Chapter 9
Pointers to methods and members usually won’t come up in your programs. However, it’s important to keep in mind that you can’t dereference a pointer to a non-static method or member without an object. Every so often, you’ll find yourself wanting to try something like passing a pointer to a non-static method to a function such as qsort() that requires a function pointer, which simply won’t work.
Note that C++ permits you to dereference a pointer to a static member or method without an object.
Chapter 22 discusses pointers to methods further in the context of the STL.
Building Abstract Classes
Now that you understand all the gory syntax of writing classes in C++, it helps to revisit the design principles from Chapters 3 and 5. Classes are the main unit of abstraction in C++. You should apply the principles of abstraction to your classes to separate the interface from the implementation as much as possible. Specifically, you should make all data members protected or private and provide getter and setter methods for them. This is how the SpreadsheetCell class is implemented. mValue and mString are protected, and set(), getValue(), and getString() retrieve those values. That way you can keep mValue and mString in synch internally without worrying about clients delving in and changing those values.
Using Interface and Implementation Classes
Even with the preceding measures and the best design principles, the C++ language is fundamentally unfriendly to the principle of abstraction. The syntax requires you to combine your public interfaces and private (or protected) data members and methods together in one class definition, thereby exposing some of the internal implementation details of the class to its clients.
The good news is that you can make your interfaces a lot cleaner and hide your implementation details. The bad news is that it takes a bit of hacking. The basic principle is to define two classes for every class you want to write: the interface class and the implementation class. The implementation class is identical to the class you would have written if you were not taking this approach. The interface class presents public methods identical to those of the implementation class, but it only has one data member: a pointer to an implementation class object. The interface class method implementations simply call the equivalent methods on the implementation class object. To use this approach with the Spreadsheet class, simply rename the old Spreadsheet class to SpreadsheetImpl. Here is the new SpreadsheetImpl class (which is identical to the old Spreadsheet class, but with a different name):
// SpreadsheetImpl.h #include “SpreadsheetCell.h”
class SpreadsheetApplication; // Forward reference
class SpreadsheetImpl
{
public:
SpreadsheetImpl(const SpreadsheetApplication& theApp, int inWidth = kMaxWidth, int inHeight = kMaxHeight);
SpreadsheetImpl(const SpreadsheetImpl& src);
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Mastering Classes and Objects
~SpreadsheetImpl();
SpreadsheetImpl &operator=(const SpreadsheetImpl& rhs);
void setCellAt(int x, int y, const SpreadsheetCell& inCell); SpreadsheetCell getCellAt(int x, int y);
int getId();
static const int kMaxHeight = 100; static const int kMaxWidth = 100;
protected:
bool inRange(int val, int upper);
void copyFrom(const SpreadsheetImpl& src);
int mWidth, mHeight; int mId;
SpreadsheetCell** mCells;
const SpreadsheetApplication& mTheApp;
static int sCounter;
};
Then define a new Spreadsheet class that looks like this:
#include “SpreadsheetCell.h”
// Forward declarations class SpreadsheetImpl;
class SpreadsheetApplication;
class Spreadsheet
{
public:
Spreadsheet(const SpreadsheetApplication& theApp, int inWidth, int inHeight);
Spreadsheet(const SpreadsheetApplication& theApp); Spreadsheet(const Spreadsheet& src); ~Spreadsheet();
Spreadsheet& operator=(const Spreadsheet& rhs);
void setCellAt(int x, int y, const SpreadsheetCell& inCell); SpreadsheetCell getCellAt(int x, int y);
int getId();
protected: SpreadsheetImpl* mImpl;
};
This class now contains only one data member: a pointer to a SpreadsheetImpl. The public methods are identical to the old Spreadsheet with one exception: the Spreadsheet constructor with default arguments has been split into two constructors because the values for the default arguments were const members that are no longer in the Spreadsheet class. Instead, the SpreadsheetImpl class will provide the defaults.
