Part D - Inheritance
Functions in a Hierarchy
Relate classes using inheritance hierarchies to minimize the duplication of object code
Shadow a base class function using a derived class function
Pass initialization data across the constructors of a class hierarchy
Shadowing
| Constructors
| Destructors
| Helpers
| Summary
| Exercises
The logic that a derived class inherits from its base class is limited
to the normal member functions of the base class. A derived class
does not by default inherit the constructors, the destructor or the copy
assignment operator - that is, the special member functions - of the base
class.
The special member functions in a class hierarchy define the logic for the
creation, destruction and copying of different parts of an object and are
necessarily different. A derived class' constructor automatically
calls the base class' default constructor. A derived class' destructor
automatically calls the base class' destructor. A derived class' copy
assignment operator automatically calls the base class' copy assignment
operator.
This chapter describes how member functions shadow one another in a hierarchy
and the order in which constructors and destructors call one another.
This chapter shows how to define a derived class' constructor to access a
specific base class constructor and how to overload a helper operator for a
derived class.
Shadowing
A member function of a derived class shadows the base class member function
with the same identifier. The C++ compiler binds a call to the member function
of the derived class, if one exists.
To access the base class version of a member function that a derived class
version has shadowed, we use scope resolution. A call to a shadowed
function takes the form
Base::identifier(arguments)
where Base identifies the class to which the
shadowed function belongs.
Example
Consider the following hierarchy. The base class and the derived class
define distinct versions of the display() member
function. The Student class version shadows
the Person class version for any object of
Student type:
// Student.h
#include <iostream>
const int NC = 30;
const int NG = 20;
class Person {
char name[NC+1];
public:
void set(const char* n);
void display(std::ostream&) const;
};
class Student : public Person {
int no;
float grade[NG];
int ng;
public:
Student();
Student(int);
Student(int, const float*, int);
void display(std::ostream&) const;
};
|
We access the base class version from the derived version using scope resolution:
// Student.cpp
#include <cstring>
#include "Student.h"
using namespace std::
void Person::set(const char* n) {
strncpy(name, n, NC);
name[NC] = '\0';
}
void Person::display(std::ostream& os) const {
os << name << ' ';
}
Student::Student() {
no = 0;
ng = 0;
}
Student::Student(int n) {
float g[] = {0.0f};
*this = Student(n, g, 0);
}
Student::Student(int sn, const float* g, int ng_) {
bool valid = sn > 0 && g != nullptr && ng_ >= 0;
if (valid)
for (int i = 0; i < ng_ && valid; i++)
valid = g[i] >= 0.0f && g[i] <= 100.0f;
if (valid) {
// accept the client's data
no = sn;
ng = ng_ < NG ? ng_ : NG;
for (int i = 0; i < ng; i++)
grade[i] = g[i];
} else {
*this = Student();
}
}
void Student::display(ostream& os) const {
if (no > 0) {
Person::display(os);
os << no << ":\n";
os.setf(ios::fixed);
os.precision(2);
for (int i = 0; i < ng; i++) {
os.width(6);
os << grade[i] << endl;
}
os.unsetf(ios::fixed);
os.precision(6);
} else {
os << "no data available" << endl;
}
}
|
The following client code produces the output shown on the right:
// Shadowing
// shadowing.cpp
#include <iostream>
#include "Student.h"
int main() {
Person jane;
jane.set("Jane Doe");
float gh[] = {89.4f, 67.8f, 45.5f};
Student harry(1234, gh, 3);
harry.set("Harry"); // inherited
harry.display(std::cout); // not inherited
jane.display(std::cout);
std::cout << std::endl;
}
|
Harry 1234:
89.40
67.80
45.50
Jane Doe
|
harry.display(std::cout) calls Student::display(), which calls the shadowed
Person::display(), while
jane.display()
calls Person::display() directly. The derived
version shadows the base version when called on harry.
Good Design Tip
By calling Person::display() within
Student::display(), we hide the
hierarchy from the client code. The main()
function is hierarchy agnostic.
Exposing an Overloaded Member Function
The C++ language shadows member functions on their identifier and not on their
signature. To expose an overloaded member function in the base class with the
same identifier but a different signature we insert a using
declaration into the definition of the derived class.
A using declaration takes the form
using Base::identifier;
where Base identifies the base class and
identifier is the name of the shadowed function.
Example
Let us overload the display() member function in the
Person class to take two arguments: a modifiable reference
to the output stream and the address of a C-style null-terminated character string
containing a prefix message. We insert the using
declaration in the definition of the derived class to expose this member function
and any other with the same identifier for objects of the derived class.
