Thread: How to learn Object Oriented Programming / Classes?

Are you learning C, or another programming language that has built-in object oriented programming support, e.g., C++?

Originally Posted by JorgeChemE

I am only learning C/C++.

You should read Bjarne Stroustrup's answer to the FAQ: What do you think of C/C++?

As such, I shall assume that you are actually learning C++, so I shall move this to the C++ programming forum.

Originally Posted by JorgeChemE

I have an easy exercise to calculate the volume of a furniture (length x width x deep) using OOP but I need to understand the neccesary theory to solve it. Where to start?

OOP sounds like a wrong tool for computing the volume of furniture, unless you are modeling different (sub)types of furniture (table, chair, bean bag, odd-shaped sofa, etc), and wish to present an interface for computing the volume of furniture without having to inspect the exact subtype of furniture in order to make the computation. In such a case, you might have an abstract base class named Furniture that has a pure virtual member function, e.g., compute_volume() that subclasses override with the precise volume computation implementation appropriate for them.

I suggest reading these articles (PDF documents):
The Open-Closed Principle
The Liskov Substitution Principle
The Dependency Inversion Principle
The Interface Segregation Principle

Originally Posted by JorgeChemE

I only need to make this program: calculate the volume of a furniture supposing that the volume is: length, width, depth and can be calculated as: length x width x depth.

Well, if this exercise can be done in int main () or using functions or structure I would solve it in a matter of minutes.

You should solve it by writing a main function that just does the job.

If that is unacceptable, then your instructor is teaching you OOP badly, and I propose this Java-inspired "solution":

Code:

class Main
{
public:
    double compute_volume(double length, double width, double depth)
    {
        // ...
    }

    void doWork()
    {
        // call compute_volume somewhere
        // ...
    }
};

int main()
{
    Main object;
    object.doWork();
}

Tada! "Object oriented programming"!

I am afraid that I cannot explain to you how this is OOP because... it isn't really OOP. It is just your usual procedural programming under the veneer of a class and an object of the class.

EDIT:

Well, okay. I suspect that what your instructor had in mind was the creation of a Furniture object with the three attributes and a compute_volume member function to compute the volume. This is arguably "OOP", but at a level that isn't very different from the typical kind of structure manipulation that is also part of primarily procedural languages like C. Plus, the name "Furniture" is arguably too generic to make assumptions that the volume is that of a cuboid.

Laser, the best way I have to explain you what I need is with an example. This is what I have done, my first approach without any success. Hope that is easier to understand if I put you a code, but since I don't know how to post it since I have three files I will put them separately:

First file: main.cpp

Code:

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include "Furniture.h"


using namespace std;


void print(Furniture *j)
{
    printf("\nPrint using a function\n");
    printf("Furniture: %s ", j->Getfurniture());
    printf("Volume of furniture: %.2f ", j->Getvolume_furniture());
    printf("Length: %.2f ", j->Getlength());
    printf("Width: %.2f ", j->Getwidth());
    printf("Depth: %.2f", j->Getdepth());
}


int main()
{
    Furniture *j1 = new Furniture();
    j1->Setfurniture("Furniture");
    j1->Setvolume_furniture(volume_furniture);
    j1->print();


    j1->Setlength(10);
    j1->print();


    j1->Setwidth(20);
    j1->print();


    j1->Setdepth(30);
    j1->print();


    return 0;
}

Second file: Furniture.h

Code:

#ifndef FURNITURE_H
#define FURNITURE_H


class Furniture
{
    public:
        Furniture();
        Furniture(char* furniture, float volume_furniture, float length, float width, float depth);
        virtual ~Furniture();//Destructor


        char* Getfurniture();
        void Setfurniture(char* val);
        float Getvolume_furniture();
        void Setvolume_furniture(float val);
        float Getlength();
        void Setlength(float val);
        float Getwidth();
        void Setwidth(float val);
        float Getdepth();
        void Setdepth(float val);


        void print();


    protected:


    private:
        char* furniture;
        float volume_furniture;
        float length;
        float width;
        float depth;
};


#endif // FURNITURE_H

Third File: Furniture.cpp

Code:

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include "Furniture.h"


Furniture::Furniture()
{
    this->furniture=NULL;
    this->volume_furniture=0.0;
    this->length=0.0;
    this->width=0.0;
    this->depth=0.0;
}
Furniture::Furniture(char* furniture, float volume_furniture, float length, float width, float depth)
{
    this->furniture=new char[strlen(furniture)+1];
    strcpy(this->furniture,furniture);
    this->volume_furniture=volume_furniture;
    this->length=length;
    this->width=width;
    this->depth=depth;
}


Furniture::~Furniture()
{
    if (this->furniture!=NULL) {
        delete[] this->furniture;
    }
}


char* Furniture:: Getfurniture()
{
    return furniture;
}
void Furniture:: Setfurniture(char* val)
{
    furniture=val;
}
float Furniture:: Getvolume_furniture()
{
    return volume_furniture;
}
void Furniture:: Setvolume_furniture(float val)
{
    volume_furniture=length*width*depth;
}
float Furniture:: Getlength()
{
    return length;
}
void Furniture:: Setlength(float val)
{
    if (val<0) {
        length=0;
    } else {
    length=val;
    }
}
float Furniture:: Getwidth()
{
    return width;
}
void Furniture:: Setwidth(float val)
{
    if (val<0) {
        width=0;
    } else {
    width=val;
    }
}
float Furniture:: Getdepth()
{
    return depth;
}
void Setdepth(float val)
{
    if (val<0) {
        depth=0;
    } else {
    depth=val;
    }
}

This is only an attempt to adapt a code that stores names, ages and heigths of people to this exercise that asks a description and a volume of an object. By the way, I failed even declaring the description and the values of measurements of the object and I should ask the user to input them through keyboard but I am still trying this way before proceeding to ask data through keyboard.

