What is value_type&& in C++ [duplicate]

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• What does T&& (double ampersand) mean in C++11? 4 answers

I was looking at the declaration of push_back in vector STL.

void push_back (const value_type& val);
void push_back (value_type&& val);

Questions :

1. what is && in 2nd declaration?
2. In first declaration, we are passing by CONST reference val variable. const means that we can't change the val, then why is it being passed as a reference if we don't want to change that. Why it's not declared as void push_back(value_type val)