String and Char in function template

["learning c++" <learning_c at hotmail dot com>]

Hi, Everyone:

I have a short code about function template like this:

#include <iostream>
#include <string>
using std::cout;

template <typename T> // template parameter list
T const &greater(T const &first, T const &second)
{
   return (first > second) ? first : second;
}

int main()
{
   int a = 1, b = 2;
   cout << greater(a, b) << '\n';
   long c = 4L, d = 3L;
   cout << greater(c, d) << '\n';
   double e = 5.62, f = 3.48;
   cout << greater(e, f) << '\n';
   char g = 'A', h = 'a';
   cout << greater(g, h) << '\n';
   std::string two("two"), three("three");
   cout << greater(two, three) << '\n';
   char const *two("two"), *three("three");
   cout << greater(two, three) << '\n';
   char const *two("two"), *three("three");
   cout << greater(two, three) << '\n';
   return 0;
}

when I compile it, the errors are the following:
/v/dunix51_alpha/gnu/gcc/lib/gcc-lib/alphaev5-dec-osf5/3.3/include/c++/bits/stl_function.h: 
In
  function `int main()':
/v/dunix51_alpha/gnu/gcc/lib/gcc-lib/alphaev5-dec-osf5/3.3/include/c++/bits/stl_function.h:188: 
error: `
  template<class _Tp> struct std::greater' is not a function,
greater.cpp:7: error:   conflict with `template<class T> const T& 
greater(const
  T&, const T&)'
greater.cpp:22: error:   in call to `greater'
greater.cpp:23: error: conflicting types for `const char*two'
greater.cpp:21: error: previous declaration as `std::string two'
greater.cpp:23: error: conflicting types for `const char*three'
greater.cpp:21: error: previous declaration as `std::string three'
greater.cpp:23: warning: statement with no effect
greater.cpp:23: confused by earlier errors, bailing out

What is wrong with my code? In addition, I hope to know something about 
"const" in "char const *two("two"), *three("three");" and the difference 
between "using std::cout;" and "using namespace std;"

Thanks in advance!

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