// Student.h
#include <iostream>
const int NC = 30;
const int NG = 20;
class Person {
char name[NC+1];
public:
void set(const char* n);
void display(std::ostream&) const;
void display(std::ostream&, const char*) const;
};
class Student : public Person {
int no;
float grade[NG];
int ng;
public:
Student();
Student(int);
Student(int, const float*, int);
void display(std::ostream&) const;
using Person::display;
};
|
We define the overloaded display() function
for the base class:
// Student.cpp
#include <cstring>
#include "Student.h"
using namespace std;
void Person::set(const char* n) {
strncpy(name, n, NC);
name[NC] = '\0';
}
void Person::display(ostream& os) const {
os << name << ' ';
}
void Person::display(ostream& os, const char* msg) const {
os << msg << name << ' ';
}
Student::Student() {
no = 0;
ng = 0;
}
Student::Student(int n) {
float g[] = {0.0f};
*this = Student(n, nullptr, 0);
}
Student::Student(int sn, const float* g, int ng_) {
bool valid = sn > 0 && g != nullptr && ng_ >= 0;
if (valid)
for (int i = 0; i < ng_ && valid; i++)
valid = g[i] >= 0.0f && g[i] <= 100.0f;
if (valid) {
// accept the client's data
no = sn;
ng = ng_ < NG ? ng_ : NG;
for (int i = 0; i < ng; i++)
grade[i] = g[i];
} else {
*this = Student();
}
}
void Student::display(ostream& os) const {
if (no > 0) {
Person::display(os);
os << no << ":\n";
os.setf(ios::fixed);
os.precision(2);
for (int i = 0; i < ng; i++) {
os.width(6);
os << grade[i] << endl;
}
os.unsetf(ios::fixed);
os.precision(6);
} else {
os << "no data available" << endl;
}
}
|
The following client produces the result shown on the right:
// Overloading and Shadowing
// overloading.cpp
#include <iostream>
#include "Student.h"
int main() {
Person jane;
float gh[] = {89.4f, 67.8f, 45.5f};
Student harry(1234, gh, 3);
harry.set("Harry");
harry.display(std::cout);
harry.display(std::cout, "Name is ");
std::cout << std::endl;
jane.set("Jane Doe");
jane.display(std::cout);
std::cout << std::endl;
}
|
Harry 1234:
89.40
67.80
45.50
Name is Harry
Jane Doe
|
Constructors
A derived class does not inherit a base class constructor by default.
That is, if we do not declare a constructor in our definition of the derived class,
the compiler inserts an empty no-argument constructor by default.
The compiler constructs an instance of the derived class in four steps in
two distinct stages:
- construct the base class portion of the complete object
- allocate memory for the instance variables in the
order of their declaration
- execute the base class constructor
- construct the derived class portion of the object
- allocate memory for the instance variables in the
order of their declaration
- execute the derived class constructor
In our example, let us define a no-argument constructor for the
base class. The header file declares the no-argument constructor:
// Student.h
#include <iostream>
const int NC = 30;
const int NG = 20;
class Person {
char name[NC+1];
public:
Person();
void set(const char* n);
void display(std::ostream&) const;
};
class Student : public Person {
int no;
float grade[NG];
int ng;
public:
Student();
Student(int);
Student(int, const float*, int);
void display(std::ostream&) const;
};
|
The implementation file defines the base class constructor:
// Student.cpp
#include <cstring>
#include "Student.h"
using namespace std;
Person::Person() {
cout << "Person()" << endl;
name[0] = '\0';
}
void Person::set(const char* n) {
cout << "Person(const char*)" << endl;
strncpy(name, n, NC);
name[NC] = '\0';
}
void Person::display(ostream& os) const {
os << name << ' ';
}
Student::Student() {
cout << "Student()" << endl;
no = 0;
ng = 0;
}
Student::Student(int n) {
cout << "Student(int)" << endl;
float g[] = {0.0f};
*this = Student(n, g, 0);
}
Student::Student(int sn, const float* g, int ng_) {
cout << "Student(int, const float*, int)" << endl;
bool valid = sn > 0 && g != nullptr && ng_ >= 0;
if (valid)
for (int i = 0; i < ng_ && valid; i++)
valid = g[i] >= 0.0f && g[i] <= 100.0f;
if (valid) {
// accept the client's data
no = sn;
ng = ng_ < NG ? ng_ : NG;
for (int i = 0; i < ng; i++)
grade[i] = g[i];
} else {
*this = Student();
}
}
void Student::display(ostream& os) const {
if (no > 0) {
Person::display(os);
os << no << ":\n";
os.setf(ios::fixed);
os.precision(2);
for (int i = 0; i < ng; i++) {
os.width(6);
os << grade[i] << endl;
}
os.unsetf(ios::fixed);
os.precision(6);
} else {
os << "no data available" << endl;
}
}
|
