C++11 binding rules for const &&

问题

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

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).

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?

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

回答1:

'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

So then reading the quote:

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

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

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

来源：https://stackoverflow.com/questions/27760290/c11-binding-rules-for-const

标签

c++

c++11

const

rvalue-reference