- •Contents
- •Introduction
- •Who This Book Is For
- •What This Book Covers
- •How This Book Is Structured
- •What You Need to Use This Book
- •Conventions
- •Source Code
- •Errata
- •p2p.wrox.com
- •The Basics of C++
- •The Obligatory Hello, World
- •Namespaces
- •Variables
- •Operators
- •Types
- •Conditionals
- •Loops
- •Arrays
- •Functions
- •Those Are the Basics
- •Diving Deeper into C++
- •Pointers and Dynamic Memory
- •Strings in C++
- •References
- •Exceptions
- •The Many Uses of const
- •C++ as an Object-Oriented Language
- •Declaring a Class
- •Your First Useful C++ Program
- •An Employee Records System
- •The Employee Class
- •The Database Class
- •The User Interface
- •Evaluating the Program
- •What Is Programming Design?
- •The Importance of Programming Design
- •Two Rules for C++ Design
- •Abstraction
- •Reuse
- •Designing a Chess Program
- •Requirements
- •Design Steps
- •An Object-Oriented View of the World
- •Am I Thinking Procedurally?
- •The Object-Oriented Philosophy
- •Living in a World of Objects
- •Object Relationships
- •Abstraction
- •Reusing Code
- •A Note on Terminology
- •Deciding Whether or Not to Reuse Code
- •Strategies for Reusing Code
- •Bundling Third-Party Applications
- •Open-Source Libraries
- •The C++ Standard Library
- •Designing with Patterns and Techniques
- •Design Techniques
- •Design Patterns
- •The Reuse Philosophy
- •How to Design Reusable Code
- •Use Abstraction
- •Structure Your Code for Optimal Reuse
- •Design Usable Interfaces
- •Reconciling Generality and Ease of Use
- •The Need for Process
- •Software Life-Cycle Models
- •The Stagewise and Waterfall Models
- •The Spiral Method
- •The Rational Unified Process
- •Software-Engineering Methodologies
- •Extreme Programming (XP)
- •Software Triage
- •Be Open to New Ideas
- •Bring New Ideas to the Table
- •Thinking Ahead
- •Keeping It Clear
- •Elements of Good Style
- •Documenting Your Code
- •Reasons to Write Comments
- •Commenting Styles
- •Comments in This Book
- •Decomposition
- •Decomposition through Refactoring
- •Decomposition by Design
- •Decomposition in This Book
- •Naming
- •Choosing a Good Name
- •Naming Conventions
- •Using Language Features with Style
- •Use Constants
- •Take Advantage of const Variables
- •Use References Instead of Pointers
- •Use Custom Exceptions
- •Formatting
- •The Curly Brace Alignment Debate
- •Coming to Blows over Spaces and Parentheses
- •Spaces and Tabs
- •Stylistic Challenges
- •Introducing the Spreadsheet Example
- •Writing Classes
- •Class Definitions
- •Defining Methods
- •Using Objects
- •Object Life Cycles
- •Object Creation
- •Object Destruction
- •Assigning to Objects
- •Distinguishing Copying from Assignment
- •The Spreadsheet Class
- •Freeing Memory with Destructors
- •Handling Copying and Assignment
- •Different Kinds of Data Members
- •Static Data Members
- •Const Data Members
- •Reference Data Members
- •Const Reference Data Members
- •More about Methods
- •Static Methods
- •Const Methods
- •Method Overloading
- •Default Parameters
- •Inline Methods
- •Nested Classes
- •Friends
- •Operator Overloading
- •Implementing Addition
- •Overloading Arithmetic Operators
- •Overloading Comparison Operators
- •Building Types with Operator Overloading
- •Pointers to Methods and Members
- •Building Abstract Classes
- •Using Interface and Implementation Classes
- •Building Classes with Inheritance
- •Extending Classes
- •Overriding Methods
- •Inheritance for Reuse
- •The WeatherPrediction Class
- •Adding Functionality in a Subclass
- •Replacing Functionality in a Subclass
- •Respect Your Parents
- •Parent Constructors
- •Parent Destructors
- •Referring to Parent Data
- •Casting Up and Down
- •Inheritance for Polymorphism
- •Return of the Spreadsheet
- •Designing the Polymorphic Spreadsheet Cell
- •The Spreadsheet Cell Base Class
- •The Individual Subclasses
- •Leveraging Polymorphism
- •Future Considerations
- •Multiple Inheritance
- •Inheriting from Multiple Classes
- •Naming Collisions and Ambiguous Base Classes
- •Interesting and Obscure Inheritance Issues
- •Special Cases in Overriding Methods
- •Copy Constructors and the Equals Operator
- •The Truth about Virtual
- •Runtime Type Facilities
- •Non-Public Inheritance
- •Virtual Base Classes
- •Class Templates
- •Writing a Class Template
- •How the Compiler Processes Templates
- •Distributing Template Code between Files
- •Template Parameters
- •Method Templates
- •Template Class Specialization
- •Subclassing Template Classes
- •Inheritance versus Specialization
- •Function Templates
- •Function Template Specialization
- •Function Template Overloading
- •Friend Function Templates of Class Templates
- •Advanced Templates
- •More about Template Parameters
- •Template Class Partial Specialization
- •Emulating Function Partial Specialization with Overloading
- •Template Recursion
- •References
- •Reference Variables
- •Reference Data Members
- •Reference Parameters
- •Reference Return Values
- •Deciding between References and Pointers
- •Keyword Confusion
- •The const Keyword
- •The static Keyword
- •Order of Initialization of Nonlocal Variables
- •Types and Casts
- •typedefs
- •Casts
- •Scope Resolution
- •Header Files
- •C Utilities
- •Variable-Length Argument Lists
- •Preprocessor Macros
- •How to Picture Memory
- •Allocation and Deallocation
- •Arrays
- •Working with Pointers
- •Array-Pointer Duality
- •Arrays Are Pointers!
