- •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 10
Respect Your Parents
When you write a subclass, you need to be aware of the interaction between parent classes and child classes. Issues such as order of creation, constructor chaining, and casting are all potential sources of bugs.
Parent Constructors
Objects don’t spring to life all at once; they must be constructed along with their parents and any objects that are contained within them. C++ defines the creation order as follows:
1.The base class, if any, is constructed.
2.Non-static data members are constructed in the order in which they were declared.
3.The body of the constructor is executed.
These rules can apply recursively. If the class has a grandparent, the grandparent is initialized before the parent, and so on. The following code shows this creation order. As a reminder, we generally advise against inlining methods, as we’ve done in the code that follows. In the interest of readable and concise examples, we have broken our own rule. The proper execution will output the result 123.
#include <iostream> using namespace std;
class Something
{
public:
Something() { cout << “2”; }
};
class Parent
{
public:
Parent() { cout << “1”; }
};
class Child : public Parent
{
public:
Child() { cout << “3”; }
protected:
Something mDataMember;
};
int main(int argc, char** argv)
{
Child myChild;
}
When the myChild object is created, the constructor for Parent is called first, outputting the string “1”. Next, mDataMember is initialized, calling the Something constructor which outputs the string “2”. Finally, the Child constructor is called, which outputs 3.
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Discovering Inheritance Techniques
Note that the Parent constructor was called automatically. C++ will automatically call the default constructor for the parent class if one exists. If no default constructor exists in the parent class, or if one does exist but you wish to use an alternate constructor, you can chain the constructor just as when initializing data members in the initializer list.
The following code shows a version of Super that lacks a default constructor. The associated version of Sub must explicitly tell the compiler how to call the Super constructor or the code will not compile.
//Super.h class Super
{
public: Super(int i);
};
//Sub.h
class Sub : public Super
{
public:
Sub();
};
// Sub.cpp Sub::Sub() : Super(7)
{
// Do Sub’s other initialization here.
}
In the preceding code, the Sub constructor passes a fixed value (7) to the Super constructor. Sub could also pass a variable if its constructor required an argument:
Sub::Sub(int i) : Super(i) {}
Passing constructor arguments from the subclass to the superclass is perfectly fine and quite normal. Passing data members, however, will not work. The code will compile, but remember that data members are not initialized until after the superclass is constructed. If you pass a data member as an argument to the parent constructor, it will be uninitialized.
Parent Destructors
Because destructors cannot take arguments, the language can automatically call the destructor for parent classes. The order of destruction is conveniently the reverse of the order of construction:
1.
2.
3.
The body of the destructor is called.
Any data members are destroyed in the reverse order of their construction.
The parent class, if any, is destructed.
Again, these rules apply recursively. The lowest member of the chain is always destructed first. The following code adds destructors to the previous example. If executed, this code will output “123321”.
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Chapter 10
#include <iostream> using namespace std;
class Something
{
public:
Something() { cout << “2”; }
virtual ~Something() { cout << “2”; }
};
class Parent
{
public:
Parent() { cout << “1”; }
virtual ~Parent() { cout << “1”; }
};
class Child : public Parent
{
public:
Child() { cout << “3”; }
virtual ~Child() { cout << “3”; }
protected:
Something mDataMember;
};
int main(int argc, char** argv)
{
Child myChild;
}
Notice that the destructors are all virtual. As a rule of thumb, all destructors should be declared virtual. If the preceding destructors were not declared virtual, the code would continue to work fine. However, if code ever called delete on a superclass pointer that was really pointing to a subclass, the destruction chain would begin in the wrong place. For example, the following code is similar to the previous example but the destructors are not virtual. This becomes a problem when a Child object is accessed as a pointer to a Parent and deleted.
class Something
{
public:
Something() { cout << “2”; }
~Something() { cout << “2”; } // Should be virtual, but will work
};
class Parent
{
public:
Parent() { cout << “1”; }
~Parent() { cout << “1”; } // BUG! Make this virtual!
};
class Child : public Parent
{
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Discovering Inheritance Techniques
public:
Child() { cout << “3”; }
~Child() { cout << “3”; } // Should be virtual, but will work
protected:
Something mDataMember;
};
int main(int argc, char** argv)
{
Parent* ptr = new Child();
delete ptr;
}
The output of this code is a shockingly terse “1231”. When the ptr variable is deleted, only the Parent destructor is called because the Child destructor was not declared virtual. As a result, the Child destructor is not called and the destructors for its data members are not called.
Technically, you could fix the above problem by simply making the Parent destructor virtual. The “virtualness” would automatically be used by any children. However, we advocate making all destructors virtual so that you never have to worry about it.
Always make your destructors virtual!
Referring to Parent Data
Names can become ambiguous within a subclass, especially when multiple inheritance (see below) comes into play. C++ provides a mechanism to disambiguate names between classes: the scope resolution operator. The syntax (two colons) is the same as referencing static data in a class.
When you override a method in a subclass, you are effectively replacing the original as far as other code is concerned. However, that parent version of the method still exists and you may want to make use of it. If you simply called the method by name, however, the compiler would assume that you meant the subclass version. This could easily lead to an infinite loop, as in the example that follows;
Sub::doSomething()
{
cout << “In Sub’s version of doSomething()” << endl;
doSomething(); |
// BUG! This will recursively call this method! |
}
To call the parent’s version of the method explicitly, simply prepend the parent’s name and two colons:
Sub::doSomething()
{
cout << “In Sub’s version of doSomething()” << endl; Super::doSomething(); // call the parent version.
}
Calling the parent version of the current method is a commonly used pattern in C++. If you have a chain of subclasses, each might want to perform the operation already defined by the superclass but add their
own additional functionality as well.
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Chapter 10
For example, imagine a class hierarchy of types of books. A diagram showing such a hierarchy is shown in Figure 10-4.
Book
Paperback Technical
Romance
Figure 10-4
Since each lower class in the hierarchy further specifies the type of book, a method that gets the description of a book really needs to take all levels of the hierarchy into consideration. This can be accomplished by chaining to the parent method as above. The following code illustrates this pattern:
#include <iostream> #include <string>
using namespace std;
class Book
{
public:
virtual string getDescription() { return “Book”; }
};
class Paperback : public Book
{
public:
virtual string getDescription() {
return “Paperback “ + Book::getDescription();
}
};
class Romance : public Paperback
{
public:
virtual string getDescription() {
return “Romance “ + Paperback::getDescription();
}
};
class Technical : public Book
{
public:
virtual string getDescription() {
return “Technical “ + Book::getDescription();
}
};
int main()
238