Как работает std::bind?

#include <iostream>
#include <functional>
#include <vector>
 
using namespace std;
 
class Base 
{
public:
    void show()
    {
        cout << "Base" << endl;
    }
};
 
////////////////////////////////////////////////////////////////////////////////////////////////////////
 
double my_divide(double x, double y) { return x / y; };
 
////////////////////////////////////////////////////////////////////////////////////////////////////////
 
void f(int x, int y)
{
    cout << "x = " << x << '\n';
    cout << "y = " << y << '\n';
}
 
int main()
{
    vector<Base*>v{ new Base{},new Base{} };
    for_each(v.begin(), v.end(), bind(&Base::show, std::placeholders::_1));
 
    auto fn_half = std::bind(my_divide, std::placeholders::_1, 2);               // returns x/2
    std::cout << fn_half(10) << '\n';
 
    bind(f, std::placeholders::_1, std::placeholders::_2)(15, 10);
    bind(f, std::placeholders::_2, std::placeholders::_1)(15, 10);
    bind(f, std::placeholders::_1, 100)(150);
    bind(f, std::placeholders::_2, 1000)(222, 1500);
    bind(f, 2222, std::placeholders::_2)(3333, 4444);
 
    system("pause");
    return 0;
}

   auto f = [](int a){};
   auto binded_f = std::bind(f, 5);
   binded_f(); // OK
   binded_f(1, 2, 3); // olso ok, unused

   using namespace std::placeholders;
   auto f = [](int a){};
   auto binded_f= std::bind(f, _3);
   binded_f(1, 2, 3); // 1, 2 - unused

   using namespace std::placeholders;
   auto f = [](int a){ std::cout << a << std::endl;};
   auto c = [](int a, int b, int c){ return a + b + c;};
   auto binded_more_arg = std::bind(f, std::bind(c, _1, _2, _3));
   binded_more_arg(1, 2, 3);

int main()
{
  auto bbb = std::bind(&Base::show, std::placeholders::_1);
    
  std::vector<Base *> vp = { new Base{}, new Base{} };
  std::for_each(vp.begin(), vp.end(), bbb);
 
  std::vector<Base> vb(2);
  std::for_each(vb.begin(), vb.end(), bbb);
}

/////////////////////////////////////////////////////////////// SERVER ///// Принимает данные ////////////////////////////////////
int main(int argc, char* argv[])
     {
    ip::tcp::endpoint ep(ip::tcp::v4(), 2001); // listen on 2001
    ip::tcp::acceptor acc(service, ep);
    std::cout << " =============== Server  ===============" << std::endl;
//------------------------- 
 
    while (true) {
        socket_ptr sock(new ip::tcp::socket(service));
                acc.accept(*sock);// <------------------- Сервер здесь висит
        boost::thread(boost::bind(client_session, sock) );// Отработавший поток рассыпался. Это другой
                 }
    std::cout <<" press any key "<< std::endl;
    _getch();
    return 0;
     }

typedef std::function<void(const std::string& str)> str_print_callback;
 
class TestBase
{
public:
    TestBase(const str_print_callback& callback = nullptr) : callback_(callback) {};
 
    void consume(std::string const& message)
    {
        if (callback_ != nullptr) callback_(message);
    }
 
private:
    str_print_callback callback_ = nullptr;
};
 
class TestPrint
{
public:
    TestPrint()
    {
        tb_ptr = boost::make_shared<TestBase>(bind(&TestPrint::print_log_str, this, std::placeholders::_1));
    }
    ~TestPrint() {}
private:
    boost::shared_ptr<TestBase> tb_ptr = nullptr;
 
    void print_log_str(std::string str)
    {
        std::cout << str << std::endl;
    }
};

auto fun = bind(&TestPrint::print_log_str, this, std::placeholders::_1);
str_print_callback cb = fun;
TestBase *tb = new TestBase(cb);
tb_ptr = shared_ptr(tb);