Callback casting and set from another object-CodePudding

I just wanted to ask if my approach is wrong or one is the right way and it can be done.

In the project, I have one hal and several types of Dir based on Base. When I create some dir I pass hal to it because each dir uses it in its own way. Everyone also reacts in their own way to events in the hal. so I wanted to use the callback mechanism. I one moment I have only one specific controller, and I change it, delete, and create another, at this moment I must connect callback. I create a solution with one dir type, and it's working fine. But what path choose when I want to use a few different dir, Can I cast to base and use base in Hal something like this:

class Base;

class Hal
{
  public:
  void set_callback(void (Base::*callback)(int), Base* base)
  {
    m_callback = callback;
    m_base = base;
  }
  void fun()
  {
    (m_base->*m_callback)(some_int);
  }

  private:
  Base* m_base;
  void (Base::*m_callback)(int);
};

#include "Hal.h"

class Base
{
  public:
  virtual void active() = 0;
  virtual void foo(int variable) = 0;
};

class Dir : public Base
{
  public:
  Dir(Hal& hal)
  : m_hal(hal)
  {}
  void active()
  {    
    auto callback = &Dir::foo;
    //add some casting?
    m_hal.set_callback(callback, this);
  }
  void foo(int variable)
  {
    // some code
  }

  private:
  Hal& m_hal;
};

Maybe I should add a separate class for routing such callbacks? Or use a different mechanism? Unfortunately, I don't have functional lib available in this project /:

CodePudding user response：

Member function pointers for one class are not convertible to member function pointers of another class, even if the functions look compatible. You can approximate this conversion using a layer of abstraction, for example using a std::function<void(Base*, int)> but it cannot be achieved using a cast.

However, member function pointers respect polymorphism. The actual function being called will depend on the dynamic type of the object with which it is called.

You only need to pass &Bar::foo, which is virtual. Since it will be called with a Dir the Dir::foo override will be used.

Changing to m_hal.set_callback(&Bar::foo, this); should do what you want.

Live example : https://godbolt.org/z/fd8hsbanv