CODEWITHSUNDEEP
c++
Angus
Angus

Reputation: 12621

Callback in c++ - Could not understand

I tried to understand how call back works...In the below program passing the (void*)&obj and receiving it in (void* pt2Object) is confusing me... *pt2Object will hold the value and not the address of some variable but here the reference is paased to it.. I could not understand how the ClassA::Wrapper_To_Call_Display func is passed and get accepted in void (*pt2Function)(void* pt2Object,char* text))... Please help me to understand about it..

//---Program To use Function Pointers As A CallBack---

#include "stdafx.h"
#include <iostream>
using namespace std;

class ClassA
{
public:
  void Display(const char* text)
  {
    cout<<text<<endl;
  }
  static void Wrapper_To_Call_Display(void *pt2Object,char* text);
};
void ClassA::Wrapper_To_Call_Display(void *pt2Object,char* text)
{
  ClassA* myself = (ClassA*)pt2Object;
  myself->Display("string");
  cout<<text<<endl;
}
void DoIt(void* pt2Object, void(*pt2Function)(void* pt2Object, char* text)) {
  pt2Function(pt2Object, "Hi I am Calling back using an argument!!");
}
void Callback_Using_Argument() {
  ClassA obj;
DoIt((void*)&obj,ClassA::Wrapper_To_Call_Display);
}
int main() {
  Callback_Using_Argument();
  return 0;
}

output

string
Hi I am Calling back using an argument!!
Press any key to continue . . .

Upvotes: 1

Views: 223

Answers (1)

littleadv
littleadv

Reputation: 20272

DoIt((void*)&obj,ClassA::Wrapper_To_Call_Display);

The &obj is not a reference. & returns the address of the obj which is an instance of the ClassA class. The next parameter (ClassA::Wrapper_To_Call_Display) is the address of the function (note that it comes without the () that are required for a function call).

*pt2Object will hold the value and not the address of some variable

*pt2Object is a variable which holds a value. The value is an address of some other variable (that's a pointer).

The whole void* and casting is a mess and that's not the way it should be done in C++, use casting operators instead.

Upvotes: 1

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