Question

Hi I am trying to learn some function pointers in C/C++ and I was trying to write the following C++ code with gcc on Ubuntu.

This code should execute the multiply or or the add function depending on the
preprocessor flag -DADD or -DMULTIPLY provided during compilation

#include <iostream>
#include <iomanip>
//Adds two numbers
int add(int a, int b)
{
  return a+b;
}
//Multiplies two numbers
int multiply(int a, int b)
{
  return a*b;
}

//Function to set the correct function to be executed.
//All functions here should have the same signature.
void functionsetter( void (*ptr2fun)(int,int)  )
{
#ifdef ADD
  ptr2fun   = add;
#endif

#ifdef MULTIPLY 
   ptr2fun  = multiply
#endif
} 

int main(int argc, char *argv[])
{
  int a = 5;
  int b = 6;

  void (*foo)(int,int);
  functionsetter(foo);

   return 0;
}

I cannot figure out how to pass the function pointer foo to the function-setter function by reference. Can someone help me out on this.I am sure the declaration of

functionsetter is wrong in the code, Please let me know how to fix it.

I am trying to compile this with g++ -O2 -g -Wall -DADD main.cpp -o main

Note: I want to use such preprocessor flags and function pointers in some other-code elsewhere.
Please let me know if such a thing is a good idea / practice.

Answer 1

Use a typedef:

typedef void (*MyFunctionPointer)(int,int);
void functionsetter(MyFunctionPointer& fp);

---

I want to use such preprocessor flags and function pointers in some other-code elsewhere. Please let me know if such a thing is a good idea / practice.

No, not really. It isn’t clear from your example what you are trying to accomplish, but your implementation is rather unusual. Consider using virtual member functions or std::function to switch function implementations at runtime, or (possibly) templates to switch them at compile-time. There’s nothing wrong with using conditional compilation for static selection like this, but mixing that with function pointers is a bit odd.

Without a good understanding of the problem you are trying to solve, it’s difficult to give good advice as to how best to solve it.