function pointers using functions in an object with parameters - c++

Question

I have been playing around with function pointers in c++ and seem to have found a bit of a problem. I made a demo to reproduce the error in a simple example.
I have the header file
class MyClass
{
public:
void MyFunction(int i);
MyClass();
~MyClass();
};
and the cpp file
#include "MyClass.h"
#include <iostream>
#include <functional>
using namespace std;
MyClass::MyClass()
{
//doesn't work
function<void(int)> func = &MyClass::MyFunction;
}
void MyClass::MyFunction(int i)
{
cout << i << endl;
}
In the constructor of the cpp file I am trying to create a pointer to MyFunction. It gives the error error C2664: 'void std::_Func_class<_Ret,int>::_Set(std::_Func_base<_Ret,int> *)' : cannot convert argument 1 from '_Myimpl *' to 'std::_Func_base<_Ret,int> *' in the functional file at line 506. It works fine with a parameterless method, but not with them. Does anyone know why, and how to resolve it?

Answer 1

You can use this and bind the object being constructed to the function. For instance, if your constructor looked like this:
MyClass::MyClass()
{
function<void(int)> func = bind(&MyClass::MyFunction, this, placeholders::_1);
func(6);
}
And you created a MyClass instance:
MyClass instance;
Then 6 will be printed to stdout.

Answer 2

You can also use std::mem_fn in C++11, which wraps a member function/variable into a callable closure
#include <iostream>
#include <functional>
class MyClass
{
public:
MyClass()
{
auto func = std::mem_fn(&MyClass::MyFunction);
func(this, 42); // call it on the current instance
}
void MyFunction(int i)
{
std::cout << i << std::endl;
}
};
int main()
{
MyClass foo;
}
or, you can explicitly specify the instance you're calling the pointer to member function
MyClass()
{
auto func = &MyClass::MyFunction;
(this->*func)(42); // call it on the current instance
}
In particular, note that std::function<void(int)> is not convertible to a pointer to member function. See related Using generic std::function objects with member functions in one class
That's why using auto with std::mem_fn gets rid of all the pain.