Introduction to Inheritance in C plus plus

A Condensed
Crash Course on C++
ECE 417/617:
Elements of Software Engineering
Stan Birchfield
Clemson University

Recommended C++ resources
• Bjarne Stroustrup,
The C++ Programming
Language
• Scott Meyers,
Effective C++

Why C++?
• Popular and relevant (used in nearly every application domain):
– end-user applications (Word, Excel, PowerPoint, Photoshop, Acrobat,
Quicken, games)
– operating systems (Windows 9x, NT, XP; IBM’s K42; some Apple OS X)
– large-scale web servers/apps (Amazon, Google)
– central database control (Israel’s census bureau; Amadeus; Morgan-
Stanley financial modeling)
– communications (Alcatel; Nokia; 800 telephone numbers; major
transmission nodes in Germany and France)
– numerical computation / graphics (Maya)
– device drivers under real-time constraints
• Stable, compatible, scalable

C vs. C++
• C++ is C incremented
(orig., “C with classes”)
• C++ is more expressive
(number of C++ source lines less than C for same program)
• C++ is just as permissive
(anything you can do in C can also be done in C++)
• C++ can be just as efficient
(most C++ expressions need no run-time support;
C++ allows you to
– manipulate bits directly and interface with hardware without regard for safety
or ease of comprehension, BUT
– hide these details behind a safe, clean, elegant interface)
• C++ is more maintainable
(1000 lines of code – even brute force, spaghetti code will work;
100,000 lines of code – need good structure, or new errors will be
introduced as quickly as old errors are removed)

Efficiency and Maintainability
90/10 rule: 10% of your program will take 90% of the
processor time to run
optimize what needs to be optimized, but no more
focus on design
Efficiency
(processor time)
Maintainability
(programmer time)

Design goals of C++
• Backward compatibility with C
(almost completely – every program in K&R is a C++ program)
• Simplicity, elegance
(few built-in data types, e.g., no matrices)
• Support for user-defined data types
(act like built-in types; N.B. Standard Template Library (STL))
• No compromise in efficiency, run-time or memory
(unless “advanced features” used)
• Compliation analysis to prevent accidental corruption
of data
(type-checking and data hiding)
• Support object-oriented style of programming

Compatibility with C
C++ does not allow
• old-style C function declarations
void f(a) int a; {}
• generic function declarations
void f();
void g() { f(2); }
• setting enum to int
enum Dir {Up, Down}; Dir
d=1;
• multiple declarations
int i; int i;
• assigning to void *
int* p = malloc(10);
• “implicit int”
signed a = 7;
Other differences:
• const global variables have
internal linkage in C++,
external in C
• extra keywords in C++
void main()
{ int catch = 5; }
• bizarre comments
int f(int a, int b)
{
return a//**/b
;
}
How is C++ not backward compatible with C (C89)?
(For these, C++ is backward
compatible with C99)

Purpose of a programming
language
• Programming languages serve two
purposes:
– vehicle for specifying actions to be executed
“close to the machine”
– set of concepts for thinking about what can
be done
“close to the problem being solved”
• Object-oriented C++ excels at both

Learning C++
• Goal: Don’t just learn new syntax, but become a
better programmer and designer; learn new and
better ways of building systems
• Be willing to learn C++’s style; don’t force
another style into it
• C++ supports gradual learning
– Use what you know
– As you learn new features and techniques, add those
tools to your toolbox
• C++ supports variety of programming paradigms

Programming paradigms
• procedural – implement algorithms via
functions (variables, functions, etc.)
• modular – partition program into modules
(separate compilation)
• object-oriented – divide problem into classes
(data hiding, inheritance)
• abstract – separate interface from
implementation (abstract classes)
• generic – manipulate arbitrary data types
(STL: containers, algorithms)

• Encapsulation
“black box” – internal data hidden
• Inheritance
related classes share implementation
and/or interface
• Polymorphism
ability to use a class without knowing its type
What is object-oriented?
© SDC
“C++ is an object-oriented language” =
C++ provides mechanisms that support object-oriented style of
programming

Some C++ concepts
• constructor / destructor / copy constructor
• initialization list
• inheritance
• exceptions
• overloading operators (e.g., assignment operator)
• namespace
• const
• virtual function
• pure virtual (abstract) function
• friend
• template
• standard template library (STL)
• pass by value, pass by reference
• composition versus derivation

