Operator overloading template

I am trying to print an STL container. I am trying to print container elements separated by a separator. However, I ran into several problems.

1.g ++ vs VC ++

ostream& operator<<(ostream& o, const vector<string>& v) {
    copy(v.begin(), v.end(), std::ostream_iterator<string>(o,","));
}

int main()
{

    vector<string> s_v;
    s_v.push_back("one");
    s_v.push_back("two");

    cout << s_v;

}

g ++ (gcc version 4.4.0 on mingw32) can compile it, works fine. VC ++ (Visual Studio 9) cannot compile this code.

error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'const std::string' (or there is no acceptable conversion)
1>        c:\program files (x86)\microsoft visual studio 9.0\vc\include\ostream(653): could be 'std::basic_ostream<_Elem,_Traits> &std::operator <<<char,std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,const char *)'
1>        with
1>        [

Why is this? Is this code illegal? or is it just VC ++ beign VC ++?

2. An unused template variable breaks compilation.

if now I add a template to a body like this (it is not used, it just sits there)

template <typename T>  // <----- Here
ostream& operator<<(ostream& o, const vector<string>& v) {
    copy(v.begin(), v.end(), std::ostream_iterator<string>(o,","));
}

int main()
{

    vector<string> s_v;
    s_v.push_back("one");
    s_v.push_back("two");

    cout << s_v;

}

gcc can no longer match the statement.

    error: no match for 'operator<<' in 'std::cout << s_v'

and a lot more candidates...

Why? the template is not used. Should it make a difference?

EDITOR: This is decided. I had to return o;

3. Used pattern

template <typename T>
ostream& operator<<(ostream& o, const vector<T>& v) {
    copy(v.begin(), v.end(), std::ostream_iterator<T>(o,","));

    return o; // Edited
}

int main()
{

    vector<string> s_v;
    s_v.push_back("one");
    s_v.push_back("two");

    vector<int> i_v;
    i_v.push_back(1);
    i_v.push_back(2);

    cout << s_v;
    cout << i_v;
}

, . g++ , .

terminate called after throwing an instance of 'std::bad_cast'
  what():  std::bad_cast

V++ , gcc . .

- ? .