unable to be compiled - c++

Question

#include <iostream>
using namespace std;
template < class T >
void swap (T& a, T& b)
{
T temp = a;
a = b;
b = temp;
}
int main ()
{
char a = 'a';
char b = 'b';
swap (a, b);
cout << "a = " << a << endl;
cout << "b = " << b << endl;
return 0;
}
the code can not be compiled under linux KDE command line (gcc compiler).
however if I change "using namespace std" into "using std::cout; using std::cin using std::endl" the program can be compiled and run well. what's wrong with it?
Thank you very much

Answer 1

Your definition of swap is conflicting with the already existing definition of std::swap when you bring std in the global namespace using using namespace.
Note that the conflict occurs when you try to instantiate the template, you can use
::swap (a, b);
to choose explicitly your definition.

Answer 2

Here is what VC++ says:
error C2668: 'swap' : ambiguous call to overloaded function
1> c:\lisp\other\test_meth\test_meth.cpp(7): could be 'void swap<char>(T &,T &)'
1> with
1> [
1> T=char
1> ]
1> c:\program files (x86)\microsoft visual studio 10.0\vc\include\utility(100): or 'void std::swap<char>(_Ty &,_Ty &)'
1> with
1> [
1> _Ty=char
1> ]
1> while trying to match the argument list '(char, char)'
The problem is: std namespace already contains the template function swap.