why does the following code generate a "reference to 'tm' is ambiguous" error?

#include <iostream>
using namespace std;

namespace characters {
    char tm='a';
    char tc='a';
}

using namespace characters;

class table {
    public:
        void printline (){
            char m;
            m=tm;
            //m=tc;
            cout<<m<<m<<m<<m<<m<<m<<m<<m<<m;
        }
};

int main()
{
    table myTable;
    myTable.printline();

return 0;
}

but when you comment out the m=tm; line and reinstate the line m=tc the code works fine.

what is so special about the identifier tm?

using namespace characters; is the issue, that brings characters::tm to the global namespace and makes the ambiguity with the global struct tm. The solution:

// using namespace characters;
using characters::tm;

That directs the compiler, if you meet tm, use here tm from the namespace characters.