A simple C++ program
#include <iostream> // std::cout
#include <cstdio> // printf
int main()
{
int a = 5; // 'a' is L-value
float b = 0.9f;
printf("Hello world %d %3.1f n", a, b);
std::cout << "Hello world" << a << " "
<< b << " " << std::endl;
return 0;
}

Declarations and definitions
• Declaration:
– extern char c;
– struct User;
– double sqrt(double);
 Okay to have many
• Definition:
– char c;
– int count = 1;
– double abs(double a) {
a>0 ? a : -a;
}
 Must have exactly one

Fundamental types
• bool (true  1, false  0)
• char (could be signed or unsigned
– implementation-defined)
• int (signed by default)
• double
• void (“pseudo-type”)
• enum
• class
• also short, long, struct, float,
wchar_t, etc.)
Do not rely on sizes of these!
(Implementation-dependent)
INTEGRAL
ARITHMETIC
USER-DEFINED

Macros
• Dangerous:
– compiler never sees them
source code  translation unit
– global
• Instead, use
– const
– inline
– template
– enum
• Ok to use for include guards (“header wrappers”)
• If you must use a macro, give it a long ugly name with
lots of capital letters
Example:
template<typename T>
inline T max(T t){
t>0 ? t : -t;
}

Memory allocation
• “on the stack”
– block delimited by {}
– object alive till it falls out of scope
– calls constructor / destructor
• “on the heap”
– new and delete replace malloc, calloc, free
– object exists independently of scope in which it was created
– also “on the free store” or “allocated in dynamic memory”
– be careful: new  delete, new[]  delete[]
– for safety, same object should both allocate and deallocate
• “local static store” void foo() {
static int i=0;
}

Global variables
• Built-in types initialized to 0
(but local variables uninitialized)
• Initialized before main() invoked
• Initialization order:
– within translation unit, same as definition
– between translation units, arbitrary order
file1.cpp
double pi = 3.14;
file2.cpp
double twopi = 2*pi;

A class
class Date {
public:
enum Month {Jan, Feb, Mar, ...}
Date(int year, Month month, int day);
int GetDay() const;
void SetDay(int day);
Date& operator+=(int days);
private:
Month m_month;
int m_year, m_day;
};
member
functions
member
variables

Names
• Maintain consistent naming style
– long names for large scope
– short names for small scope
• Don’t start with underscore; reserved for
special facilities
• Avoid similar-looking names: l and 1
• Choosing good names is an art

Access control
• Public: visible to everyone
• Private: visible only to the
implementer of this particular class
• Protected: visible to this class and
derived classes
• Good rule of thumb:
– member functions public or protected
– member variables private

Default class functions
• By default, each class has member functions:
– constructor Date();
– destructor ~Date();
– copy constructor
Date(const Date& other);
– assignment operator
Date& operator=(const Date& other);
• These call the appropriate functions on each
member variable
• Be careful: If this is not what you want, then
either override or disallow (by making private)

Constructor and destructor
• (Copy) constructor creates object
• Destructor destroys (“cleans up”) object
• Be aware of temporary objects
class Matrix {
Matrix(const Matrix& other);
Matrix& operator+(const Matrix& other);
Matrix& operator=(const Matrix& other);
};
void foo() {
Matrix a, b, c, d;
a = b + c + d;
}
What functions get called?
(Note: There are ways to speed this up while preserving the syntax)

Initializer lists
Matrix::Matrix(const Matrix& other)
: m_n(0), m_a(0)
{
}
Matrix::Matrix(const Matrix& other)
{
m_n = 0;
m_a = 0;
}
Use initializer list:
Assign values inside constructor:

Concrete classes
• A concrete class
– does a single, relatively small thing well and
efficiently
– hides data members (encapsulation)
– provides clean interface
– acts like a built-in type
– is a “foundation of elegant programming” –
Stroustrup
• Don’t underestimate the importance of this
basic C++/OO feature!

