Question

I would like someone to shed some light this code snippet, which confuses me.

   //-------------------------------------------------------------------------------
   // 3.5 Example B: Callback to member function using a global variable
   // Task: The function 'DoItB' does something that implies a callback to
   //       the member function 'Display'. Therefore the wrapper-function
   //       'Wrapper_To_Call_Display is used.

   #include <iostream.h>   // due to:   cout

   void* pt2Object;        // global variable which points to an arbitrary object

   class TClassB
   {
   public:

      void Display(const char* text) { cout << text << endl; };
      static void Wrapper_To_Call_Display(char* text);

      /* more of TClassB */
   };


   // static wrapper-function to be able to callback the member function Display()
   void TClassB::Wrapper_To_Call_Display(char* string)
   {
       // explicitly cast global variable <pt2Object> to a pointer to TClassB
       // warning: <pt2Object> MUST point to an appropriate object!
       TClassB* mySelf = (TClassB*) pt2Object;

       // call member
       mySelf->Display(string);
   }


   // function does something that implies a callback
   // note: of course this function can also be a member function
   void DoItB(void (*pt2Function)(char* text))
   {
      /* do something */

      pt2Function("hi, i'm calling back using a global ;-)");   // make callback
   }


   // execute example code
   void Callback_Using_Global()
   {
      // 1. instantiate object of TClassB
      TClassB objB;


      // 2. assign global variable which is used in the static wrapper function
      // important: never forget to do this!!
      pt2Object = (void*) &objB;


      // 3. call 'DoItB' for <objB>
      DoItB(TClassB::Wrapper_To_Call_Display);
   }

Question 1: Regarding this function call:

DoItB(TClassB::Wrapper_To_Call_Display)

Why does Wrapper_To_Call_Display not take any arguments, although it is supposed to take a char* argument according to its declaration?

Question 2: DoItB is declared as

void DoItB(void (*pt2Function)(char* text))

What I’ve understood so far is that DoItB takes a function pointer as argument, but why does the function call DoItB(TClassB::Wrapper_To_Call_Display) take TClassB::Wrapper_To_Call_Display as argument even tough it’s not a pointer?

Thanx in advance

Source of code snippet: http://www.newty.de/fpt/callback.html

Answer 1

In C/C++ when a function name is used with no parameters – that is no parenthesis – it is a pointer to a function. So TClassB::Wrapper_To_Call_Display is a pointer to the address in memory where the code for the function is implemented.

Since TClassB::Wrapper_To_Call_Display is a pointer to a void function that takes a single char* it’s time is void (*)(char* test) so it matches the type required by DoItB.