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Chapter 9
The implementations of the Spreadsheet methods such as setCellAt() and getCellAt() just pass the request on to the underlying SpreadsheetImpl object:
void Spreadsheet::setCellAt(int x, int y, const SpreadsheetCell& inCell)
{
mImpl->setCellAt(x, y, inCell);
}
SpreadsheetCell Spreadsheet::getCellAt(int x, int y)
{
return (mImpl->getCellAt(x, y));
}
int Spreadsheet::getId()
{
return (mImpl->getId());
}
The constructors for the Spreadsheet must construct a new SpreadsheetImpl to do its work, and the destructor must free the dynamically allocated memory. Note that the SpreadshetImpl class has only one constructor with default arguments. Both normal constructors in the Spreadsheet class call that constructor on the SpreadsheetImpl class:
Spreadsheet::Spreadsheet(const SpreadsheetApplication &theApp, int inWidth, int inHeight)
{
mImpl = new SpreadsheetImpl(theApp, inWidth, inHeight);
}
Spreadsheet::Spreadsheet(const SpreadsheetApplication& theApp)
{
mImpl = new SpreadsheetImpl(theApp);
}
Spreadsheet::Spreadsheet(const Spreadsheet& src)
{
mImpl = new SpreadsheetImpl(*(src.mImpl));
}
Spreadsheet::~Spreadsheet()
{
delete (mImpl); mImpl = NULL;
}
The copy constructor looks a bit strange because it needs to copy the underlying SpreadshetImpl from the source spreadsheet. Because the copy constructor takes a reference to a SpreadsheetImpl, not a pointer, you must dereference the mImpl pointer to get to the object itself to the constructor call can take its reference.
The Spreadsheet assignment operator must similarly pass on the assignment to the underlying
SpreadsheetImpl:
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Mastering Classes and Objects
Spreadsheet& Spreadsheet::operator=(const Spreadsheet& rhs)
{
*mImpl = *(rhs.mImpl); return (*this);
}
The first line in the assignment operator looks a little strange. You might be tempted to write this line instead:
mImpl = rhs.mImpl; // Incorrect assignment!
That code will compile and run, but it doesn’t do what you want. It just copies pointers so that the lefthand side and right-hand side Spreadsheets now both possess pointers to the same SpreadsheetImpl. If one of them changes it, the change will show up in the other. If one of them destroys it, the other
will be left with a dangling pointer. Therefore, you can’t just assign the pointers. You must force the SpreadsheetImpl assignment operator to run, which only happens when you copy direct objects. By dereferencing the mImpl pointers, you force direct object assignment, which causes the assignment operator to be called. Note that you can only do this because you already allocated memory for mImpl in the constructor.
This technique to truly separate interface from implementation is powerful. Although a bit clumsy at first, once you get used to it you will find it natural to work with. However, it’s not common practice in most workplace environments, so you might find some resistance to trying it from your coworkers.
Summar y
This chapter, along with Chapter 8, provided all the tools you need to write solid, well-designed classes, and to use objects effectively.
You discovered that dynamic memory allocation in objects presents new challenges: you must free the memory in the destructor, copy the memory in the copy constructor, and both free and copy memory in the assignment operator. You learned how to prevent assignment and pass-by-value by declaring a private copy constructor and assignment operator.
You learned more about different kinds of data members, including static, const, const reference, and mutable members. You also learned about static, inline, and const methods, and method overloading and default parameters. The chapter also described nested class definitions and friend classes and functions.
You encountered operator overloading, and learned how to overload the arithmetic and comparison operators, both as global friend functions and as class methods.
Finally, you learned how to take abstraction to an extreme by providing separate interface and implementation classes.
Now that you’re fluent in the language of object-oriented programming, it’s time to tackle inheritance and templates, which are covered in Chapters 10 and 11, respectively.
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Discovering Inheritance
Techniques
Without inheritance, classes would simply be data structures with associated behaviors. That alone would be a powerful improvement over procedural languages, but inheritance adds an entirely new dimension. Through inheritance, you can build new classes based on existing ones. In this way, your classes become reusable and extensible components. This chapter will teach you the different ways to leverage the power of inheritance. You will learn about the specific syntax of inheritance as well as sophisticated techniques for making the most of inheritance.
After finishing this chapter, you will understand:
How to extend a class through inheritance
How to employ inheritance to reuse code
How to build interactions between superclasses and subclasses
How to use inheritance to achieve polymorphism
How to work with multiple inheritance
How to deal with unusual problems in inheritance
The portion of this chapter relating to polymorphism draws heavily on the spreadsheet example discussed in Chapters 8 and 9. If you have not read Chapters 8 and 9, you may wish to skim the sample code in those chapters to get a background on this example. This chapter also refers to the object-oriented methodologies described in Chapter 3. If you have not read that chapter and are unfamiliar with the theories behind inheritance, you should review Chapter 3 before continuing.