15.3. Virtual Functions
by Barbara E. Moo, Josée Lajoie, Stanley B. Lippman
C++ Primer, Fifth Edition
- Title Page
- Copyright Page
- Dedication Page
- Contents
- New Features in C++11
- Preface
- Chapter 1. Getting Started
- Part I. The Basics
- Contents
- Chapter 2. Variables and Basic Types
- Chapter 3. Strings, Vectors, and Arrays
- Chapter 4. Expressions
- Contents
- 4.1. Fundamentals
- 4.2. Arithmetic Operators
- 4.3. Logical and Relational Operators
- 4.4. Assignment Operators
- 4.5. Increment and Decrement Operators
- 4.6. The Member Access Operators
- 4.7. The Conditional Operator
- 4.8. The Bitwise Operators
- 4.9. The sizeof Operator
- 4.10. Comma Operator
- 4.11. Type Conversions
- 4.12. Operator Precedence Table
- Chapter Summary
- Defined Terms
- Chapter 5. Statements
- Chapter 6. Functions
- Chapter 7. Classes
- Part II. The C++ Library
- Contents
- Chapter 8. The IO Library
- Chapter 9. Sequential Containers
- Chapter 10. Generic Algorithms
- Chapter 11. Associative Containers
- Chapter 12. Dynamic Memory
- Part III. Tools for Class Authors
- Contents
- Chapter 13. Copy Control
- Chapter 14. Overloaded Operations and Conversions
- Contents
- 14.1. Basic Concepts
- 14.2. Input and Output Operators
- 14.3. Arithmetic and Relational Operators
- 14.4. Assignment Operators
- 14.5. Subscript Operator
- 14.6. Increment and Decrement Operators
- 14.7. Member Access Operators
- 14.8. Function-Call Operator
- 14.9. Overloading, Conversions, and Operators
- Chapter Summary
- Defined Terms
- Chapter 15. Object-Oriented Programming
- Contents
- 15.1. OOP: An Overview
- 15.2. Defining Base and Derived Classes
- 15.3. Virtual Functions
- 15.4. Abstract Base Classes
- 15.5. Access Control and Inheritance
- 15.6. Class Scope under Inheritance
- 15.7. Constructors and Copy Control
- 15.8. Containers and Inheritance
- 15.9. Text Queries Revisited
- Chapter Summary
- Defined Terms
- Chapter 16. Templates and Generic Programming
- Part IV. Advanced Topics
- Contents
- Chapter 17. Specialized Library Facilities
- Chapter 18. Tools for Large Programs
- Chapter 19. Specialized Tools and Techniques
- Appendix A. The Library
- Contents
- A.1. Library Names and Headers
- A.2. A Brief Tour of the Algorithms
- A.2.1. Algorithms to Find an Object
- A.2.2. Other Read-Only Algorithms
- A.2.3. Binary Search Algorithms
- A.2.4. Algorithms That Write Container Elements
- A.2.5. Partitioning and Sorting Algorithms
- A.2.6. General Reordering Operations
- A.2.7. Permutation Algorithms
- A.2.8. Set Algorithms for Sorted Sequences
- A.2.9. Minimum and Maximum Values
- A.2.10. Numeric Algorithms
- A.3. Random Numbers
- Index
- Add Pages
15.3. Virtual Functions
As we’ve seen, in C++ dynamic binding happens when a virtual member function is called through a reference or a pointer to a base-class type (§15.1, p. 593). Because we don’t know which version of a function is called until run time, virtual functions must always be defined. Ordinarily, if we do not use a function, we don’t need to supply a definition for that function (§6.1.2, p. 206). However, we must define every virtual function, regardless of whether it is used, because the compiler has no way to determine whether a virtual function is used.
Exercises Section 15.2.3
Exercise 15.8: Define static type and dynamic type.
Exercise 15.9: When is it possible for an expression’s static type to differ from its dynamic type? Give three examples in which the static and dynamic type differ.
Exercise 15.10: Recalling the discussion from §8.1 (p. 311), explain how the program on page 317 that passed an ifstream to the Sales_data read function works.
Key Concept: Conversions among Types Related by Inheritance
There are three things that are important to understand about conversions among classes related by inheritance:
• The conversion from derived to base applies only to pointer or reference types.
• There is no implicit conversion from the base-class type to the derived type.
• Like any member, the derived-to-base conversion may be inaccessible due to access controls. We’ll cover accessibility in §15.5 (p. 613).
Although the automatic conversion applies only to pointers and references, most classes in an inheritance hierarchy (implicitly or explicitly) define the copy-control members (Chapter 13). As a result, we can often copy, move, or assign an object of derived type to a base-type object. However, copying, moving, or assigning a derived-type object to a base-type object copies, moves, or assigns only the members in the base-class part of the object.
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