Why is `using namespace std` considered bad practice?

Consider this code:

#include <iostream>
#include <algorithm>

using namespace std;

int swap = 0;

int main() {
	cout << swap << endl; // ERROR: reference to "swap" is ambiguous
}

Here, the compiler throws an error because it doesn’t know whether you refer to your swap global variable, or the std::swap function inside the <algorithm> header.

That is to say, using namespace is considered bad practice because it leads to name collisions, and it also makes your code less clear. If you absolutely must, you can use a using-declaration inside a local scope:

#include <iostream>

int main() {
	using std::cout; // This only affects the current function

	cout << "Hello" <<'\n';
}

Generally speaking, it is better to specify the namespace.

See also#

See also