Rvalue References and Perfect Forwarding

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http://www.justsoftwaresolutions.co.uk/cplusplus/rvalue_references_and_perfect_forwarding.html

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X x;
X& lref = x;          // bind lvalue reference to object: fine
X&& rref = x;       // bind rvalue reference to object: fine too.
X&& rref2 = lref;  // bind rvalue reference to lvalue ref: fine
X& lref2 = X();     // bind lvalue reference to temporary (rvalue): error
X&& rref3 = X();  // bind rvalue reference to temporary (rvalue): fine, extends lifetime of temporary