- •Not All Pointers Are Arrays!
- •Dynamic Strings
- •C-Style Strings
- •String Literals
- •The C++ string Class
- •Pointer Arithmetic
- •Custom Memory Management
- •Garbage Collection
- •Object Pools
- •Function Pointers
- •Underallocating Strings
- •Memory Leaks
- •Double-Deleting and Invalid Pointers
- •Accessing Out-of-Bounds Memory
- •Using Streams
- •What Is a Stream, Anyway?
- •Stream Sources and Destinations
- •Output with Streams
- •Input with Streams
- •Input and Output with Objects
- •String Streams
- •File Streams
- •Jumping around with seek() and tell()
- •Linking Streams Together
- •Bidirectional I/O
- •Internationalization
- •Wide Characters
- •Non-Western Character Sets
- •Locales and Facets
- •Errors and Exceptions
- •What Are Exceptions, Anyway?
- •Why Exceptions in C++ Are a Good Thing
- •Why Exceptions in C++ Are a Bad Thing
- •Our Recommendation
- •Exception Mechanics
- •Throwing and Catching Exceptions
- •Exception Types
- •Throwing and Catching Multiple Exceptions
- •Uncaught Exceptions
- •Throw Lists
- •Exceptions and Polymorphism
- •The Standard Exception Hierarchy
- •Catching Exceptions in a Class Hierarchy
- •Writing Your Own Exception Classes
- •Stack Unwinding and Cleanup
- •Catch, Cleanup, and Rethrow
- •Use Smart Pointers
- •Common Error-Handling Issues
- •Memory Allocation Errors
- •Errors in Constructors
- •Errors in Destructors
- •Putting It All Together
- •Why Overload Operators?
- •Limitations to Operator Overloading
- •Choices in Operator Overloading
- •Summary of Overloadable Operators
- •Overloading the Arithmetic Operators
- •Overloading Unary Minus and Unary Plus
- •Overloading Increment and Decrement
- •Overloading the Subscripting Operator
- •Providing Read-Only Access with operator[]
- •Non-Integral Array Indices
- •Overloading the Function Call Operator
- •Overloading the Dereferencing Operators
- •Implementing operator*
- •Implementing operator->
- •What in the World Is operator->* ?
- •Writing Conversion Operators
- •Ambiguity Problems with Conversion Operators
- •Conversions for Boolean Expressions
- •How new and delete Really Work
- •Overloading operator new and operator delete
- •Overloading operator new and operator delete with Extra Parameters
- •Two Approaches to Efficiency
- •Two Kinds of Programs
- •Is C++ an Inefficient Language?
- •Language-Level Efficiency
- •Handle Objects Efficiently
- •Use Inline Methods and Functions
- •Design-Level Efficiency
- •Cache as Much as Possible
- •Use Object Pools
- •Use Thread Pools
- •Profiling
- •Profiling Example with gprof
- •Cross-Platform Development
- •Architecture Issues
- •Implementation Issues
- •Platform-Specific Features
- •Cross-Language Development
- •Mixing C and C++
- •Shifting Paradigms
- •Linking with C Code
- •Mixing Java and C++ with JNI
- •Mixing C++ with Perl and Shell Scripts
- •Mixing C++ with Assembly Code
- •Quality Control
- •Whose Responsibility Is Testing?
