Question

typedef void (__thiscall* LPVOIDPROC) (void);

class ClassA
{
  LPVOIDPROC m_pProc;

  void SetProc(LPVOIDPROC pProc)  { m_pProc = pProc; }

  void OnSomeEvent() { m_pProc(); }
}

class ClassB
{
  ClassA* pCA;

  void Proc() { /* ... */ }

  void Init()
  {
    // Assume pCA != NULL
    pCA->Set((LPVOIDPROC)&ClassB::Proc); // error C2440
  }
}

How to get rid of this error C2440: ‘type cast’ : cannot convert from ‘void (__thiscall ClassB::* )(void)’ to ‘LPVOIDPROC’ ? I don’t want to limit LPVOIDPROC signature to ClassB only. This should be any class and referenced proc should not be static.

Answer 1

Workaround:

typedef void (* CLASSPROC) (void *); 

template<class T, void (T::*proc)()>
void class_proc(void * ptr) 
{
 (static_cast<T*>(ptr)->*proc)();
}

class ClassA 
{ 
 CLASSPROC m_pProc;
 void    * m_pInstance;

public:
 void SetProc(void *pInstance, CLASSPROC pProc)  { 
          m_pInstance = pInstance; 
          m_pProc = pProc; 
     } 

 void OnSomeEvent() { m_pProc(m_pInstance); } 
};

class ClassB 
{ 
 ClassA* pCA; 

 void Proc() { /* ... */ } 

 void Init() 
 { 
  // Assume pCA != NULL 
  pCA->SetProc(this, class_proc<ClassB, &ClassB::Proc>);      
 } 
};