The following client uses this implementation to produce the
result shown on the right:
// Derived Class Constructors
// derivedCtors.cpp
#include <iostream>
#include "Student.h"
int main() {
Person jane;
float gh[] = {89.4f, 67.8f, 45.5f};
Student harry(1234, gh, 3);
harry.set("Harry");
harry.display(std::cout);
jane.set("Jane");
jane.display(std::cout);
}
|
Person()
Person()
Student(int, const float*, int);
Person(const char*);
Harry 1234:
89.40
67.80
45.50
Person(const char*);
Jane
|
In this example, the compiler constructs the two objects as follows:
- allocates memory for jane
- allocates memory for person
- the base class constructor initializes person to an empty string
- allocates memory for harry
- allocates memory for name
- the base class constructor initializes name to an empty string
- allocates memory for no,
grade and ng
- the derived class constructor initializes
- no to 1234
- grade to {89.40f, 67.80f, 45.50f}
- ng to 3
Passing Arguments to a Base Class Constructor
Each constructor of a derived class, other than the no-argument constructor,
receives in its parameters all of the values passed by the client.
Each constructor forwards the values for the base class part of the object to
the base class constructor. The base class constructor uses the values
received to build the base class part of the object. The derived class
constructor uses the values received to complete building the derived class
part of the object.
A call to the base class constructor from a derived class constructor that
forwards values takes the form
Derived( parameters ) : Base( arguments )
where Derived is the name of the derived class and
Base is the name of the base class.
The single colon separates the header of the derived-class constructor
from its call to the base class constructor. If we omit this call,
the compiler inserts a call to the default base class constructor.
Example
Let us replace the set() member function
in the base class with a one-argument constructor and upgrade the
Student's three-argument constructor
to receive the student's name. The header file declares
a single-argument base class constructor and a four-argument
derived class constructor:
// Student.h
#include <iostream>
const int NC = 30;
const int NG = 20;
class Person {
char name[NC+1];
public:
Person();
Person(const char*);
void display(std::ostream&) const;
};
class Student : public Person {
int no;
float grade[NG];
int ng;
public:
Student();
Student(int);
Student(const char*, int, const float*, int);
void display(std::ostream&) const;
};
|
The implementation of the single-argument constructor copies the name to the
instance variable:
// Student.cpp
#include <cstring>
#include "Student.h"
using namespace std;
Person::Person() {
name[0] = '\0';
}
Person::Person(const char* nm) {
strncpy(name, nm, NC);
name[NC] = '\0';
}
void Person::display(ostream& os) const {
os << name << ' ';
}
Student::Student() {
no = 0;
ng = 0;
}
Student::Student(int n) {
float g[] = {0.0f};
*this = Student("", n, g, 0);
}
Student::Student(const char* nm, int sn, const float* g, int ng_) : Person(nm) {
bool valid = sn > 0 && g != nullptr && ng_ >= 0;
if (valid)
for (int i = 0; i < ng_ && valid; i++)
valid = g[i] >= 0.0f && g[i] <= 100.0f;
if (valid) {
// accept the client's data
no = sn;
ng = ng_ < NG ? ng_ : NG;
for (int i = 0; i < ng; i++)
grade[i] = g[i];
} else {
*this = Student();
}
}
void Student::display(ostream& os) const {
if (no > 0) {
Person::display(os);
os << no << ":\n";
os.setf(ios::fixed);
os.precision(2);
for (int i = 0; i < ng; i++) {
os.width(6);
os << grade[i] << endl;
}
os.unsetf(ios::fixed);
os.precision(6);
} else {
os << "no data available" << endl;
}
}
|
The following client uses this implementation to produce the
output shown on the right:
// Derived Class Constructors with Arguments
// drvdCtorsArgs.cpp
#include <iostream>
#include "Student.h"
int main() {
Person jane("Jane");
float gh[] = {89.4f, 67.8f, 45.5f};
Student harry("Harry", 1234, gh, 3);
harry.display(std::cout);
jane.display(std::cout);
}
|
Harry 1234:
89.40
67.80
45.50
Jane
|
Inheriting Base Class Constructors
C++11 introduced syntax for inheriting a base class constructor in cases where
the derived class constructor does not execute any logic on the instance variables
of the derived class and only passes to the base class constructor values
received from the client. In such cases, the derived class may inherit
the base class constructors.
