creating a c++ class file in both a .h and .cpp file

Question

The assignment involves using many class files to output volume and areas of figures.

My program works perfectly how it is now, but my instructor wants the class files to use both a .h and .cpp file for each class. At the moment I am only using a .h file for each class.

Would appreciate help on how to split it into two files correctly. Let me know if there are any questions!

Below is one of the class .h files that needs to become a .h and a .cpp file.

#ifndef CYLINDER_H
#define CYLINDER_H

class Cylinder {
    double height, radius;
    double PI=3.1415926535898;

public:
   void set_values (double,double);
   double volume () {return (radius*radius)*(height)*PI;}
   double area() {return ((2.0000*PI)*(radius)*(radius))+((2.0000*PI)*(radius*height));}

 };

void Cylinder::set_values (double x, double y) {
   height = x;
   radius = y;
 }

  #endif    /* CYLINDER_H */

Please edit your question to include the code. Posting a link to an image of text is not really the done thing! — enhzflep, Dec 10 '13 at 22:06
looks like you already have it split up in your header, just start with moving the Cylinder::set_values() function to the .cpp file and include the header, then you should be fine. you should try it out yourself. when that compiles move the other two functions into the .cpp and leave the prototypes in the same form as set_values() — AndersK, Dec 10 '13 at 22:22

odinthenerd · Accepted Answer · 2013-12-10T22:57:16.183

divide the code as follows:

in the Cylinder.h

#ifndef CYLINDER_H
#define CYLINDER_H

class Cylinder {
    double height, radius;
    double PI=3.1415926535898;

public:
   void set_values(double,double);
   double volume();
   double area();    
 };
 #endif    /* CYLINDER_H */

in the Cylinder.cpp

#include "Cylinder.h"
void Cylinder::set_values(double x, double y) {
   height = x;
   radius = y;
 }

double Cylinder::volume() {
    return (radius*radius)*(height)*PI;
}
double Cylinder::area() {
    return ((2.0000*PI)*(radius)*(radius))+((2.0000*PI)*(radius*height));
}

The reason this is good practice is that a function can be declared as many times as you like but only defined once. void set_values(double,double); declares the function (notice no curly braces) whereas

void Cylinder::set_values(double x, double y) { 
   height = x;
   radius = y;
}

defines it. By deviding the declarations and the definitions into two different files you are able to include the cylender.h in many other files so that they can use the class without having to worry about it being defined in two places.

It is actually technically more complicated than that because function that are defined inside the body (between the curly braces) of the class are allowed to be defined more than once as long as all the definitions are exactly the same. This is called the One Definition Rule or ODR, it is actually more complicated then it seems at first because macros and the preprocessor can actually change the definition depending on what has been previously defined. That is why it is good advice to divide things into .h and .cpp files in this way until you fully understand the ODR and the preprocessor.

This method also has the advantage that the order in which things are written is not as important. If you declare everything first and then define it you are much less likely to have to shuffle the order around so that everything is known by the compiler before it is used.

I would also suggest reading about operator precetance (for example here: http://en.wikipedia.org/wiki/Operators_in_C_and_C++) since you are making many unneeded parenthesis I think you may benefit from it. For example in volume all the operators are * which is evaluated left to right, which is perfectly fine so radius*radius*height*PI is fine. In area it is a little more complicated, however because operator * has a higher precedence than + 2.0000*PI*radius*radius+2.0000*PI*radius*height should produce the same result.

You also need to include the header in the cpp file also the file that uses the class. — drescherjm, Dec 10 '13 at 22:32
@PorkyBrain I am getting an error saying ":set_values(double,double)' in the .h file. do you know why that could be? sorry if it is obvious, i am very much a beginner — user2493923, Dec 10 '13 at 22:33
@PorkyBrain thank you for the resource, will look at that for sure — user2493923, Dec 10 '13 at 22:36
@PorkyBrain does this help? /cygdrive/c/Users/Laura/Documents/NetBeansProjects/proj4/cylinder.h:22: multiple definition of `Cylinder::set_values(double, double)' — user2493923, Dec 10 '13 at 22:37
@user2493923 do you maybe have the function definition (the part with {} braces) in the headder and in the .cpp at the same time? — odinthenerd, Dec 10 '13 at 22:39
@PorkyBrain ah yes that helped me figure it out and it works well now! thank you so much, your help is very very appreciated!! — user2493923, Dec 10 '13 at 22:46

creating a c++ class file in both a .h and .cpp file

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