I created a new class in Code Blocks, I have the classes public, protected and private (we still don't use protected until the next week, we are still learning classes/OOP), "Getters" and "Setters".

I hope with this information you have a little idea of what I am doing.

Sorry for omitting where is the error: is in int main(), line 25

Code:

j1->Setvolume_furniture(volume_furniture);

Originally Posted by jimblumberg

Also where have you defined a variable named volume_furniture?

And where have you defined and implemented a class member function named print(), all I see is the global function print().

And you really should be pushing your instructor to start using modern C++ classes like std::string instead of using the more dangerous C style strings.


Jim

Fixed the pointers. That's one of my questions, I don't know where to define volume_furniture (to calculate the volume), this is my first attempt using classes. All I know is that I calculate the volume in Furniture.cpp.

The function print() is in public and a global function as you mentioned is in main. Should I implement "a class member function named print()"? where?

Edited: if helps this is the error I get:

|23|error: no matching function for call to 'Mueble::Setvolume_furniture()'|
|23|note: candidate is:|
include\Furniture.h|15|note: void Mueble::Setvolumen_mueble(float)|

First file: main.cpp

Code:

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include "Furniture.h"


using namespace std;


void print(Furniture *j)
{
    printf("\nPrint using a function\n");
    printf("Furniture: %s ", j->Getfurniture());
    printf("Volume of furniture: %.2f ", j->Getvolume_furniture());
    printf("Length: %.2f ", j->Getlength());
    printf("Width: %.2f ", j->Getwidth());
    printf("Depth: %.2f", j->Getdepth());
}


int main()
{
    Furniture *j1 = new Furniture();
    j1->Setfurniture("Furniture");
    j1->Setvolume_furniture(volume_furniture);
    j1->print();


    j1->Setlength(10);
    j1->print();


    j1->Setwidth(20);
    j1->print();


    j1->Setdepth(30);
    j1->print();

    delete j1;

    return 0;
}

Second file: Furniture.h

Code:

#ifndef FURNITURE_H
#define FURNITURE_H


class Furniture
{
    public:
        Furniture();
        Furniture(char* furniture, float volume_furniture, float length, float width, float depth);
        virtual ~Furniture();


        char* Getfurniture();
        void Setfurniture(char* val);
        float Getvolume_furniture();
        void Setvolume_furniture();
        float Getlength();
        void Setlength(float val);
        float Getwidth();
        void Setwidth(float val);
        float Getdepth();
        void Setdepth(float val);


        void print();


    protected:


    private:
        char* furniture;
        float volume_furniture;
        float length;
        float width;
        float depth;
};


#endif // FURNITURE_H

Third File: Furniture.cpp

Code:

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "Furniture.h"


Furniture::Furniture()
{
    this->furniture=NULL;
    this->volume_furniture=0.0;
    this->length=1.0;
    this->width=1.0;
    this->depth=1.0;
}
Furniture::Furniture(char* furniture, float volume_furniture, float length, float width, float depth)
{
    this->furniture=new char[strlen(furniture)+1];
    strcpy(this->furniture,furniture);
    this->volume_furniture=volume_furniture;
    this->length=length;
    this->width=width;
    this->depth=depth;
}


Furniture::~Furniture()
{
    if (this->furniture!=NULL) {
        delete[] this->furniture;
    }
}


char* Furniture:: Getfurniture()
{
    return furniture;
}
void Furniture:: Setfurniture(char* val)
{
    furniture=val;
}
float Furniture:: Getvolume_furniture()
{
    return volume_furniture;
}
void Furniture:: Setvolume_furniture()
{
    volume_furniture=length*width*depth;
}
float Furniture:: Getlength()
{
    return length;
}
void Furniture:: Setlength(float val)
{
    if (val<0) {
        length=0;
    } else {
    length=val;
    }
}
float Furniture:: Getwidth()
{
    return width;
}
void Furniture:: Setwidth(float val)
{
    if (val<0) {
        width=0;
    } else {
    width=val;
    }
}
float Furniture:: Getdepth()
{
    return depth;
}
void Furniture:: Setdepth(float val)
{
    if (val<0) {
        depth=0;
    } else {
    depth=val;
    }
}

This is the code of the 3 files (main.cpp, Furniture.h, Furniture.cpp). Note that I am still using pointers but I will fix it later, I think the pointers are ok but I will change them with your recommendations if I achieve to solve this. Even if the program runs, there is still one thing I need to fix: data should be inputted through keyboard instead of declaring values for length, width, depth. Another thing I fixed is that in constructor I declared all the starting values with 1 because I read somewhere else is better than 0 (except the volume to calculate).

I appreciate any feedback. Thanks!