Separate classes into headers and source files in C++

Question

How do I separate classes into multiple files? Here is my understanding so far:

1. Create new class, and a ".h" and a ".cpp" file for it.
2. You use #include classname.h in your main source file to import its contents.
3. The Classname::Classname at the beginning of the source file is a Scope Resolution Operator.
4. You can call functions from main by using the objects that you have declared.

I'm just confused about how to implement this in practice. I have created a working calculator program with all the classes in one source file:

#include <iostream>
using namespace std;

class Addition {
    public:
    float add(float x, float y) {
        float sum;
        sum = x + y;
        return sum;
    }
};

class Subtraction {
    public:
    float subtract(float x, float y) {
        float dif;
        dif = x - y;
        return dif;
    }
};

class Multiplication {
    public:
    float multiply(float x, float y) {
        float prod;
        prod = x * y;
        return prod;
    }
};

class Division {
    public:
    float divide(float x, float y) {
        float quot;
        quot = x / y;
        return quot;
    }
};

int op;
char cont;

int main() {
    do {
    cout << "Welcome to C++ Calculator v2!" << endl;
    cout << "Select the number for which operation you want to use: " << endl;
    cout << "1-Addition" << endl;
    cout << "2-Subtraction" << endl;
    cout << "3-Mutliplication" << endl;
    cout << "4-Division" << endl;
    cin >> op;

    if (op == 1) {
        float num1;
        float num2;
        Addition addObj;
        cout << "You have chosen Addition!" << endl;
        cout << "Enter the first number you want to add: " << endl;
        cin >> num1;
        cout << "Enter the second number you wat to add: " << endl;
        cin >> num2;
        float ans = addObj.add(num1, num2);
        cout << "The sum is " << ans << endl;
        cout << "Do you wish to continue? Y/N" << endl;
        cin >> cont;
    }

    if (op == 2) {
        float num1;
        float num2;
        Subtraction subObj;
        cout << "You have chosen Subtraction!" << endl;
        cout << "Enter the first number you want to subtract: " << endl;
        cin >> num1;
        cout << "Enter the second number you want to subtract: " << endl;
        cin >> num2;
        float ans = subObj.subtract(num1, num2);
        cout << "The difference is " << ans << endl;
        cout << "Do you wish to continue? Y/N" << endl;
        cin >> cont;
    }

    if (op == 3) {
        float num1;
        float num2;
        Multiplication multObj;
        cout << "You have chosen Multiplication!" << endl;
        cout << "Enter the first number you want to multiply: " << endl;
        cin >> num1;
        cout << "Enter the second number you want to multiply: " << endl;
        cin >> num2;
        float ans = multObj.multiply(num1, num2);
        cout << "The product is " << ans << endl;
        cout << "Do you wish to continue? Y/N" << endl;
        cin >> cont;
    }

    if (op == 4) {
        float num1;
        float num2;
        Division divObj;
        cout << "You have chosen Division!" << endl;
        cout << "Enter the first number you want to divide: " << endl;
        cin >> num1;
        cout << "Enter the second number you want to divide: " << endl;
        cin >> num2;
        float ans = divObj.divide(num1, num2);
        cout << "The quotient is " << ans << endl;
        cout << "Do you wish to continue? Y/N" << endl;
        cin >> cont;
    }

    } while (cont == 'Y' || cont == 'y');

    if (cont == 'N' || 'n') {
    cout << "Thanks for using my program, goodbye!" << endl;
    }

    return 0;
}

I know there is a lot easier way to make something like this, but I've used classes and objects instead for the sole purpose of practice.

