CPSC 120 C++ pointers to functions and member functions

cpsc120_function_pointer.cc

// Gist https://gist.github.com/mshafae/396db34b08fb9d3b553358778d176f56
// Filename cpsc120_function_pointer.cc
// CompileCommand clang++ -std=c++17 cpsc120_function_pointer.cc
// Demonstrates how to work with functions and member functions as pointers.
// Generally speaking, don't do this unless you need to. You will probably
// find that function objects and lambda expressions will get you more mileage
// in C++ however this knowledge is useful.

#include <iostream>
#include <string>
#include <cstdlib>

class Foo {
private:
  int value_;
public:
  Foo() : value_{3} {};
  void Sum(int other_value) { value_ += other_value; }
  int Value() { return value_; }
};

int Square(int num) {
  return num * num;
}

int AscendingCompare(const void* lhs, const void* rhs) {
  int lhs_int = *static_cast<const int*>(lhs);
  int rhs_int = *static_cast<const int*>(rhs);
  return lhs_int - rhs_int;
}


int main(int argc, char const *argv[]) {
  int (*func_ptr)(int) = Square;
  // You can also say
  // The difference is like the difference between
  // int arr[4];
  // int* p = arr;
  // int* q = &arr;
  int (*func_ptr_variation)(int) = &Square;
  std::cout << func_ptr(4) << "\n";
  std::cout << func_ptr_variation(5) << "\n";
  
  Foo f;
  // error: call to non-static member function without an object argument
  // void (Foo::*member_function_ptr)(int) = Foo::Sum;
  void (Foo::*member_function_ptr)(int) = &Foo::Sum;
  // To call the member function (that is not static), we need to start
  // with an instance and then follow the pointer from there.
  (f.*member_function_ptr)(17);
  // We expect to see 20
  std::cout << f.Value() << "\n";
  // What if we have a pointer to an instance
  Foo* foo_ptr = &f;
  (foo_ptr->*member_function_ptr)(4);
  // We expect to see 24
  std::cout << f.Value() << "\n";
  
  int numbers[10]{10, 6, 8, 7, 4, 5, 3, 2, 9, 1};
  
  for(const auto num : numbers) {
    std::cout << num << ", ";
  }
  std::cout << "\n";
  
  // The prototype for qsort is typically
  // void qsort(void *base, size_t nel, size_t width,
  //           int (*compar)(const void *, const void *));
  qsort(numbers, 10, sizeof(int), AscendingCompare);
  for(const auto num : numbers) {
    std::cout << num << ", ";
  }
  std::cout << "\n";
  
  return 0;
}