Passing std::function as a parameter - c++

Question

I am trying to pass a std::function as a parameter. I am having a couple of problems with the syntax.
The code is simple. I want to save a function handler into a std::map. I don't want to make the registerHandler method a template. Here is the code example.
#include <map>
#include <functional>
#include <iostream>
using namespace std;
class message
{
public:
void print(string command)
{
cout << "message id: " << id_ << " content: " << command << std::endl;
}
int id_ = 0;
};
std::map<int, function<void(string)>> functionMap;
void registerHandler(int id, message& messageClass, std::function<void(string)>& func)
{
auto messageHandler = bind(&func, &messageClass, placeholders::_1);
// ERROR #2
// functionMap.insert({id, messageHandler});
}
int main()
{
message msg1;
msg1.id_ = 5000;
// ERROR #1
// registerHandler(msg1.id_, msg1, message::print);
std::map<int, function<void(string)>>::iterator iter;
iter = functionMap.begin();
while (iter != functionMap.end())
{
int key = iter->first;
auto messageHandler = iter->second;
messageHandler("Junk Payload");
iter++;
}
}
ERROR#1
message.cc:37:44: error: invalid use of non-static member function ‘void message::print(std::string)’
registerHandler(msg1.id_, msg1, message::print);
ERROR#2
message.cc: In function ‘void registerHandler(int, message&, std::function<void(std::basic_string<char>)>&)’:
message.cc:24:42: error: no matching function for call to ‘std::map<int, std::function<void(std::basic_string<char>)> >::insert(<brace-enclosed initializer list>)’
functionMap.insert({id, messageHandler});
^

Answer 1

You can write your program like this to work:
#include <map>
#include <functional>
#include <iostream>
using namespace std;
class message
{
public:
void print(string command)
{
cout << "message id: " << id_ << " content: " << command << std::endl;
}
int id_ = 0;
};
std::map<int, function<void(string)>> functionMap;
void registerHandler(int id, message& messageClass, const std::function<void(string)>& func)
{
functionMap.insert({id, func});
}
int main()
{
message msg1;
msg1.id_ = 5000;
registerHandler(msg1.id_, msg1, bind(&message::print, &msg1, placeholders::_1));
std::map<int, function<void(string)>>::iterator iter;
iter = functionMap.begin();
while (iter != functionMap.end())
{
int key = iter->first;
auto messageHandler = iter->second;
messageHandler("Junk Payload");
iter++;
}
}