Answer 1

Ok, I will show you by doing your example:

subtraction.h

class Subtraction 
{
public:
 float subtract (float x, float y);
};

subtraction.cxx

#include "subtraction.h"

float Subtraction::subtract (float x, float y)
{     
  float dif;
  dif=x-y;
  return dif;    
}

multiplication.h

class Multiplication 
{
public:
  float multiply (float x, float y);
};

multiplication.cxx

#include "multiplication.h"

float Multiplication::multiply (float x, float y)
{
  float prod;
  prod=x*y;
  return prod;
}

and so on...

main.cxx

#include "subtraction.h"
#include "multiplication.h"

int main()
{
 //use the classes just as before.
}

Also, I didn't put it in the code here, for simplicity, but go ahead and get into the habit of ensuring that your declarations are only included once. On a large project, this can get very nasty if you don't put these safeguards in.

#ifndef SUBTRACTION_H
#define SUBTRACTION_H

class Subtraction
{
     ....
};
#endif /*SUBTRACTION_H*/

Answer 2

here's something like Jonathan's example (I did not add the include guards for brevity, definitely do so though!), but with some duplication removed and some OO added for your learning pleasure. Note that while this is certainly not how an actual calculator would be implemented, it will hopefully give you some more understanding of C++ if you study it well enough.

mathoperation.h:

  //a base class!
class MathOperation
{
public:
  virtual float doit( float x, float y ) const = 0;
};

subrtaction.h:

class Subtraction : public MathOperation
{
public:
  float doit( float x, float y ) const;
};

addition.h:

class Addition : public MathOperation
{
public:
  float doit( float x, float y ) const;
};

subtraction.cpp:

#include "subtraction.h"
float Subtraction::doit( float x, float y ) const { return x - y; }

addition.cpp:

#include "addition.h"
float Subtraction::doit( float x, float y ) const { return x + y; }

main.cpp:

#include <iostream>
#include <string>
  //yes this saves typing, but there reasons not to use it.. search SO!
using namespace std;

  //this one avoids you having to copy/paste the similar parts
void DoIt( const MathOperation& op, const std::string& opName, const std::string& verb, const std::string& resultName )
{
  cout << "You have chosen " << opName << "!" << endl;

  cout<<"Enter the first number you want to " << verb << ": "<< endl;

    //here you should actually check if the user really types in a number, and not    "blablabla"
    //and off course, put it in a seperate function so you can reuse it for num2
  float num1;
  cin>>num1;

  float num2;
  cout<<"Enter the second number you wat to " << verb << ": "<< endl;
  cin>>num2;

  cout<<"The " << resultName << " is " << op.doit( num1, num2 ) <<endl;
}

int main()
{
int op;
char cont = 'n';

do {
  cout<<"Welcome to C++ Calculator v2!"<<endl;
  cout<<"Select the number for which operation you want to use: "<<endl;
  cout<<"1-Addition"<<endl;
  cout<<"2-Subtraction"<<endl;
  cout<<"3-Mutliplication"<<endl;
  cout<<"4-Division"<<endl;
  cin>>op;

    //see how much shorter this is?
  if( op == 1 )
    DoIt( Addition(), "Addition", "add", "sum" );
  else if (op==2)
    DoIt( Subtraction(), "Subtraction", "subtract", "difference" );
  else
    cout << "Sorry I don't know this number" << endl;

  cout<<"Do you wish to continue? Y/N"<<endl;
  cin>>cont;

} while (cont=='Y'||cont=='y');

cout<<"Thanks for using my program, goodbye!"<<endl;
return 0;
}

Answer 3

It's good to name your files the same way as the class. It's not such a good idea in your case, beacause your classes are short, but each class should have its own 2 files - for example Subtraction.h and Subtraction.cpp. In the .h file you should put only the decaration - in your case: class Subtraction { public: float subtract (float , float); }

In the .cpp file you should include the .h file and put the implementation. In your case:

float Substraction::subtract (float x, float y) { float dif; dif=x-y; return dif; }

See also C++ Header Files, Code Separation and Why have header files and .cpp files in C++?

Hope this helps a bit! :)

Answer 4

It's good to have only one public class per file. By convention filename is the same as the classname (with an extension like .h .hh or .hpp if some definitions are done in the file).

if you want to put different classes in different files, a simple text editor will help you to do that.