using-declaration

using-declaration is used to alias an unqualified identifier to a qualified identifier.

For example, to refer std::cout just cout, we can use

#include <iostream>
 
int main(int argc, char const *argv[])
{
    using std::cout;
 
    cout << "Hello world";
    return 0;
}

References

1. https://www.learncpp.com/cpp-tutorial/using-declarations-and-using-directives/