C++ Calling a function from the header file in the main

Question

I'm writing an integral calculator. I'm trying to get the main.cpp file to read in the functions from the functions class which is in the functions.h.cpp files. I have a function defined as the following for the easy case:

double square(double aX)
 {
   return aX*aX;
 }

This function works when I include it in the main file, though it doesn't want to work when I try and call it from the main file when it's in the functions file. Here is the code that I have:

Main.cpp file:

#include <iostream>
#include "IntegralCalculator.h"
#include "functions.h"

using namespace std;

int main()
  {
    IntegralCalculator Function(square);
    Function.print();
  }

As you can see, there is another class called IntegralCalculator which actually does the calculation of the integral, though nothing should be wrong with that as it works when the function definition is included in the main, and the function is passed as a parameter. The functions.h file is defined as the following:

#ifndef FUNCTIONS_H
#define FUNCTIONS_H

#include <iostream>
#include <cmath>
#include "IntegralCalculator.h"


class functions : public IntegralCalculator
 {
  public:
   functions();
   double square(double aX);

 };

#endif

and the functions.cpp file is defined as such:

#include <iostream>
#include <cmath>
#include "functions.h"


double functions::square(double aX)
 {
   return aX*aX;
 }

I know that as of right now I don't need the <cmath> library, though I'm going to need it for more complicated functions when I add them.

I have tried inheriting the functions class in the main, and I have tried creating a namespace for the functions class and defining the function, however none of these have worked. When I go to compile, I get the following error:

Main.cpp: In function ‘int main()’:

Main.cpp:9: error: ‘square’ was not declared in this scope

If someone could please help me figure out this problem. This code that I'm trying to write is more advanced than my class has gone over, so I'm not sure how to fix it, though I want to write the program this way.

Edit: In case you were curious, I am including the functions.h and functions.cpp files in the Makefile. I was getting the same error when I was trying to put the square(double aX) definition in the IntegralCalculator class as well.

Solution

Your problem is, square is a member function AKA method. In other words, there is no plain symbol square anywhere, it's functions::square. And because it is not a static method, it makes no sense alone, it always needs a corresponding object instance, which you do not have and which IntegralCalculator couldn't use even if you did.

3 solutions, in the order of simplest, closest to your question, best:

---

From the constructor delcaration, move the square out of the class, make it a global function:

In functions.h outside any class:

double square(double aX);

In functions.cpp

double square(double aX) {
    return aX*aX;
}

Usage in main.cpp:

IntegralCalculator Function(square);

---

Or make it static method (which is how you would have to do if you used a language like Java, which didn't allow "naked" functions, but is not really C++ way):

in functions.h:

class functions // inheriting IntegralCalculator removed as not needed
{
    public:
    static double square(double aX);
};

and in functions.cpp:

//static
double functions::square(double aX) {
    return aX*aX;
}

Then you can pass it like this to `Function constructor in main.cpp:

IntegralCalculator Function(functions::square);

---

Finally, perhaps the best solution, sort of combining the best of the two above: put square in a namespace. So functions.h becomes

namespace functions {
    double square(double aX)
}

functions.cpp becomes

namespace functions {
    double square(double aX) {
        return aX*aX;
    }
}

You still use it in main like this:

IntegralCalculator Function(functions::square);