param of asio timer async_wait difference lambda, bind, function pointer - c++

Question

When I use boost::asio::steady_timer, I find something difference between lambda, bind, function pointer.
#include <iostream>
#include <boost/asio.hpp>
void print() { std::cout << "Hello, world!" << std::endl; }
int main()
{
boost::asio::io_context io;
boost::asio::steady_timer t(io, boost::asio::chrono::seconds(5));
t.async_wait(&print); // Error
t.async_wait([]{print();}) // Error
t.async_wait(std::bind(print)); // Done
io.run();
return 0;
}
I read asio manual, async_wait handler need const boost::system::error_code& error param. So if I changed print to void print(const boost::system::error_code & /*e*/), all things was right. But asio example of timer4/timer.cc && timeouts/server.cc used handler creating by bind without void print(const boost::system::error_code & /*e*/). When I changed to lambda, compile was wrong. So, what difference of signature been had between bind && lambda.
#include <iostream>
#include <functional>
#include <boost/asio.hpp>
#include <boost/bind.hpp>
class printer
{
public:
printer(boost::asio::io_context &io)
: timer_(io, boost::asio::chrono::seconds(1)), count_(0)
{
timer_.async_wait(std::bind(&printer::print, this));
}
~printer() { std::cout << "Final count is " << count_ << std::endl; }
void print()
{
if (count_ < 5) {
std::cout << count_ << std::endl;
++count_;
timer_.expires_at(timer_.expiry() +
boost::asio::chrono::seconds(1));
timer_.async_wait(boost::bind(&printer::print, this));
// timer_.async_wait([this]{print();}); Error
}
}
private:
boost::asio::steady_timer timer_;
int count_;
};
int main()
{
boost::asio::io_context io;
printer p(io);
io.run();
return 0;
}

Answer 1

The "partial" generated by std::bind detects and ignores arguments provided by the invocation point and not explicitly wired to the bound code.
A minimalistic example (godbolted):
#include <functional>
#include <iostream>
void callme(std::function<void(int, float)> arg) {
arg(42, 4.2);
}
// or like this
// template <typename F> void callme(F&& arg) {
// arg(42, 4.2);
// }
int main()
{
auto fn = std::bind([](){std::cout << "hi there" << std::endl; });
// auto fn = std::bind([](auto&& x){std::cout << "x=" << x << std::endl; }, std::placeholders::_1); <-- this works too and prints 42
// auto fn = std::bind([](auto&& x){std::cout << "x=" << x << std::endl; }, std::placeholders::_2); <-- and works too and prints 4.2
callme(fn);
return 0;
}