The declaration for inheriting a base class constructor takes
the form:
using Base::Base;
where Base is the name of the base class.
Example
Let us derive an Instructor class from the
Person base class and inherit all of the constructors
of the base class. The header file overrides the no-inheritance default:
// Student.h
// compiles with GCC 4.8 or greater or equivalent
#include <iostream>
const int NC = 30;
const int NG = 20;
class Person {
char name[NC+1];
public:
Person();
Person(const char*);
void display(std::ostream&) const;
};
class Student : public Person {
int no;
float grade[NG];
int ng;
public:
Student();
Student(int);
Student(const char*, int, const float*, int);
void display(std::ostream&) const;
};
class Instructor : public Person {
public:
using Person::Person;
};
|
The implementation file remains unchanged. The following client
uses this new class definition to produce the output shown on the right:
// Inherited Constructors
// inheritCtors.cpp
#include <iostream>
#include "Student.h"
int main() {
Instructor john("John");
Person jane("Jane");
float gh[] = {89.4f, 67.8f, 45.5f};
Student harry("Harry", 1234, gh, 3);
john.display(std::cout);
std::cout << std::endl;
harry.display(std::cout);
jane.display(std::cout);
}
|
John
Harry 1234:
89.40
67.80
45.50
Jane
|
Destructors
A derived class does not inherit the destructor of its base class.
Destructors execute in opposite order to the order of their object's construction.
That is, the derived class destructor always executes before the base class
destructor.
Example
Let us define destructors for our base and derived classes that insert
tracking messages to standard output. We declare each destructor
in its class definition:
// Student.h
#include <iostream>
const int NC = 30;
const int NG = 20;
class Person {
char name[NC+1];
public:
Person();
Person(const char*);
~Person();
void display(std::ostream&) const;
};
class Student : public Person {
int no;
float grade[NG];
int ng;
public:
Student();
Student(int);
Student(const char*, int, const float*, int);
~Student();
void display(std::ostream&) const;
};
|
We specify the messages in the destructor definitions:
// Student.cpp
#include <cstring>
#include "Student.h"
using namespace std;
Person::Person() {
name[0] = '\0';
}
Person::Person(const char* nm) {
strncpy(name, nm, NC);
name[NC] = '\0';
}
Person::~Person() {
std::cout << "Leaving " << name << std::endl;
}
void Person::display(ostream& os) const {
os << name << ' ';
}
Student::Student() {
no = 0;
ng = 0;
}
Student::Student(int n) {
float g[] = {0.0f};
*this = Student("", n, g, 0);
}
Student::Student(const char* nm, int sn, const float* g, int ng_) : Person(nm) {
bool valid = sn > 0 && g != nullptr && ng_ >= 0;
if (valid)
for (int i = 0; i < ng_ && valid; i++)
valid = g[i] >= 0.0f && g[i] <= 100.0f;
if (valid) {
// accept the client's data
no = sn;
ng = ng_ < NG ? ng_ : NG;
for (int i = 0; i < ng; i++)
grade[i] = g[i];
} else {
*this = Student();
}
}
Student::~Student() {
std::cout << "\nLeaving " << no << std::endl;
}
void Student::display(ostream& os) const {
if (no > 0) {
Person::display(os);
os << no << ":\n";
os.setf(ios::fixed);
os.precision(2);
for (int i = 0; i < ng; i++) {
os.width(6);
os << grade[i] << endl;
}
os.unsetf(ios::fixed);
os.precision(6);
} else {
os << "no data available" << endl;
}
}
|
The following client uses this implementation to produce the
output shown on the right:
// Derived Class Destructors
// drvdDtors.cpp
#include <iostream>
#include "Student.h"
int main() {
Person jane("Jane");
float gh[] = {89.4f, 67.8f, 45.5f};
Student harry("Harry", 1234, gh, 3);
harry.display(std::cout);
jane.display(std::cout);
}
|
Harry 1234:
89.40
67.80
45.50
Jane
Leaving 1234
Leaving Harry
Leaving Jane
|
Helper Operators
Helper functions support the classes identified by their parameter types.
