Passing a pointer to a function as an argument to a function

Question

Just wondering if anyone can give me some advice regarding where I'm going wrong here. My program works OK if I run it as is, but as soon as I swap the commented line with the one below it, I get errors. My goal is to be able to use the commented line because I want to create a program which let's me pass a pointer to a function as an argument to another function, but so far I'm having no luck.

#include <iostream>
using namespace std;

double arith_op(double left, double right, double (*f)(double, double));
double addition(double left, double right);
double subtraction(double left, double right);
double multiplication(double left, double right);

int main()
{
  double left, right;
  int choice;
  double (*f[3])(double, double) = { addition, subtraction, multiplication };
  
  cout << "Enter 1 for addition, 2 for subtraction, 3 for multiplication "
       << "(-1 to end): " << endl;
  cin >> choice;

  while (choice != -1) {

    cout << "Enter a floating-point number: " << endl;
    cin >> left;

    cout << "Enter another floating-point number: " << endl;
    cin >> right;

    // double* result = arith_op(left, right, f[choice - 1](left, right));
     double result = f[choice - 1](left, right);

    if (choice == 1) {
      cout << left << " + " << right << " = " << result;
    }
    else if (choice == 2) {
      cout << left << " - " << right << " = " << result;    
    }
    else {
      cout << left << " * " << right << " = " << result;    
    }
    cout << endl;

    cout << "Enter 1 for addition, 2 for subtraction, 3 for multiplication "
     << "(-1 to end): " << endl;
    cin >> choice;
  }
}

double arith_op(double left, double right, double (*f)(double, double))
{
  return (*f)(left, right);
}

double addition(double left, double right)
{
  return left + right;
}
double subtraction(double left, double right)
{
  return left - right;
}

double multiplication(double left, double right)
{
  return left * right;
}

I should add that my end goal is to package the function arith_op and the other functions in a seperate file, then use them by including their prototypes with 'extern'. This may be an odd way to approach the problem - it's for an assignment, and they're always odd.

Thank you :)

Wade

What type of errors do you get? Besides of that your program is C-style, but not C++ style: You would use classes and inheritance... — U. Windl, Mar 26 '19 at 23:35
I've just finished a C course, so you are correct. The structure of the new course I'm doing is such that we are reviewing C first, then moving on to C++ styles. Consequently we aren't allowed to use any OOP paradigms to solve this problem. — Wade Shiell, Mar 27 '19 at 00:05

score 3 · Accepted Answer · answered Mar 26 '19 at 23:31

3

Your problem is in the 3rd argument here

arith_op(left, right, f[choice - 1](left, right));

f[i](left, right) is calling the function to give a double, rather than passing the function pointer to arith_op. Simply remove the parameter list

arith_op(left, right, f[choice - 1]);

answered Mar 26 '19 at 23:31

Peter Bell

371
2
6

3

Also, on that line the declaration `double* result` should be `double result` – Chris Taylor Mar 26 '19 at 23:34
Correct! Thank you sir. – Wade Shiell Mar 27 '19 at 00:05

score 0 · Answer 2 · answered Mar 27 '19 at 00:07

smacks head I feel stupid now. It works. Brilliant, thank you!

Sadly my program still doesn't actually get me full marks, but that's because we have to submit it using an automated online marking system. So if your format is not precisely correct, you'll lose marks even if the program actually works fine.

Post script - I also successfully moved my functions to a seperate file, as per the assignment instructions. It's telling me I didn't name my functions correctly, but the program works, which is what matters.

Passing a pointer to a function as an argument to a function

2 Answers2