STL friendly operator << doesn't work

[rodis]

I want to overload ostream& operator << so that it prints content of whatever container I want. I wrote something like this:

#include <iostream>
#include <list>
#include <set>
template <class T>
ostream& operator << (ostream& strm, T l)
{
	for (class T::iterator it = l.begin(); it != l.end();++it)
	{
		strm << *it;
// 		if (++it != l.end())
// 			strm << " ";
	}
	return strm;
}
int main()
{
	list<int> l;
	int t[] = {1,2,3,-1,-2,-3};
	l.insert(l.begin(), t, t + 6);
	cout << l;
	set<int> s;
	s.insert(l.begin(), l.end());
	cout << s;
}

It works. However, it'd be nice to actually have these spaces between numbers. There's the problem: when I uncomment the code (and remove ++it from the for loop), the compilers gives me a bunch of messages with the "main" reading:

stl_test.cpp: In function ‘std::ostream& operator<<(std::ostream&, T)’:
stl_test.cpp:19: error: ambiguous overload for ‘operator<<’ in ‘strm << " "’

Does anyone have any idea why does it not work? (it seems like << isn't overloaded for const char *, but simple cout << " "; works, so...)

I use g++ 4.4.5