强制自动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 这是一个填充的 std :: vector< double> 。

我需要对这个向量的元素进行操作。我有动力写下

  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：已经采用了向量元素的深拷贝。

我可以给 auto 一些指导，使 it 引用类型吗？然后我可以直接在 it 上进行操作。

我怀疑我缺少一些微不足道的语法。

解决方案

最少自动参考 b
$ b

循环可以声明如下：

  for（auto& it：foo） {
 // ^附加&需要
 / * ToDo  - 操作'it'* / 
} 
  

这将允许 it 是对 foo 中每个元素的引用。

对于这些循环的规范形式有一些争论，但 auto& 在这种情况下应该做。

一般自动参考

一个更一般的意义（在容器的细节之外），以下循环工作（并且可能是更好的）。

  （auto&& it：container）{
 // ^&&用于此处
} 
  

自动& c $ c>允许绑定到左值和右值。当在一般或一般（例如模板情况）中使用时，这种形式可以达到期望的平衡（即引用，副本，prvalue / xvalue返回（例如代理对象）等）。

喜欢一般的 auto&&& ，但如果您必须具体说明表单，请使用更具体的变体（例如 auto

为什么<$ <$ p $ <$> c $ c> auto&&&< / code>更好？

为什么 auto&&& 更好？只要它会做你认为它应该在大多数情况下，看到此提案和其更新。

一如往常，Scott Meyers的

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

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

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

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.

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

I suspect I'm missing some trivial syntax.

解决方案

A minimal auto reference

The loop can be declared as follows:

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

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

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

General auto reference

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
}

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.).

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.).

Why is auto&& better?

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.

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

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