Help With Class Inhertance

doom1992 · #1 20-Oct-2007, 07:05

I need help accessing some numbers from the public class. I have not defined the intergers anywere becuase i dont know where. If you can help me anything you be appriciated. Sorry for bad spelling

CPP / C++ / C Code:

#include <iostream>
#include <string>

class Maths {
      public:
             void addition();
             void subtraction();
             };
             
      class addition : public Maths {
            public:
            //NEED HELP HERE
                   };
      
      class subtraction : public Maths {
            public:
                   //NEED HELP HERE
                   };          
      
      int main () {
          
          int x;
          int y;
          std::cout <<"Enter the value of x;
          std::cout <<"Enter the valu of y;
          ///do the addition function here.
          ///do the subtraction function here.
          
          return 0;
          }

Kimmo · #2 22-Oct-2007, 10:31

Quote:

Originally Posted by doom1992

I need help accessing some numbers from the public class. I have not defined the intergers anywere becuase i dont know where. If you can help me anything you be appriciated. Sorry for bad spelling

You need to tell a bit more about what you want. What integers are you talking about? Do you want to make the integers data members to the classes?

Before you go any further, consider your class hierarchy.

You have a base class Maths and derived class Addition. Now in your base class you declare a method addition(). Do you think that makes sense? If you have addition() in your base Maths class, why have a class Addition at all?

In my opinion Maths as a class is way too broad and abstract. Mathematics involves a lot more than just regular addition and division etc, things I wouldn't ever get a clue about. (I've tried, and failed.)

Here's what I would suggest:

Rename your Maths class to BinaryOperators or something. Then think about what is common with all binary operators. Two operands, right? So in your BinaryOperators class you would create two data members for two numbers.

You would NOT, however, create methods for addition and subtraction as you do in your example. In your example, the addition class has operators for addition and subtraction, but that doesn't really make any sense does it. Addition can't subract.

So, your classes could look like something as follows.

CPP / C++ / C Code:

class BinaryOperators
{
public:
    // maybe a constructor here? Maybe make this an abstract base class?

private:

    int operand1;
    int operand2;

};

class Addition : public BinaryOperators
{
public:
    
    // methods here

private:

    // doesn't probably need more data members?
};

class Subtraction : public BinaryOperators
{
    // ...
};

doom1992 · #3 22-Oct-2007, 10:58

thanks but that not wt im afgter, i will figure it out some how.

Algar · #4 22-Oct-2007, 11:48

if you want to have public access to public members (not usually recommended):

somefile.cpp

CPP / C++ / C Code:

class CMyNums {
public:
  int i;
  int j;
};

int main() {
 CMyNums objMyNums;
  objMyNums.i = 253; //some integer
  objMyNums.j = 234; //same deal
  return 0;
}

Generally when you make classes though you try and use information hiding, or encapsulation, or whatever you wanna call it by preventing direct access to member variables, but providing public methods (functions) to work with those private variables. Those are often called accessor and mutator methods.

somefile.cpp

CPP / C++ / C Code:

class CMyNums {
private:
  int i;
  int j;
public:
  void setI(int newi) { i = newi; } 
  void setJ(int newj) { j = newj; }
  int getI() { return i; } //returns a copy of I
  int getJ() { return j; } //returns a copy of J
};

int main() {
  CMyNums objMyNums;
  objMyNums.setI(14);  //internaly set i in CMyNums to 14
  objMyNums.setJ(objMyNums.getI()); //gets 14 from i and puts 14 in j
}

I didn't compile any of that or whatever but it should demonstrate the basic uses

doom1992 · #5 22-Oct-2007, 11:59

thanks m8, i now have a wider insite to this.

#6 22-Oct-2007, 20:06

Quote:

Originally Posted by doom1992

CPP / C++ / C Code:

class Maths {
      public:
             void addition();
             void subtraction();
             };

Here's a hint. If you are designing "functions" that take no parameters and return nothing, they are usually worthless. In otherwords, they tend to represent bad design choices. I'm not saying that these are your choices, but it is something to be considered.

You need to learn a lot more about OOP. Your base class is specifying behavior for your derived classes...but you define operations named "addition" but how is that useful in a class named Subtraction? Likewise, how is the operation "subtraction" useful in a class named Addition?

As Kimmo mentions, there are probably some useful ABCs to be implemented here...but what will we expect from the base? Overloaded operators? Aren't we talking about simple mathematics? I don't see any "natural" ABC from what it appears you're trying to implement. If there was one, I'd expect it to be a prototypical, but I'm not sure what it would be. I can only guess.

Without a stated purpose for what this is, I'm going to have to say that everywhere you see:

NEED HELP HERE

...in your code, you need help there as well as both above and below those lines.

:davis:

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Help With Class Inhertance

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