std::bind to std::function conversion problem

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;
}

Can we see the exact delcaration of the various types, especially `driver_manager::test_callback`? — Mooing Duck, Jul 07 '21 at 17:36
Unrelated: you can shorten the code, by adding `using namespace std::placeholders;` right before the line (please keep it inside the method its in though) — Mooing Duck, Jul 07 '21 at 17:37
What is `driverManager`? Normally I'd assume it's an instance of `driver_manager`, but your error message makes it look like its type is `driver_manager (*)()` — Patrick Roberts, Jul 07 '21 at 17:47
You have to pass a pointer to an instance of `driver_manager` as the second parameter to `std::bind()`. You can't just pass a function that returns one. — Patrick Roberts, Jul 07 '21 at 17:53
@Patrick Roberts Removing brackets from `driver_manager driverManager();` worked. I am sorry, I forgot about the difference in instantiation without arguments. — Rexon Nucleide, Jul 07 '21 at 17:59
@RexonNucleide You have just been a victim of [most vexing parse](https://en.wikipedia.org/wiki/Most_vexing_parse). — Patrick Roberts, Jul 07 '21 at 18:01

score 4 · Answer 1 · answered Jul 07 '21 at 18:16

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.

std::bind to std::function conversion problem

1 Answers1