Dealing w/ virtual functions

[crystalattice]

I've been fighting w/ this problem for about 3 weeks. My school's forum and another programming forum I belong to have been unable to help me (I can't get ahold of my instructor but the school is looking into that).

When running the attached program, I get the errors

/home/cody/Projects/wavelength/src/wavelength.cpp:19: cannot allocate an object of type `Color'
*/home/cody/Projects/wavelength/src/wavelength.cpp:19: because the following virtual functions are abstract:
*/home/cody/Projects/wavelength/src/colors.h:26: virtual Color* Color:: previous() const
*/home/cody/Projects/wavelength/src/colors.h:25: virtual Color* Color::next() const
*/home/cody/Projects/wavelength/src/colors.h:23: virtual double Color::wavelength() const

(For ease, the main file is below)

#include <iostream>
#include "colors.h"

using namespace std;

int main()
{
  cout << "To compute the wavelength corresponding to a given color,\n"
    << "enter a color (e.g. Red): ";

  //string theColor;
  //cin >> theColor;
  Color* colorHandle = new Color();

  cout << "\nThe corresponding wavelength is " << colorHandle->wavelength() << endl;
  return 0;
}

I vaguely understand why this is happening; because the Color class and subclasses are abstract, the subclasses must provide a definition of the virtual method. But that's what I thought they did, for example:

// ***** Violet operations *************************************
inline Violet::Violet() : Color("Violet")
{}

inline double Violet::wavelength() const
{
  return 4.1e-7;
}

inline Color* Violet::next() const
{
  return NULL;
}

inline Color* Violet::previous() const
{
  return new Indigo();
}

Now, the colors.h and colors.cpp files were taken from the textbook website, as the actual problem is to use the Colors class hierarchy to create a program that gives the wavelength of light for a given color. The 2 files were originally for a program that used handles to output a table of color properties.

If I change the Color* to

Color* colorHandle = new theColor();

then all I get is a "syntax error before the ';' token".

Even if I don't use handles, I still get "virtual function" errors if I try to create a Color instance. Is there a correct way to input a color using the Color hierarchy? I've looked through the website's code and corrected what I think were deliberate errors but may have missed some.

[Chainsaw]

When you do a new on Color() the compiler notes that the Color() object has abstract virtuals. You defined an object called Violet. So, you need to do a new on Violet(), not Color()

You can still assign it to a Color* pointer, but the new has to be on the subclass:

Color* violet = new Violet;
Color* green = new Green;
Violet* violet = new Violet; // also ok, of course

[crystalattice]

Ideally, the user decides what wavelength is output and inputs the color to make it happen. Should I not be using object handles to do this?

When I tried it originally, I got many errors (hence the

//string theColor;
//cin >> theColor;

in the main code).

I guess what I'm asking is, would the code look like:

string theColor;
cin >> theColor;
Color* theColor = new theColor;

Is this valid?

[Chainsaw]

no, but I guess I don't understand the problem enough. Color has abstract virtuals, which means that you must declare a subclass of Color that has those virtuals defined, and it is the SUBCLASS that you must use.

Then you showed a Violet subclass of Color. Why can't you construct that?

Or, if you ask the customer to enter the wavelength, you could have a subclass with just a variable for the wavelength, but I don't understand what 'next' and 'prior' have to do with colors, so I'm not sure I can help here.

[crystalattice]

The problem used the Colors class heirarchy developed from the textbook's website. "next" and "previous" were included in those files as the textbook created it to print a table of colors and wavelengths. What I want to do is take the previously made files and just change the main program file. The change would allow a user to input a color and get the wavelength for it vs. a "hard-coded" table output.

In a nutshell, how are subclasses used with interactive input? I hope this makes more sense.

[Chainsaw]

Instead of your 'Violet' class, then, maybe have a 'VariableColor' class with a number for the wavelength (or RGB for color or whatever), and you would construct one of THOSE and give it the input wavelength.