- •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
Discovering Inheritance Techniques
If the dontTell() method is called on a Blabber object, it will output I’ll tell all!
myBlabber.dontTell(); // Outputs “I’ll tell all!”
In this case, however, the protected method in the superclass stays protected because any attempts to call Secret’s dontTell() method through a pointer or reference will not compile.
Blabber myBlabber;
Secret& ref = myBlabber;
Secret* ptr = &myBlabber;
ref.dontTell(); // BUG! Attempt to access protected method. ptr->dontTell(); // BUG! Attempt to access protected method.
The only truly useful way to change a method’s access level is by providing a less restrictive accessor to a protected method.
Copy Constructors and the Equals Operator
In Chapter 9, we said that providing a copy constructor and assignment operator is considered a good programming practice when you have dynamically allocated memory in the class. When defining a subclass, you need to be careful about copy constructors and operator=.
If your subclass does not have any special data (pointers, usually) that require a nondefault copy constructor or operator=, you don’t need to have one, regardless of whether or not the superclass has one. If your subclass omits the copy constructor, the parent copy constructor will still be called when the object is copied. Similarly, if you don’t provide an explicit operator=, the default one will be used, and operator= will still be called on the parent class.
On the other hand, if you do specify a copy constructor in the subclass, you need to explicitly chain to the parent copy constructor, as shown in the following code. If you do not do this, the default constructor (not the copy constructor!) will be used for the parent portion of the object.
class Super
{
public:
Super();
Super(const Super& inSuper);
};
class Sub : public Super
{
public:
Sub();
Sub(const Sub& inSub);
};
Sub::Sub(const Sub& inSub) : Super(inSub)
{
}
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Similarly, if the subclass overrides operator=, it is almost always necessary to call the parent’s version of operator= as well. The only case where you wouldn’t do this is if there is some bizarre reason why you only want part of the object assigned when an assignment takes place. The following code shows how to call the parent’s assignment operator from the subclass:
Sub& Sub::operator=(const Sub& inSub)
{
if (&inSub == this) { return *this;
}
Super::operator=(inSub) // Call parent’s operator=.
// Do necessary assignments for subclass.
return (*this);
}
If your subclass does not specify its own copy constructor or operator=, the parent functionality continues to work. If the subclass does provide its own copy constructor or operator=, it needs to explicitly reference the parent versions.
The Truth about Virtual
When you first encountered method overriding above, we told you that only virtual methods can be properly overridden. The reason we had to add the qualifier properly is that if a method is not virtual, you can still attempt to override it but it will be wrong in subtle ways.
Hiding Instead of Overriding
The following code shows a superclass and a subclass, each with a single method. The subclass is attempting to override the method in the superclass, but it is not declared to be virtual in the superclass.
class Super
{
public:
void go() { cout << “go() called on Super” << endl; }
};
class Sub : public Super
{
public:
void go() { cout << “go() called on Sub” << endl; }
};
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Discovering Inheritance Techniques
Attempting to call the go() method on a Sub object will initially appear to work.
Sub mySub;
mySub.go();
The output of this call is, as expected, go() called on Sub. However, since the method was not virtual, it was not actually overridden. Rather, the Sub class created a new method, also called go() that is completely unrelated to the Super class’s method called go(). To prove this, simply call the method in the context of a Super pointer or reference.
Sub mySub;
Super& ref = mySub;
ref.go();
You would expect the output to be, go() called on Sub, but in fact, the output will be, go() called on Super. This is because the ref variable is a Super reference and because the virtual keyword was omitted. When the go() method is called, it simply executes Super’s go() method. Because it is not virtual, there is no need to consider whether a subclass has overridden it.
Attempting to override a non-virtual method will “hide” the superclass definition and will only be used in the context of the subclass.
How virtual Is Implemented
To understand why method hiding occurs, you need to know a bit more about what the virtual keyword actually does. When a class is compiled in C++, a binary object is created that contains all of the data members and methods for the class. In the non-virtual case, the code to jump to the appropriate method is hard-coded directly where the method is called based on the compile-time type.
If the method is declared virtual, the implementation is looked up in a special area of memory called the vtable, for “virtual table.” Each class that has one or more virtual methods has a vtable that contains pointers to the implementations of the virtual methods. In this way, when a method is called on an object, the pointer is followed into the vtable and the appropriate version of the method is executed based on the type of the object, not the type of the variable used to access it.
Figure 10-11 shows a high-level view of how the vtable makes the overriding of methods possible. The diagram shows two classes, Super and Sub. Super declares two virtual methods, foo() and bar(). As you can see by looking at Super’s vtable, each method has its own implementation defined by the Super class. The Sub class does not override Super’s version of foo(), so the Sub vtable points to the same implementation of foo(). Sub does, however, override bar(), so the vtable points to the new version.
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Super |
|
|
|
vtable |
foo |
Super::foo() |
|
implementation |
|||
|
|
||
|
bar |
Super::bar() |
|
|
|
||
|
|
implementation |
Sub
vtable |
foo |
|
|
bar |
Sub::bar() |
|
|
|
|
|
implementation |
Figure 10-11
The Justification for virtual
Given the fact that you are advised to make all methods virtual, you might be wondering why the virtual keyword even exists. Can’t the compiler automatically make all methods virtual? The answer is yes, it could. Many people think that the language should just make everything virtual. The Java language effectively does this.
The argument against making everything virtual, and the reason that the keyword was created in the first place, has to do with the overhead of the vtable. To call a virtual method, the program needs to perform an extra operation by dereferencing the pointer to the appropriate code to execute. This is a miniscule performance hit for most cases, but the designers of C++ thought that it was better, at least at the time, to let the programmer decide if the performance hit was necessary. If the method was never going to be overridden, there was no need to make it virtual and take the performance hit. There is also a small hit to code size. In addition to the implementation of the method, each object would also need a pointer, which takes up a small, but measurable, amount of space.
The Horror of Non-virtual Destructors
Even programmers who don’t adopt the guideline of making all methods virtual still adhere to the rule when it comes to destructors. The reason is that making your destructors non-virtual can easily cause memory leaks.
For example, if a subclass uses memory that is dynamically allocated in the constructor and deleted in the destructor, it will never be freed if the destructor is never called. As the following code shows, it is easy to “trick” the compiler into skipping the call to the destructor if it is non-virtual.
266