Classes and private:

This is a discussion on Classes and private: within the C++ Programming forums, part of the General Programming Boards category; I know I can use what I put into private with in the class but I am having issues tying ...

  1. #1
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    Classes and private:

    I know I can use what I put into private with in the class but I am having issues tying it all together. Can anyone clear this up?

    Code:
    #include <iostream>
    
    class Plot {
        
        void area(float x, float y)
        {
            
            int Area = x * y;
            
            std::cout<< "The area is: "<< Area << std::endl;
        
        }
        void parameter(float x, float y)
        {
            
            int parameter = 2 * (x + y);
            
            std::cout<<"The parameter is: "<< parameter << std::endl;
        }
        
    public:
        float length;           //not using these right
        float width;
    };
    
    int main(int argc, const char * argv[])
    {
     
       Plot calculations;
        
        float inLength;
        float inWidth;
    
        std::cout<<"Please enter the lenth and width of the object in question?"<<std::endl;
        
        std::cout<<"Lenth:"<<std::endl;
        std::cin>>inLength;
        std::cout<<"Width: "<<std::endl;
        std::cin>>inWidth;
        
      
        calculations.area(inLength, inWidth);          //member of a private member, failing
        calculations.parameter(inLength, inWidth);     //member of a private member, failing
        
        
        return 0;
    }

  2. #2
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    By default the access to a class in C++ is private, if you want public or protected members you must use the public: and protected: specifiers. I always recommend you always state your intentions by always using the access specifiers:

    Code:
    class bogus
    {
       public:
          // some public members.
    
       protected:
          // some protected members.
    
       private:
          // some private members.
    };
    Jim

  3. #3
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    ugh....

    I thought declared public and private that I was blind to looking for it. But look at this. I want length to be private so if I just write it a variables within the class that would be correct?
    Code:
     #include <iostream>
    
    class Plot {
    public:
        void area(float x, float y)
        {
            
            int Area = x * y;
            
            std::cout<< "The area is: "<< Area << std::endl;
        
        }
        void parameter(float x, float y)
        {
            
            int parameter = 2 * (x + y);
            
            std::cout<<"The parameter is: "<< parameter << std::endl;
        }
        
    private:
        float length;
        float width;
    };
    
    int main(int argc, const char * argv[])
    {
     
       Plot calculations;
        
        float inLength;
        float inWidth;
    
        std::cout<<"Please enter the lenth and width of the object in question?"<<std::endl;
        
        std::cout<<"Lenth:"<<std::endl;
        std::cin>>inLength;
        std::cout<<"Width: "<<std::endl;
        std::cin>>inWidth;
        
      
        calculations.area(inLength, inWidth);
        calculations.parameter(inLength, inWidth);
        
        
        return 0;
    }
    Like this:
    Code:
     class Plot {
    public:
        void area(float length, float width)
        {
            
            int Area = length * width;
            
            std::cout<< "The area is: "<< Area << std::endl;
        
        }
        void parameter(float length, float width)
        {
            
            int parameter = 2 * (length + width);
            
            std::cout<<"The parameter is: "<< parameter << std::endl;
        }
        
    private:
        float length;
        float width;
    };

  4. #4
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    I don't really see any point of your private class variables, you never use them so why even have them.

    Jim

  5. #5
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    I want to but I cannot find a place to use them.

  6. #6
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    Perhaps you need to rethink the purpose of your class? Maybe you should create a function to assign a value to these variables? Then instead of passing parameters into your member functions just use the private member variables.


    Jim

  7. #7
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    Am I wrong in thinking I am assigning a value to these variables this way?

    Code:
     #include <iostream>
    
    class Plot {
    public:
        
        float getLength(float)
        {
        
            return length;
        }
        float getWidth(float)
        {
        
            return width;
        }
        void area()
        {
            
            int Area = length * width;
            
            std::cout<< "The area is: "<< Area << std::endl;
        
        }
        void parameter()
        {
      
            int parameter = 2 * (length + width);
            
            std::cout<<"The parameter is: "<< parameter << std::endl;
        }
        
    private:
        float length;
        float width;
    };
    
    int main(int argc, const char * argv[])
    {
     
       Plot calculations;
        
        float inLength;
        float inWidth;
    
        std::cout<<"Please enter the lenth and width of the object in question?"<<std::endl;
        std::cout<<" "<<std::endl;
        std::cout<<"Lenth:"<<std::endl;
        std::cin>>inLength;
        std::cout<<"Width: "<<std::endl;
        std::cin>>inWidth;
        
        calculations.getLength(inLength);
        calculations.getWidth(inWidth);
      
        calculations.area();
        calculations.parameter();
        
        
        return 0;
    }

  8. #8
    C++まいる!Cをこわせ! Elysia's Avatar
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    Yes, you are. GetLength() and GetWidth() takes unnecessary parameters with which it does nothing with and returns the private member variables (it does absolutely not assign them).
    Area() and Parameter() (perimeter?) calculates the area and perimeter from these variables, but they are never assigned.
    Perhaps you meant to say that GetLength and GetWidth be SetLength and SetWidth and actually assign the private variables instead of returning them.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  9. #9
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    Like this?

    Code:
     #include <iostream>
    
    class Plot {
    public:
        void setLength(float x)
        {
        
            length = x;
        
        }
        void setWidth(float y)
        {
            
            width = y;
            
        }
        void area()
        {
            
            int Area = length * width;
            
            std::cout<< "The area is: "<< Area << std::endl;
        
        }
        void perimeter()
        {
      
            int perimeter = 2 * (length + width);
            
            std::cout<<"The parameter is: "<< perimeter << std::endl;
        }
        
    private:
        
        float length;
        float width;
    };
    
    int main(int argc, const char * argv[])
    {
     
       Plot calculations;
        
        float inLength;
        float inWidth;
    
        std::cout<<"Please enter the lenth and width of the object in question?"<<std::endl;
        std::cout<<" "<<std::endl;
        std::cout<<"Lenth:"<<std::endl;
        std::cin>>inLength;
        std::cout<<"Width: "<<std::endl;
        std::cin>>inWidth;
        
        calculations.setLength(inLength);
        calculations.setWidth(inWidth);
      
        calculations.area();
        calculations.perimeter();
        
        
        return 0;
    }

  10. #10
    C++まいる!Cをこわせ! Elysia's Avatar
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    Yes, precisely so.
    area and perimeter should probably be getters (ie, returning said calculations instead of printing it).
    It might also be good to document your member variables by prefixing them with something (like m_). This allows you to distinguish between local variables and class member variables more easily.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  11. #11
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    Thank you very much...

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