Question

What are the differences in compilability or generated code (if any) between the following two source files:

Exhibit A:

namespace std {};
using namespace std;
#include <vector>
#include <string>

<any code here>

Exhibit B:

#include <vector>
#include <string>
using namespace std;

<any code here>

Assuming the two <any code here> placeholders are replaced with any identical user code.

Put another way: Is there any user visible difference if “using namespace std;” is put before the standard #includes (assuming namespace std has been introduced as above)?

Answer 1

As unlikely as it may be, the following code could possibly be in your implementation’s vector header:

namespace __AA
{
    class vector {};
}

namespace std
{
    // actual std::vector implementation here
}

namespace __BB
{
    using namespace __AA;
    vector x;
}

Now with Exhibit A, you’ve got an ambiguity.