Use `using namespace` directives for transition, for foundation libraries (such as `std`), or within a local scope (only)

Reason

using namespace can lead to name clashes, so it should be used sparingly. However, it is not always possible to qualify every name from a namespace in user code (e.g., during transition) and sometimes a namespace is so fundamental and prevalent in a code base, that consistent qualification would be verbose and distracting.

Example

#include <string>
#include <vector>
#include <iostream>
#include <memory>
#include <algorithm>

using namespace std;

// ...

Here (obviously), the standard library is used pervasively and apparently no other library is used, so requiring std:: everywhere could be distracting.

Example

The use of using namespace std; leaves the programmer open to a name clash with a name from the standard library

#include <cmath>
using namespace std;

int g(int x)
{
    int sqrt = 7;
    // ...
    return sqrt(x); // error
}

However, this is not particularly likely to lead to a resolution that is not an error and people who use using namespace std are supposed to know about std and about this risk.

Note

A .cpp file is a form of local scope. There is little difference in the opportunities for name clashes in an N-line .cpp containing a using namespace X, an N-line function containing a using namespace X, and M functions each containing a using namespace X with N lines of code in total.

Note

Don't write using namespace at global scope in a header file.