C++ stl库bind函数模板用法的注意事项

#include <iostream>
#include <memory>
#include <functional>
using namespace std;

struct TesSt {
    TesSt()
    {}

    void update(const string &in_str) {
        str = in_str;
        cout << "str:" << str << endl;;
    }

    string str;
};

TesSt g_test_st;

int main () {
    auto func1 = bind(&TesSt::update, &g_test_st, "hihi"); 
    auto func2 = bind(TesSt::update, &g_test_st, "hihi");
    auto func3 = bind(&TesSt::update, g_test_st, "hihi");
    auto func4 = bind(TesSt::update, g_test_st, "hihi");
    return 0;
}

    auto func3 = bind(&TesSt::update, std::ref(g_test_st), "hihi");