Function matching and Argument-dependent lookup

[Nikko YL]

#include <iostream>
#include <string>

std::ostream& operator<<(std::ostream& os, const std::string& str) {
    os << '[';
    std::operator<<(os, str);
    os << ']';
    return os;
}

int main() {
    std::string str = "hi";
    std::cout << str;
}

In the above program, I define an operator<< function in global namespace which has exactly the same signature as the one define in <string> header : http://www.cplusplus.com/reference/string/string/operator%3C%3C/ .
In the main function, I call operator<< on cout and a string, after which I guess it would cause ambiguity. The result is out of my anticipation, it compiles and prints [hi]. So I guess my function is a better match because it does not require argument-dependent lookup (ADL) (Please let me know if I'm wrong).
Moreover, even if I add using namespace std; at the beginning of the main function, the program still compiles and prints [hi].

In short, I have two question:
#1 : Why is my function a better match (therefore no ambiguity) in the two cases respectively (with and without using namespace std;)?

#2 : Does using namespace affect ADL? Is ADL still performed after applying using namespace, or is it not performed because the function is thrown into global namespace?

Thanks in advance.

[cire]

http://stackoverflow.com/questions/14666219/why-does-overload-of-template-and-non-template-function-with-the-same-signature

[Nikko YL]

As I see in the reference, the operator<< function defined in <string> is not template, isn't it?

[cire]

The declaration shown in the link you provided is a simplification. You can always visit your <string> header and check it out for yourself.