- •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 26
cout << inPlayer->getName() << “ wants to play again” << endl; return true;
}else {
//The player said no, or is offline.
cout << inPlayer->getName() << “ does not want to play again” << endl; return false;
}
}
The Adapter Pattern
The motivation for changing the abstraction given by a class is not always driven by a desire to hide functionality or protect against performance concerns. Sometimes, the underlying abstraction cannot be changed but it doesn’t suit the current design. In this case, you can build an adapter or wrapper class. The adapter provides the abstraction that the rest of the code uses and serves as the bridge between the desired abstraction and the actual underlying code. You’ve already seen adapters in use by the STL. Recall that the STL provides container adapters, such as the stack and queue, which are wrappers around other containers like deque and list.
Example: Adapting an XML Library
In Chapter 24, you read about the Xerces XML parsing library. Xerces is a great general-purpose tool — it implements many obscure XML standards and provides much flexibility. However, there are several reasons why you might want a wrapper around Xerces. Your use case might be simple enough that you require only a subset of Xerces’ functionality. By writing a wrapper, you can maximize ease of use for the features that are relevant to you. Also, putting a wrapper around Xerces gives you the freedom to switch between different XML libraries. Perhaps you foresee a move to custom XML code down the road, or wish to allow users to write their own XML parsing code. As long as their code supports the same interface as your wrapper, it will work.
Implementation of an Adapter
The first step in writing an adapter is reading and understanding the class or library that you’re going to adapt. If you are unfamiliar with Xerces, you should review Chapter 24 before continuing.
The next step is to define the new interface to the underlying functionality. For this example, we will assume that users only need the Xerces features that were discussed in Chapter 24 — the ability to read XML elements, attributes, and text nodes. A single class, ParsedXMLElement, serves as an adapter to Xerces. The client creates a ParsedXMLElement from a file, which represents the root node. All subelements of that element are also represented as ParsedXMLElements. The following class definition shows the public functionality of ParsedXMLElement:
// ParsedXMLElement.h
#include <string> #include <vector>
class ParsedXMLElement
{
768
Applying Design Patterns
public:
ParsedXMLElement(const std::string& inFilename); ~ParsedXMLElement();
std::string getName() const; std::string getTextData() const;
std::string getAttributeValue(const std::string& inKey) const; std::vector<ParsedXMLElement*> getSubElements() const;
};
Because the adapter will be using Xerces behind the scenes, some additions are needed to this class definition. The ParsedXMLElement will be responsible for initializing the Xerces library when the first ParsedXMLElement root object is created and terminating the library when the last root object is deleted. In order to implement this functionality, the ParsedXMLElement needs to keep a static count of the number of root element objects in existence. Additionally, each ParsedXMLElement will contain a pointer to a Xerces DOMElement, which is used to actually obtain the parsed data. The XercesDOMParser object will need to remain in existence as long as associated DOMElements exist. The parser will live in the root object, so a comment warns clients that subelements are invalid once the root element is destroyed. Here is the modified definition of ParsedXMLElement:
// ParsedXMLElement.h
#include <string> #include <vector>
#include <xercesc/util/PlatformUtils.hpp> #include <xercesc/parsers/XercesDOMParser.hpp> #include <xercesc/dom/DOM.hpp>
XERCES_CPP_NAMESPACE_USE
/**
*Note: If the root element is deleted, subelements become
*invalid.
*/
class ParsedXMLElement
{
public:
ParsedXMLElement(const std::string& inFilename); ~ParsedXMLElement();
std::string getName() const; std::string getTextData() const;
std::string getAttributeValue(const std::string& inKey) const;
//The caller is responsible for freeing the ParsedXMLElements
//pointed to by the elements of the vector. std::vector<ParsedXMLElement*> getSubElements() const;
protected:
// This constructor is used internally to create subelements. ParsedXMLElement(DOMElement* inElement);
XercesDOMParser* mParser;
DOMElement* mElement;
769
Chapter 26
static int |
sReferences; |
private:
// Disallow copy construction and op=. ParsedXMLElement(const ParsedXMLElement&); ParsedXMLElement& operator=(const ParsedXMLElement& rhs);
};
The implementation of the wrapper is very similar to the examples in Chapter 24, so we won’t go into too much detail here: the code below should speak for itself. The important point is that every public method of ParsedXMLElement is really fronting calls to Xerces. We hope you agree that ParsedXMLElement provides a friendlier interface to this subset of Xerces functionality:
#include “ParsedXMLElement.h”
#include <xercesc/util/XMLString.hpp>
#include <iostream>
XERCES_CPP_NAMESPACE_USE using namespace std;
// No references by default
int ParsedXMLElement::sReferences = 0;
ParsedXMLElement::ParsedXMLElement(const std::string& inFilename)
{
if (sReferences == 0) {
// First element--initialize the library XMLPlatformUtils::Initialize();
}
sReferences++;
mParser = new XercesDOMParser(); mParser->parse(inFilename.c_str());
DOMNode* node = mParser->getDocument();
DOMDocument* document = dynamic_cast<DOMDocument*>(node); if (document == NULL) {
cerr << “WARNING: No XML document!” << endl; return;
}
mElement = dynamic_cast<const DOMElement*>(document->getDocumentElement()); if (mElement == NULL) {
cerr << “WARNING: XML Document had no root element!” << endl;
}
}
ParsedXMLElement::~ParsedXMLElement()
{
if (mParser != NULL) {
// This is the root element.
770
Applying Design Patterns
delete mParser;
sReferences--;
if (sReferences == 0) {
// Last element destroyed XMLPlatformUtils::Terminate();
}
}
}
string ParsedXMLElement::getName() const
{
char* tagName = XMLString::transcode(mElement->getTagName()); string result(tagName);
XMLString::release(&tagName);
return result;
}
string ParsedXMLElement::getTextData() const
{
//We assume that the first text node we reach is the one we want. DOMNodeList* children = mElement->getChildNodes();
for (int i = 0; i < children->getLength(); i++) {
DOMText* textNode = dynamic_cast<DOMText*>(children->item(i)); if (textNode != NULL) {
char* textData = XMLString::transcode(textNode->getData()); string result(textData);
XMLString::release(&textData); return result;
}
}
//No text nodes were found.
return “”;
}
string ParsedXMLElement::getAttributeValue(const std::string& inKey) const
{
XMLCh* key = XMLString::transcode(inKey.c_str());
const XMLCh* value = mElement->getAttribute(key); XMLString::release(&key);
char* valueString = XMLString::transcode(value); string result(valueString); XMLString::release(&valueString);
return result;
}
vector<ParsedXMLElement*> ParsedXMLElement::getSubElements() const
{
771