Class relationships
• OK:
– A calls function from B
– A creates B
– A has a data member of type B
• Bad:
– A uses data directly from B
(without using B’s interface)
• Even worse:
– A directly manipulates data in B

Pointers, arrays, references
• Use 0, not NULL (stronger type checking)
• Name of array is equivalent to pointer to
initial element
• Access array using * or []; same
efficiency with modern compiler
• use std::vector, not built-in array,
when possible
• Reference is like a pointer

References
• Reference: alternate
name for an object
• There is no null
reference
• No pointer to a reference
• No reference to a
temporary
• Syntax confusing
• Basically a const
dereferenced pointer
with no operations
int &a;
int a; int& b=a; b++;
int* c = &a;
int &a = b; int* c= &a;
int& a = 1;

Argument passing / return
• Pass / return by value
– calls copy constructor
– ok for built-in types
int foo(int a) { return 0; }
– performance penalty for structs and classes (temporary objects)
• Pass / return by reference or pointer
– does not call copy constructor
– pass inputs by const reference
– never pass inputs by “plain” reference
void update(int& a); update(2); // error
– pass outputs by pointer
int x = 1; next(x); // should not change x
int x = 1; next(&x); // may change x
– ok to return a ref, or const ref

C++ function mechanisms
• Overloaded function names
– Cleaner and safer
print(int);
print(float);
– But beware
print(int); print(int*); print(0);
• Default parameters
void print(int a, int b=0, int c=0);
• Operator overloading
Matrix& operator+=(const Matrix& other);
• Implicit conversion operator
operator int() const {} // converts to int
– Provides convenient syntax, but potentially dangerous so use
sparingly

Explicit type conversion
• C++ casts
– static_cast between 2 related types
(int/float, int/enum, 2 pointers in class hierarchy)
– reinterpret_cast between 2 unrelated types
(int/ptr, pointers to 2 unrelated classes)
– const_cast cast away constness
– dynamic_cast used for polymorphic types
Run-time type info (RTTI)
• Avoid casts, but use these instead of C casts
– e.g., compiler can perform minimal checking for
static_cast, none for reinterpret_cast

Namespaces
• Namespace expresses logical grouping
• using declaration
– Don’t use global using except for transition to
older code
– Ok in namespace for composition
– Ok in function for notational convenience
• Namespaces are open
• Unnamed namespaces restrict code to local
translation unit
• Aliases

Const
• Const prevents object from being modified (orig., readonly)
• Avoid magic numbers
char a[128];
const int maxn = 128;
char a[maxn];
• Logical constness vs. physical constness
• Const is your friend; use it extensively and consistently
• can cast away constness, but be sure to use mutable
• const pointers:
– const int * const ptr = &a[0]; // const ptr to a const int
– int const * const ptr = &a[0]; // ”
– int * const p2 = &a[0]; // const ptr to an int
– const int * p1 = &a[0]; // ptr to a const int
– int const * p2 = &a[0]; // ”

Inheritance
• Subclass derived from base class
• Two classes should pass the “ISA” test:
derived class is a base class
class Shape {
};
class Circle : public Shape {
};
• Class hierarchy: means of building classes
incrementally, using building blocks
(subclass becomes base class for someone else)
• Facilitates code reuse

Inheritance vs. composition
• Inheritance: “is a”
class Circle : public Shape {
};
• Composition: “has a”
class Circle {
private:
Shape m_shape;
};
• Decision should be based on commonality of
interface

Virtual functions
• Function of derived class is called even if you have only a pointer to the base class
File.h
class Shape
{
virtual void Draw();
};
class Circle : public Shape
{
virtual void Draw();
};
File.cpp
void Func1()
{
Circle mycirc;
Func2(&mycirc);
}
void Func2(Shape* s)
{
s->Draw();
}
// calls Circle::Draw()

How a virtual function works
Shape vtable
vfunc1 addr
vfunc2 addr
...
vfuncN addr
vfunc1 addr
vfunc2 addr
...
vfuncN addr
Circle vtable
var1
...
varM
vtable ptr
mycirc

What is the penalty of a virtual
function?
• Space:
– one vtable per class with virtual function(s)
– one pointer per instance
• Time:
– one extra dereference if type not known at
compile time
– no penalty if type known at compile time
(ok to inline a virtual function)

