C++ :: STL Friendly Operator Not Working

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

Code:
#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)

[Code] ...

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:

Code:
stl_test.cpp: In function "std::ostream& operator<<(std::ostream&, T)":
stl_test.cpp:19: error: ambiguous overload for "operator<<" in "strm << " " "

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