Reference of Reference in C++

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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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