Why is "using namespace std" considered bad practice?

Question

I've been told by others that writing `using namespace std` in code is wrong, and that I should use `std::cout` and `std::cin` directly instead. Why is `using namespace std` considered a bad practice? Is it inefficient or does it risk declaring ambiguous variables (variables that share the same name as a function in `std` namespace)? Does it impact performance?

Answer 1

Adding using namespace std; to your code pulls in all of the that library, even if you're only using one or two items. However, writing std::cout everywhere gets old fast, so you can add using for those elements of the namespace you are going to use:

#include <iostream>

using std::cout;

using std::endl;

int main()

{

cout << "Hello World!" << endl;

return 0;

}