Question

“error: invalid use of non-static data member ‘thread::tfun’”

Class thread {

  typedef void* (th_fun) (void*);

  th_fun *tfun;

  void create(th_fun *fun=tfun) {

    pthread_create(&t, NULL, fun, NULL);

}

}

How to have a function pointer inside a class?

Please note:- static deceleration will make the code compile. But my requirement is to hold the function per object.

Answer 1

Your use of pthreads is fine, and you have no pointer-to-member-function here.

The problem is that you’re trying to use a non-static member variable as a default parameter for a function, and you can’t do that:

struct T {
   int x;

   void f(int y = x) {}
};
// Line 2: error: invalid use of non-static data member 'T::x'
// compilation terminated due to -Wfatal-errors.

The default argument must be something that’s — essentially — a global, or at least a name that doesn’t require qualification.

Fortunately it’s an easy fix!

Class thread {

  typedef void* (th_fun) (void*);
  th_fun* tfun;

  void create(th_fun* fun = NULL) {      // perfectly valid default parameter
     if (fun == NULL) {
        fun = tfun;                      // works now because there's an object
     }                                   // context whilst we're inside `create`

     pthread_create(&t, NULL, fun, NULL);
  }
};