cannot convert 'int (Scheduler::*)(int, void*)' to 'int (*)(int, void*)' for argument '2' to 'bool irq_InstallISR(int, int (*)(int, void*), void*)'

Question

I have this function in irq header

/* irq.h */
bool irq_InstallISR(int irq, int (*isr)(int, void*), void* isr_data);

and a class Scheduler

/* Scheduler.cpp */
using namespace x86Duino;
void Scheduler::init(){
...
irq_InstallISR(RTCIRQ, &Scheduler::timerrtc_isr_handler, isrname_rtc);
...
}

int Scheduler::timerrtc_isr_handler(int irq, void* data){
...
}

and I got this error

error: cannot convert 'int (x86Duino::Scheduler::*)(int, void*)' to 'int (*)(int, void*)' for argument '2' to 'bool irq_InstallISR(int, int (*)(int, void*), void*)'

I have tried this in init function

using namespace std::placeholders;
irq_InstallISR(RTCIRQ, std::bind(&Scheduler::timerrtc_isr_handler, this, _1, _2), isrname_rtc);

but I also got a similar error

 error: cannot convert 'std::_Bind_helper<false, int (x86Duino::Scheduler::*)(int, void*), x86Duino::Scheduler*, const std::_Placeholder<1>&, const std::_Placeholder<2>&>::type {aka std::_Bind<int (x86Duino::Scheduler::*(x86Duino::Scheduler*, std::_Placeholder<1>, std::_Placeholder<2>))(int, void*)>}' to 'int (*)(int, void*)' for argument '2' to 'bool irq_InstallISR(int, int (*)(int, void*), void*)'

Please tell me what I'm doing wrong?

Answer 1

You haven't included the declaration of Scheduler::timerrtc_isr_handler, so my first guess would be that you forgot to declare that method as static?

i.e. In the class definition, it should look like:

class Scheduler
{
  static int timerrtc_isr_handler(int irq, void* data);
};

The irq_InstallISR takes a global function. The error message says it cannot convert this prototype:

int (Scheduler::*)(int, void*)

Which is a member function, to

int (*)(int, void*)

Which is a normal C function (or non-member function, e.g. a static method).

/edit I'm guessing you wanted something along the lines of this pattern:

class Scheduler
{
public:
  Scheduler()
  {
    irq_InstallISR(RTCIRQ, timerrtc_isr_handler, this);
  }
  ~Scheduler()
  {
    // probably want to uninstall the callback here!
  }

  int isr_handler(int irq)
  {
    /// do you handling here
  }
private:
  static int timerrtc_isr_handler(int irq, void* data)
  {
    // cast user data to correct class type
    Scheduler* sch = (Scheduler*)data;
    assert(sch); // just in case we get null for some reason?

    // thunk call to member function
    return sch->isr_handler(irq);
  }
};

/edit 2

You aren't choosing between declaring the method as static or int, but the choice is between a static function (i.e. does not have access to 'this'), or a member function (which requires 'this'). Consider the following:

struct Foo
{
  void func1() { std::cout << "func1\n"; }
  static void func2() { std::cout << "func2\n"; }
};

void bar()
{
  // call the static method - does not require an object!
  Foo::func2(); 

  // to call func1, we ALWAYS need an object... 
  Foo obj;
  obj.func1(); 
}

So let's take it another step. Let's imagine I've written a C-API library. C does not have support for C++ classes, so typically in order to inter-operate with C++ class based code, you often resort to a fairly common user data pattern. So I'll try to strip this down to the simplest possible example....

/// MyLib.h


// I want to call this method, and it will inturn call any 
// callback functions registered with the system. 
void callMyRegisteredFunction();

// the type of function I want to call in the previous method
// The C++ way of doing this is to say :
// 
//   someObject->func();
//
// However in C, without classes, the way you'd call it would be:
//
//   func(someObject); 
// 
// so the second pointer here is the object to call it on. 
typedef void (CallbackFunc*)(void*);

// register a callback function, and an associated user-defined object
void registerFunc(CallbackFunc funcPtr, void* userData);

// reset the internal callback
void unregisterFunc();

/// MyLib.cpp
#include "MyLib.h"

// the currently registered callback function
CallbackFunc g_func = NULL;

// the 'object' it is registered against. From 'C' we don't know that 
// what type of object this is, we just know it's address. 
void* g_userData = NULL;

void registerFunc(CallbackFunc funcPtr, void* userData)
{
  g_func = funcPtr;
  g_userData = userData;
}
void unregisterFunc()
{
  g_func = NULL;
  g_userData = NULL;
}

void callMyRegisteredFunction()
{
  // don't call invalid method
  if(!g_func) return;

  // call the function, and pass it the userData pointer
  // This code does NOT know about C++ code, or the class 
  // type that you registered. 
  g_func(g_userData);
}

class MyCallbackObject
{
public:
  MyCallbackObject()
  {
    registerFunc(C_callback, this); //< NOTE: !!this!!
  }

  ~MyCallbackObject()
  {
    unregisterFunc();
  }

  // everything else prior exists PURELY to be able to call this C++ 
  // class method, from C code, that has absolutely NO idea about how 
  // your class is defined. 
  // NOTE: I'm making this method virtual so that instead of duplicating
  // the boiler plate code everywhere, you can just inherit from this 
  // class, and override the doThing method. 
  virtual void doThing()
  {
    /// do you handling here
  }
private:
  static void C_callback(void* userData)
  {
    // cast user data to correct class type
    MyCallbackObject* obj = (MyCallbackObject*)userData;

    // now call the method
    obj->doThing();
  }
};

Honestly, I can't make the above example any simpler. This userData pattern exists simply so that you can call member functions of C++ objects from a C library. Hopefully the above makes sense, if not you'll probably need to read up on static methods, and the limitations of C.