Each helper function is dedicated to the class that it supports. The
compiler binds a call to a helper function on the basis of its parameter
type(s). That is, the helper functions of a base class do not directly
support classes derived from the supported base class.
Example
Let us upgrade our Student class to include overloads
of the insertion and extraction operators for both base and derived classes.
The header file contains:
// Student.h
#include <iostream>
const int NC = 30;
const int NG = 20;
class Person {
char name[NC+1];
public:
Person();
Person(const char*);
void display(std::ostream&) const;
};
std::istream& operator>>(std::istream&, Person&);
std::ostream& operator<<(std::ostream&, const Person&);
class Student : public Person {
int no;
float grade[NG];
int ng;
public:
Student();
Student(int);
Student(const char*, int, const float*, int);
void read(std::istream&);
void display(std::ostream&) const;
};
std::istream& operator>>(std::istream&, Student&);
std::ostream& operator<<(std::ostream&, const Student&);
|
The implementation file defines the helper operators:
// Student.cpp
#include <cstring>
#include "Student.h"
using namespace std;
Person::Person() {
name[0] = '\0';
}
Person::Person(const char* nm) {
strncpy(name, nm, NC);
name[NC] = '\0';
}
void Person::display(ostream& os) const {
os << name << ' ';
}
istream& operator>>(istream& is, Person& p) {
char name[NC+1];
cout << "Name: ";
is.getline(name, NC+1);
p = Person(name);
return is;
}
std::ostream& operator<<(ostream& os, const Person& p) {
p.display(os);
return os;
}
Student::Student() {
no = 0;
ng = 0;
}
Student::Student(int n) {
float g[] = {0.0f};
*this = Student("", n, g, 0);
}
Student::Student(const char* nm, int sn, const float* g, int ng_) : Person(nm) {
bool valid = sn > 0 && g != nullptr && ng_ >= 0;
if (valid)
for (int i = 0; i < ng_ && valid; i++)
valid = g[i] >= 0.0f && g[i] <= 100.0f;
if (valid) {
// accept the client's data
no = sn;
ng = ng_ < NG ? ng_ : NG;
for (int i = 0; i < ng; i++)
grade[i] = g[i];
} else {
*this = Student();
}
}
void Student::display(std::ostream& os) const {
if (no > 0) {
Person::display(os);
os << no << ":\n";
os.setf(ios::fixed);
os.precision(2);
for (int i = 0; i < ng; i++) {
os.width(6);
os << grade[i] << endl;
}
os.unsetf(ios::fixed);
os.precision(6);
} else {
os << "no data available" << endl;
}
}
void Student::read(istream& is) {
char name[NC + 1]; // will hold the student's name
int no; // will hold the student's number
int ng; // will hold the number of grades
float grade[NG]; // will hold the grades
std::cout << "Name: ";
is.getline(name, NC+1);
cout << "Student Number : ";
is >> no;
cout << "Number of Grades : ";
is >> ng;
if (ng > NG) ng = NG;
for (int i = 0; i < ng; i++) {
cout << "Grade " << i + 1 << " : ";
is >> grade[i];
}
// construct a temporary Student
Student temp(name, no, grade, ng);
// if data is valid, the temporary object into the current object
if (temp.no != 0)
*this = temp;
}
istream& operator>>(istream& is, Student& s) {
s.read(is);
return is;
}
ostream& operator<<(ostream& os, const Student& s) {
s.display(os);
return os;
}
|
The following client uses this implementation to produce the
output shown on the right:
// Helpers to Derived Classes
// drvdHelpers.cpp
#include <iostream>
#include "Student.h"
int main() {
Person jane;
Student harry;
std::cin >> jane;
std::cin >> harry;
std::cout << jane << std::endl;
std::cout << harry << std::endl;
}
|
Name: Jane Doe
Name: Harry
Student Number : 1234
Number of Grades : 3
Grade 1 : 89.40
Grade 2 : 67.80
Grade 3 : 45.50
Jane Doe
Harry 1234
89.40
67.80
45.50
|
Summary
- a member function of a derived class shadows an identically named member function
of a base class
- a derived class does not inherit the destructor, assignment operators
or helper functions of a base class
- a derived class does not by default inherit the constructor of a base
class, but we may add syntax to allow inheritance where the derived class
constructor does not contain logic to set its instance variables
- constructors in an inheritance hierarchy execute in order from the base
class to the derived class
- destructors in an inheritance hierarchy execute in order from the derived
class to the base class
Exercises
|
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