Passing member function pointer to member object in c++

Question

I have a problem with using a pointer to function in C++. Here is my example:

#include <iostream>  using namespace std;  class bar { public:     void (*funcP)(); };  class foo { public:     bar myBar;     void hello(){cout << "hello" << endl;}; };  void byebye() {     cout << "bye" << endl; }   int main() {     foo testFoo;      testFoo.myBar.funcP = &byebye;         //OK     testFoo.myBar.funcP = &testFoo.hello;  //ERROR     return 0; } 

Compilator returns an error at testFoo.myBar.funcP = &testFoo.hello;:

ISO C++ forbids taking the address of a bound member function to form a pointer to member function. Say '&foo::hello'

cannot convert 'void (foo::)()' to 'void ()()' in assignment

So i tried it like this:

class bar { public:     void (*foo::funcP)(); }; 

But now the compilator adds one more:

'foo' has not been declared

Is there a way make it work?

Thanks in advance for suggestions

Answer 1

Taking everyone's suggestions together, your final solution will look like:

#include <iostream>  using std::cout; usind std::endl;  class foo; // tell the compiler there's a foo out there.  class bar  {  public:      // If you want to store a pointer to each type of function you'll     // need two different pointers here:     void (*freeFunctionPointer)();     void (foo::*memberFunctionPointer)(); };   class foo  {  public:      bar myBar;      void hello(){ cout << "hello" << endl; } };   void byebye()  {      cout << "bye" << endl;  }    int main()  {      foo testFoo;       testFoo.myBar.freeFunctionPointer = &byebye;     testFoo.myBar.memberFunctionPointer = &foo::hello;      ((testFoo).*(testFoo.myBar.memberFunctionPointer))(); // calls foo::hello()     testFoo.myBar.freeFunctionPointer();   // calls byebye()     return 0;  }  

The C++ FAQ Lite has some guidance on how to simplify the syntax.

Taking Chris' idea and running with it, you could get yourself something like this:

#include <iostream> using std::cout; using std::endl;  class foo; typedef void (*FreeFn)(); typedef void (foo::*MemberFn)();  class bar { public:   bar() : freeFn(NULL), memberFn(NULL) {}   void operator()(foo* other)   {     if (freeFn != NULL) { freeFn(); }     else if (memberFn != NULL) { ((other)->*(memberFn))(); }     else { cout << "No function attached!" << endl; }   }    void setFreeFn(FreeFn value) { freeFn = value; memberFn = NULL; }   void setMemberFn(MemberFn value) { memberFn = value; freeFn = NULL; } private:   FreeFn freeFn;   MemberFn memberFn; };  class foo { public:   bar myBar;   void hello() { cout << "foo::hello()" << endl; }   void operator()() { myBar(this); } };  void bye() { cout << "bye()" << endl; }  int main() {   foo testFoo;    testFoo();    testFoo.myBar.setMemberFn(&foo::hello);   testFoo();    testFoo.myBar.setFreeFn(&bye);   testFoo();    return 0; }