std::bind to std::function conversion problem [duplicate]

Question

Trying to get the function object from std::bind() using following code:

driver_manager driverManager();

std::function<void(mqtt::const_message_ptr,
mqtt::async_client*,callback*,sql::Driver*)> fn = std::bind(&driver_manager::test_callback, &driverManager, std::placeholders::_1, 
std::placeholders::_2, std::placeholders::_3, std::placeholders::_4);

Results in error:

error: conversion from ‘std::_Bind_helper<false, void (driver_manager::*)(std::shared_ptr<const mqtt::message>, mqtt::async_client*, callback*, sql::Driver*), driver_manager (*)(), const std::_Placeholder<1>&, const std::_Placeholder<2>&, const std::_Placeholder<3>&, const std::_Placeholder<4>&>::type’ to non-scalar type ‘std::function<void(std::shared_ptr<const mqtt::message>, mqtt::async_client*, callback*, sql::Driver*)>’ requested

I have been trying to find out what's wrong for hours, what is wrong with this code?

EDIT

driver_manager.hpp

class driver_manager{
public:
    driver_manager();
    void test_callback(mqtt::const_message_ptr, mqtt::async_client*,callback*,sql::Driver*);
};

driver_manager.cpp

driver_manager::driver_manager(){

}

void driver_manager::test_callback(mqtt::const_message_ptr msg, mqtt::async_client *client, callback *cb, sql::Driver *driver){
    std::cout << "Callback triggered" << std::endl;
}

Answer 1

As @Patrick Roberts pointed out in the comments, I was trying to pass a function to std::bind, when an instance of driver_manager was needed. The solution was to change

driver_manager driverManager();

to

driver_manager driverManager;.

Referred to [StackOverflow answer] https://stackoverflow.com/a/9490384/11693641 for more details.