C ++ 11绑定规则为const&& [英] C++11 binding rules for const &&

问题描述

许多人不知道 const 右值引用是C ++ 11语言的一部分。 此博文讨论了这些问题，但出现关于约束规则的错误。引用博客：

Many people do not know that const rvalue references are part of the C++11 language. This blog post discusses them but appears to be mistaken regarding the binding rules. Quoting the blog:

struct s {};

void f (      s&);  // #1
void f (const s&);  // #2
void f (      s&&); // #3
void f (const s&&); // #4

const s g ();
s x;
const s cx;

f (s ()); // rvalue        #3, #4, #2
f (g ()); // const rvalue  #4, #2
f (x);    // lvalue        #1, #2
f (cx);   // const lvalue  #2

注意不对称：while const lvalue引用可以绑定到一个右值，
a const右值引用不能绑定到一个左值。在
特殊情况下，这使得一个常量引用能够做所有
a常量值引用可以和更多（即绑定到左值）。

Note the asymmetry: while a const lvalue reference can bind to an rvalue, a const rvalue reference cannot bind to an lvalue. In particular, this makes a const lvalue reference able to do everything a const rvalue reference can and more (i.e., bind to lvalues).

对示例代码的注释似乎在我安装的GCC 4.9（带有-std = c ++ 14标志设置）上签出。因此，与博客文本相反， const&&& 应该绑定到 const& const&& 和 const& 只绑定到 const& ？

The comments on the sample code appear to check out on my installation of GCC 4.9 (with -std=c++14 flag set). So, contrary to the blog text, is it true that const && should bind to const & and const && and const & only bind to const &? If not what is the actual rule?

这是一个演示，显示 const& ;& 绑定到GCC 4.9中的 const& ： http://coliru.stacked-crooked.com/a/794bbb911d00596e

Here is a demo that appears to show const && binding to const& in GCC 4.9: http://coliru.stacked-crooked.com/a/794bbb911d00596e

推荐答案

'binding'在这个上下文中意味着绑定对特定对象的引用。

'binding' in this context means tying a reference to a particular object.

int a;

int &b = a; // the reference is 'bound' to the object 'a'

void foo(int &c);

foo(a); // the reference parameter is bound to the object 'a'
        // for this particular execution of foo.

http://coliru.stacked-crooked.com/a/5e081b59b5e76e03

然后阅读引文：

注意不对称：虽然const lvalue引用可以绑定到右值，

Note the asymmetry: while a const lvalue reference can bind to an rvalue,

void foo(int const &);

foo(1); // the const lvalue reference parameter is bound to
        // the rvalue resulting from the expression '1'

http://coliru.stacked-crooked.com/a/12722f2b38c74c75

一个常量引用不能绑定到一个左值。

a const rvalue reference cannot bind to an lvalue.

void foo(int const &&);

int a;

foo(a); // error, the expression 'a' is an lvalue; rvalue
        //references cannot bind to lvalues

http://coliru.stacked-crooked.com/a/ccadc5307135c8e8

特别是，这使得一个常量引用能够做一个常量引用可以和更多（即绑定到左值）。

In particular, this makes a const lvalue reference able to do everything a const rvalue reference can and more (i.e., bind to lvalues).

void foo(int const &);

foo(1); // const lvalue reference can bind to rvalue

int a;
foo(a); // and lvalue

http://coliru.stacked-crooked.com/a/d5553c99e182c89b

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