Chapter 8 page 233, class SpreadsheetCell

phztfte1 · April 19th, 2016, 09:10 AM

The code for the SpreadsheetCell is as follows:

Code:

// SpreadsheetCell.cpp

#include <string>

class SpreadsheetCell
{
public:
	SpreadsheetCell();		    // default constructor
	virtual ~SpreadsheetCell();	   // destructor
	virtual void set(const std::string& inString) = 0; // setter for mString
	virtual std::string getString() const = 0;	     // getter for mString

};

The text indicates that the constructor is implemented just as a placeholder in case initialization needs to happen in the future. Why wasn't the constructor declared as virtual or pure virtual?

The text indicates that "SpreadsheetCell cell;" will not compile, and this is confirmed because I see the red squiggly line under "cell". But, why so, when there is a constructor?

Marc Gregoire · April 20th, 2016, 03:43 PM

A constructor in C++ cannot be virtual and so also not pure virtual.
Since in this example, the constructor is not actually doing anything useful, you might as well remove it in this case.
However, as the text explains, the virtual destructor is required, even though it is empty.

SpreadsheetCell cell; will not compile because SpreadsheetCell is an abstract class because it contains pure virtual methods, namely getString() and set().
So, you can only create instances of subclasses that have an implementation for these pure virtual methods.

phztfte1 · April 20th, 2016, 10:30 PM

Thank you for the explanation.