What are function pointers used for, and how would I use them?

Question

I understand I can use pointers for functions.

Can someone explain why one would use them, and how? Short example code would be very helpful to me.

Answer 1

You use a function pointer when you need to give a callback method. One of the classic example is to register signal handlers - which function will be called when your program gets SIGTERM (Ctrl-C)

Here is another example:

// The four arithmetic operations ... one of these functions is selected
// at runtime with a switch or a function pointer
float Plus    (float a, float b) { return a+b; }
float Minus   (float a, float b) { return a-b; }
float Multiply(float a, float b) { return a*b; }
float Divide  (float a, float b) { return a/b; }

// Solution with a switch-statement -  specifies which operation to execute
void Switch(float a, float b, char opCode)
{
   float result;

   // execute operation
   switch(opCode)
   {
      case '+' : result = Plus     (a, b); break;
      case '-' : result = Minus    (a, b); break;
      case '*' : result = Multiply (a, b); break;
      case '/' : result = Divide   (a, b); break;
   }

   cout << "Switch: 2+5=" << result << endl;         // display result
}  

// Solution with a function pointer -  is a function pointer and points to
// a function which takes two floats and returns a float. The function pointer
// "specifies" which operation shall be executed.
void Switch_With_Function_Pointer(float a, float b, float (*pt2Func)(float, float))
{
   float result = pt2Func(a, b);    // call using function pointer

   cout << "Switch replaced by function pointer: 2-5=";  // display result
   cout << result << endl;
}

You can learn more about function pointers here http://www.newty.de/fpt/index.html

If you are more familiar with object-oriented languages, you can think of it as C's way to implement the strategy design pattern.