using directive vs using declaration swap in C++

Question

Please refer to the code below:

#include 

namespace N
{

    template 
    class C
    {
    public:
        void SwapWit         


        

Answer 1

Note: I have removed your swap definition in namespace std. It is not relevant here. Even with out it the code will have the same issues.

---

This is due to look up rule differences between using directive(using namespace std) and the using declaration(using std::swap)

Microsoft says

If a local variable has the same name as a namespace variable, the namespace variable is hidden. It is an error to have a namespace variable with the same name as a global variable.

#include

namespace T {
    void flunk(int) { std::cout << "T";}
}

namespace V {
    void flunk(int) { std::cout << "V";}
}


int main() {
    using T::flunk;   // makes T::flunk local
    // using V::flunk;  // makes V::flunk local. This will be an error
    using namespace V;  // V::flunk will be hidden
    flunk(1);
}

According to this, due to your

template 
void swap(C & c1, C & c2)

std::swap will be hidden when you use

using namespace std; 

So the only swap available for template deduction is N::swap and it won't work for ints because it expects a template class as argument.

but not when

using std::swap;

In this case it becomes equivalent to local definition. And can be used with out problem.