`auto`说明符类型推导以供参考 [英] `auto` specifier type deduction for references

问题描述

让我们考虑以下代码段

void Test()
  {
  int x = 0;

  int& rx = x;
  int* px = &x;

  auto apx = px;    // deduced type is int*
  auto arx = rx;    // deduced type is int
  }

可以从指针类型得出一个类比，期望 arx 的推导类型是 int& ，但它是 int 实际上。

One could draw an analogy from pointer types expecting that the deduced type of arx is int&, but it is int in fact.

标准中的规则是什么？背后的原因是什么？
有时在这样的情况下会被它抓住：

What is the rule in Standard which governs that? What is the reason behind it? Sometimes I get caught by it in a case like this:

const BigClass& GetBigClass();
...
auto ref_bigclass = GetBigClass();   // unexpected copy is performed

推荐答案

最简单的思考方式

给出：

template<typename T>
void deduce(T) { }

如果您致电：

deduce(px);

然后将推导出模板参数 T 为 int * ，并且如果您致​​电

then the template argument T will be deduced as int* and if you call

deduce(rx);

然后将 T 推导为 int ，而不是 int&

then T will be deduced as int, not int&

当使用 auto 时。

可以从指针类型得出一个类比，期望 arx 的推导类型为 int&

您必须具有相当混乱的C ++语言模型才能进行类比。仅仅因为它们在语法上的声明方式相似，例如带有类型和修饰符的 Type @ 并不能使它们以相同的方式工作。指针是一个值，一个对象，可以将其复制并通过赋值更改其值。引用不是对象，而是对某些对象的引用。引用不能复制（复制引用会复制引用对象）或更改（分配给引用会更改引用对象）。返回指针的函数按值返回对象 （所讨论的对象为指针对象），但是返回引用的函数（如您的 GetBigClass（））通过引用返回对象 。它们是完全不同的语义，试图在指针和引用之间绘制类比注定会失败。

You'd have to have a fairly confused model of the C++ language to make that analogy. Just because they are declared in syntactically similar ways, as Type@ with a type and a modifier doesn't make them work the same way. A pointer is a value, an object, and it can be copied and have its value altered by assignment. A reference is not an object, it's a reference to some object. A reference can't be copied (copying it copies the referent) or altered (assigning to it alters the referent). A function that returns a pointer returns an object by value (the object in question being a pointer object), but a function that returns a reference (like your GetBigClass()) returns an object by reference. They're completely different semantics, trying to draw analogies between pointers and references is doomed to failure.

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