Using namespace used for

Answer 1

https://en.cppreference.com/w/cpp/language/namespace

Answer 2

https://www.sololearn.com/Discuss/1530209/?ref=app
https://www.sololearn.com/Discuss/905774/?ref=app
https://www.sololearn.com/Discuss/1694246/?ref=app
https://www.sololearn.com/Discuss/1858611/?ref=app
https://www.sololearn.com/Discuss/392399/?ref=app

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