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

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.