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
'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