- •The Life Cycle of a Bug
- •Bug-Tracking Tools
- •Unit Testing
- •Approaches to Unit Testing
- •The Unit Testing Process
- •Unit Testing in Action
- •Higher-Level Testing
- •Integration Tests
- •System Tests
- •Regression Tests
- •Tips for Successful Testing
- •The Fundamental Law of Debugging
- •Bug Taxonomies
- •Avoiding Bugs
- •Planning for Bugs
- •Error Logging
- •Debug Traces
- •Asserts
- •Debugging Techniques
- •Reproducing Bugs
- •Debugging Reproducible Bugs
- •Debugging Nonreproducible Bugs
- •Debugging Memory Problems
- •Debugging Multithreaded Programs
- •Debugging Example: Article Citations
- •Lessons from the ArticleCitations Example
- •Requirements on Elements
- •Exceptions and Error Checking
- •Iterators
- •Sequential Containers
- •Vector
- •The vector<bool> Specialization
- •deque
- •list
- •Container Adapters
- •queue
- •priority_queue
- •stack
- •Associative Containers
- •The pair Utility Class
- •multimap
- •multiset
- •Other Containers
- •Arrays as STL Containers
- •Strings as STL Containers
- •Streams as STL Containers
- •bitset
- •The find() and find_if() Algorithms
- •The accumulate() Algorithms
- •Function Objects
- •Arithmetic Function Objects
- •Comparison Function Objects
- •Logical Function Objects
- •Function Object Adapters
- •Writing Your Own Function Objects
- •Algorithm Details
- •Utility Algorithms
- •Nonmodifying Algorithms
- •Modifying Algorithms
- •Sorting Algorithms
- •Set Algorithms
- •The Voter Registration Audit Problem Statement
- •The auditVoterRolls() Function
- •The getDuplicates() Function
- •The RemoveNames Functor
- •The NameInList Functor
- •Testing the auditVoterRolls() Function
- •Allocators
- •Iterator Adapters
- •Reverse Iterators
- •Stream Iterators
- •Insert Iterators
- •Extending the STL
- •Why Extend the STL?
- •Writing an STL Algorithm
- •Writing an STL Container
- •The Appeal of Distributed Computing
- •Distribution for Scalability
- •Distribution for Reliability
- •Distribution for Centrality
- •Distributed Content
- •Distributed versus Networked
- •Distributed Objects
- •Serialization and Marshalling
- •Remote Procedure Calls
- •CORBA
- •Interface Definition Language
- •Implementing the Class
- •Using the Objects
- •A Crash Course in XML
- •XML as a Distributed Object Technology
- •Generating and Parsing XML in C++
- •XML Validation
- •Building a Distributed Object with XML
- •SOAP (Simple Object Access Protocol)
- •. . . Write a Class
- •. . . Subclass an Existing Class
- •. . . Throw and Catch Exceptions
- •. . . Read from a File
- •. . . Write to a File
- •. . . Write a Template Class
- •There Must Be a Better Way
- •Smart Pointers with Reference Counting
- •Double Dispatch
- •Mix-In Classes
- •Object-Oriented Frameworks
- •Working with Frameworks
- •The Model-View-Controller Paradigm
- •The Singleton Pattern
- •Example: A Logging Mechanism
- •Implementation of a Singleton
- •Using a Singleton
- •Example: A Car Factory Simulation
- •Implementation of a Factory
- •Using a Factory
- •Other Uses of Factories
- •The Proxy Pattern
- •Example: Hiding Network Connectivity Issues
- •Implementation of a Proxy
- •Using a Proxy
- •The Adapter Pattern
- •Example: Adapting an XML Library
- •Implementation of an Adapter
- •Using an Adapter
- •The Decorator Pattern
- •Example: Defining Styles in Web Pages
- •Implementation of a Decorator
- •Using a Decorator
- •The Chain of Responsibility Pattern
- •Example: Event Handling
- •Implementation of a Chain of Responsibility
- •Using a Chain of Responsibility
- •Example: Event Handling
- •Implementation of an Observer
- •Using an Observer
- •Chapter 1: A Crash Course in C++
- •Chapter 3: Designing with Objects
- •Chapter 4: Designing with Libraries and Patterns
- •Chapter 5: Designing for Reuse
- •Chapter 7: Coding with Style
- •Chapters 8 and 9: Classes and Objects
- •Chapter 11: Writing Generic Code with Templates
- •Chapter 14: Demystifying C++ I/O
- •Chapter 15: Handling Errors
- •Chapter 16: Overloading C++ Operators
- •Chapter 17: Writing Efficient C++
- •Chapter 19: Becoming Adept at Testing
- •Chapter 20: Conquering Debugging
- •Chapter 24: Exploring Distributed Objects
- •Chapter 26: Applying Design Patterns
- •Beginning C++
- •General C++
- •I/O Streams
- •The C++ Standard Library
- •C++ Templates
- •Integrating C++ and Other Languages
- •Algorithms and Data Structures
- •Open-Source Software
- •Software-Engineering Methodology
- •Programming Style
- •Computer Architecture
- •Efficiency
- •Testing
- •Debugging
- •Distributed Objects
- •CORBA
- •XML and SOAP
- •Design Patterns
- •Index
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.
219
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.
221
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.