call of overloaded 'swap(char&, char&)' is ambiguous - c++

Question

I can't figure out what is ambiguous about swap(arr[i++],arr[n--]); below. Please educate me on my wrongful ways.
#include <iostream>
#include <string>
template <typename T> void swap ( T & a, T & b )
{
T temp = b;
b = a;
a = temp;
}
template <typename T> void reverse_array ( T * arr, size_t n )
{
size_t i = 0;
while (i < n) swap(arr[i++],arr[n--]); // problem line
}
int main ()
{
char mystr [] = "Obama smokes";
reverse_array(mystr, sizeof(mystr)/sizeof(char));
return 0;
}

Answer 1

codepad has an implicit using namespace std;, which is a really bad idea and makes your swap conflict with std::swap. You can use ::swap instead, but not before fixing your bounds problem by passing sizeof mystr - 2 instead of sizeof(mystr)/sizeof(char).

Answer 2

As #minitech mentions in his answer, the problem is because of the implicit namespace declaration, e.g. using namespace std;
With that I get the following error, which shows that the conflict is with std::swap:
100%] Building CXX object CMakeFiles/csi_projects.dir/main.cpp.o
/home/vsnyc/ClionProjects/main.cpp: In instantiation of ‘void reverse_array(T*, size_t) [with T = char; size_t = long unsigned int]’:
/home/vsnyc/ClionProjects/main.cpp:151:52: required from here
/home/vsnyc/ClionProjects/main.cpp:144:43: error: call of overloaded ‘swap(char&, char&)’ is ambiguous
while (i < n) { swap(arr[i++],arr[n--]); } // problem line
^
/home/vsnyc/ClionProjects/main.cpp:144:43: note: candidates are:
/home/vsnyc/ClionProjects/main.cpp:134:28: note: void swap(T&, T&) [with T = char]
template <typename T> void swap ( T & a, T & b )
^
In file included from /usr/include/c++/4.8/bits/stl_pair.h:59:0,
from /usr/include/c++/4.8/bits/stl_algobase.h:64,
from /usr/include/c++/4.8/bits/char_traits.h:39,
from /usr/include/c++/4.8/ios:40,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from /home/vsnyc/ClionProjects/main.cpp:1:
/usr/include/c++/4.8/bits/move.h:166:5: note: void std::swap(_Tp&, _Tp&) [with _Tp = char]
swap(_Tp& __a, _Tp& __b)
^