ the additional & is needed
   /*ToDo - Operate on 'it'*/
}

这将允许 it 成为对 foo 中每个元素的引用.

This will allow it to be a reference to each element in foo.

对于这些循环的规范形式"存在一些争论，但在这种情况下 auto& 应该可以解决问题.

There is some debate as to the "canonical form" of these loops, but the auto& should do the trick in this case.

一般auto参考

General auto reference

在更一般的意义上(在容器的细节之外)，以下循环有效(并且可能更可取).

In a more general sense (outside the specifics of the container), the following loop works (and may well be preferable).

for (auto&& it : container) {
   //    ^ && used here
}

auto&& 允许绑定到左值和右值.当在通用或一般(例如模板情况)中使用时，此表单可能会达到所需的平衡(即引用、副本、prvalue/xvalue 返回(例如代理对象)等).

The auto&& allows for bindings to lvalues and rvalues. When used in a generic or general (e.g. template situation) this form may strike the desired balance (i.e. references, copies, prvalue/xvalue returns (e.g. proxy objects) etc.).

倾向于使用一般的 auto&&，但如果您必须对表单进行具体说明，则使用更具体的变体(例如 auto、autoconst& 等).

Favour the general auto&&, but if you have to be specific about the form, then use a more specific variation (e.g. auto, auto const& etc.).

为什么 auto&& 更好?

Why is auto&& better?

如此处的其他答案和评论中所述.为什么 auto&& 更好?在大多数情况下，它会做你认为应该做的事情，参见 本提案 和 更新.

As noted in other answers here and the comments. Why is auto&& better? Simply it will do what you think it should in most cases, see this proposal and its update.

一如既往，Scott Meyers 的 博客 也适合阅读此内容.

As always, Scott Meyers' blog about this also makes for a good read.

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