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摘要：
std::forward is designed for use in a template function which takes its arguments by T&&, where T is a template parameter.
std::forward preserves the rvalue-ness of the arguments, so if your function was called with an rvalue then std::forward<T> provides an rvalue. If your function was called with an lvalue then std::forward<T> provides an lvalue.
This works because of the way T is deduced from a T&& parameter: T is deduced to be "X&", for an lvalue argument of type X, whereas it is deduced to be plain "X" for an rvalue argument.