c – 奇怪的运算符重载行为？

#include <iostream>
using namespace std;

class Foo{

        string _s;
        public:
        Foo(string ss){
                _s = ss;
        }
        Foo& operator=(bool b){
                cout << "bool" << endl;
                return *this;
        }
        Foo& operator=(const string& ss){
                cout << "another one" << endl;
                return *this;
        }
};


int main(){

        Foo f("bar");
        f = "this";
        return 0;

}

我有重载=运算符.我期待f =“这个”;调用operator =(const string& ss)重载的语句.但事实并非如此.它调用operator =(bool b)重载.为什么？