How to declare class in different files in c++?

Question

I just learned that I can define function function in separate files than main.cpp as long as I declared the function in main.cpp and compile it using g++ main.cpp message.cpp.

But what if I want to define a class in separate files than main.cpp? It doesn't seems to work and I don't know what to declare in main.cpp while defining that class in separate files. Thanks.

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Edit:

Here is main.cpp code:

#include <iostream>
#include <string>
using namespace std;

int main() {
  int a, b;
  string str;
  Fraction fract;
  while (true) {
    cout << "Enter numerator: ";
    cin >> a;
    cout << "Enter denumerator: ";
    cin >> b;
    fract.set(a, b);
    cout << "Numerator is " << fract.get_num()
     << endl;
    cout << "Denumerator is " << fract.get_den()
     << endl;
    cout << "Do it again? (y or n) ";
    cin >> str;
    if (!(str[0] == 'y'))
      break;
  }
  return 0;
}

And here is Fraction.cpp

#include <cstdlib>

class Fraction {
private:
  int num, den;
public:
  void set(int n, int d) {
    num = n;
    den = d;
    normalize();
  }
  int get_num() {
    return num;
  }
  int get_den() {
    return den;
  }
private:
  void normalize() {
    if (den == 0 || num == 0) {
      num = 0;
      den = 1;
    }
    if (den < 0) {
      num *= -1;
      den *= -1;
    }
    int n = gcf(num, den);
    num = num / n;
    den = den / n;
  }
  int gcf(int a, int b) {
    if (b == 0)
      return abs(a);
    else
      return gcf(b, a%b);
  }
  int lcm(int a, int b) {
    int n = gcf(a, b);
    return a / n * b;
  }
};

But I can't compile it with g++ main.cpp Fraction.cpp.

---

Edit2:

It gives me error:

main.cpp: in function 'int main()':
main.cpp:8:3: error: 'Fraction' was not declared in this scope
main.cpp:14:15: 'fract' was not declared in this scope

Answer 1

I would create a Fraction.h file containing your class

#ifndef FRACTION_H
#define FRACTION_H

class Fraction {
private:
  int num, den;
public:
  void set(int n, int d) {
    num = n;
    den = d;
    normalize();
  }
  int get_num() {
    return num;
  }
  int get_den() {
    return den;
  }
private:
   void normalize();
   int gcf(int a, int b);
   int lcm(int a, int b);
};

#endif //ndef FRACTION_H   

Then Fraction.cpp containing

#include "Fraction.h"
void Fraction::normalize() {
  if (den == 0 || num == 0) {
    num = 0;
    den = 1;
  }
  if (den < 0) {
    num *= -1;
    den *= -1;
  }
  int n = gcf(num, den);
  num = num / n;
  den = den / n;
}

int Fraction::gcf(int a, int b) {
  if (b == 0)
    return abs(a);
  else
    return gcf(b, a%b);
}

int Fraction::lcm(int a, int b) {
  int n = gcf(a, b);
  return a / n * b;
}

Then in main.cpp make sure you add

#include "Fraction.h"

to your includes

The main reason I moved the larger functions from the class declaration to the Fraction.cpp was to keep the interface clean. However there are several other reasons to consider what member functions to implement in the header versus the translation unit.

The following question discusses that in more detail: Why have header files and .cpp files?

In the comments there was a question on the purpose of the #ifndef FRACTION_H these are include guards. A good discussion on what these are used for are here: Why are #ifndef and #define used in C++ header files?