A using declaration introduces a single name into the current scope that was previously declared elsewhere.
A using-declaration is distinct from a using directive, which tells the compiler to look in a particular namespace when looking up any name. A using-directive begins with using namespace.
A using-declaration is also distinct from an alias declaration, which gives a new name to an existing type in the same manner as [typedef](<http://stackoverflow.com/documentation/c%2B%2B/7840/variable-declaration-keywords/18785/typedef>). An alias declaration contains an equals sign.