Dealing with virtual functions

crystalattice · #1 16-Nov-2004, 22:34

When running the attached program, I get the errors

Code:

/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)

CPP / C++ / C Code:

#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:

CPP / C++ / C Code:

// ***** 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

CPP / C++ / C Code:

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.

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