在C ++中引用参考 [英] Reference of Reference in C++

问题描述

我每隔一段时间看到StackOverflow上的代码，询问一些涉及一个函数的重载模糊性：

  void foo（int&& param）; 
  

我的问题是：为什么会出现？或者，你什么时候有引用参考？这与普通的旧引用有什么不同？我从来没有遇到这种现实世界的代码，所以我很好奇什么样的代码将需要这个。

解决方案

这是一个有价值的参考资料，Bjarne在此处。

无耻的复制（quoting）：

右值参考
$ b

右值引用是一个复合类型
，非常类似于C ++的传统
引用。为了更好地区分这些
两种类型，我们将传统的
C ++引用称为左值引用。
当使用术语引用时，
引用两种引用：
lvalue引用和rvalue引用。

引用由
形成，

  A a; A& a_ref1 = a; //一个左值引用
  

右值引用由
形成，放置&&后一些类型。

  A a; A&& a_ref2 = a; //一个右值引用
  

右值引用的行为就像
一个左值引用，可以
绑定到一个临时（一个右值），
，而你不能绑定一个（非const）
左值引用一个右值。

  a_ref3 = A（）; //错误！ 
 A&& a_ref4 = A（）; // Ok 
  

I see code on StackOverflow every once in a while, asking about some overload ambiguity with something involving a function like:

void foo(int&& param);

My question is: Why does this even come up? Or rather, when would you ever have "a reference to a reference"? How is that any different from a plain old reference? I've never run across this in real-world code, so I'm curious as to what kind of code would need this.

解决方案

It's a rvalue reference, Bjarne describes it here.

Shameless copying ("quoting"):

The rvalue reference

An rvalue reference is a compound type very similar to C++'s traditional reference. To better distinguish these two types, we refer to a traditional C++ reference as an lvalue reference. When the term reference is used, it refers to both kinds of reference: lvalue reference and rvalue reference.

An lvalue reference is formed by placing an & after some type.

A a; A& a_ref1 = a;  // an lvalue reference

An rvalue reference is formed by placing an && after some type.

A a; A&& a_ref2 = a;  // an rvalue reference

An rvalue reference behaves just like an lvalue reference except that it can bind to a temporary (an rvalue), whereas you can not bind a (non const) lvalue reference to an rvalue.

A&  a_ref3 = A();  // Error! 
A&& a_ref4 = A();  // Ok

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