Pure virtual function
• Pure virtual function
– Function intentionally undefined
– Same penalty as regular virtual function
• Abstract class
– Contains at least one pure virtual function
– Cannot instantiate; must derive from base class and override
pure virtual function
– Provides an interface
(separates interface from implementation)
class Shape {
virtual void Draw() = 0;
};

Multiple inheritance
• C++ allows you to inherit from multiple
base classes
class CalculatorWatch :
public Calculator, Watch {};
• Works best if
– exactly one base class passes ISA test
– all other base classes are interfaces
• Advanced feature that you will not need
in this course

Polymorphism
• Polymorphism
– “ability to assume different forms”
– one object acts like many different types of objects
(e.g., Shape*)
– getting the right behavior without knowing the type
– manipulate objects with a common set of operations
• Two types:
– Run-time (Virtual functions)
– Compile-time (Templates)

Exceptions
• Error handling in C:
– Half of code is error handling
– Dangerous: Easy for programmer to forget
to check return value
void Func() {
int ret;
ret = OpenDevice();
if (ret != 0) error(“Unable to open device”);
ret = SetParams();
if (ret != 0) error(“Unable to set params”);
}

Exceptions (cont.)
• Error handling in C++:
– try-catch blocks safer
– separate “real code” from error handling code
void Func() {
try {
OpenDevice();
SetParams();
} catch (const MyException& e) {
e.ReportToUser();
} catch (...) {
abort(1);
}
}
void OpenDevice()
{
if (bad) throw MyException(“Cannot open device”);
}

Templates
• Define a class or function once, to work with a variety
of types
• Types may not be known until future
• Better type checking and faster (cf. qsort)
• Specialization can be used to reduce code bloat
• Templates support generic programming
T Max(T a, T b) { return a>b ? a : b; }
class Vector {
Vector(int n, T init_val);
T* m_vals;
};

Generic programming
• Drawbacks of qsort in <stdlib.h>
– requires a compare function, even if trivial
– loss of efficiency b/c dereferencing pointer
– lost type safety b/c void*
– only works with contiguous arrays
– no control over construction / destruction /
assignment; all swapping done by raw
memory moves

Standard Template Library (STL)
• Containers:
– Sequences
• vector – array in contiguous memory (replaces realloc)
• list – doubly-linked list (insert/delete fast)
• deque (“deck”) – double-ended queue
• stack, queue, priority queue
– Associative
• map – dictionary; balanced tree of (key,value) pairs
like array with non-integer indices
• set – map with values ignored (only keys important)
• multimap, multiset (allow duplicate keys)
– Other
• string, basic_string – not necessarily contiguous
• valarray – vector for numeric computation
• bitset – set of N bits

STL (cont.)
• Algorithms (60 of them):
– Nonmodifying
• find, search, mismatch, count, for_each
– Modifying
• copy, transform/apply, replace, remove
– Others
• unique, reverse, random_shuffle
• sort, merge, partition
• set_union, set_intersection, set_difference
• min, max, min_element, max_element
• next_permutation, prev_permutation

std::string
• Example:
#include <string>
void Func()
{
std::string s, t;
char c = 'a';
s.push_back(c); // s is now “a”;
const char* cc = s.c_str(); // get ptr to “a”
const char dd[] = "afaf";
t = dd; // t is now “afaf”;
t = t + s; // append “a” to “afaf”
}

std::vector
#include <vector>
void Func()
{
std::vector<int> v(10);
int a0 = v[3]; // unchecked access
int a1 = v.at(3); // checked access
v.push_back(2); // append element to end
v.pop_back(); // remove last element
size_t howbig = v.size(); // get # of elements
v.insert(v.begin()+5, 2); // insert 2 after 5th element
}
• Example:

std::vector (cont.)
#include <vector>
#include <algorithm>
void Func()
{
std::vector<int> v(10);
v[5] = 3; // set fifth element to 3
std::vector<int>::const_iterator it
= std::find(v.begin(), v.end(), 3);
bool found = it != v.end();
if (found) {
int three = *it;
int indx = it - v.begin();
int four = 4;
}
}
• Example:

Iterators
• iterator – generalized pointer
• Each container has its own type of
iterator
void Func() {
stl::vector<int> v;
stl::vector<int>::const_iterator it = v.begin();
for (it = v.begin() ; it != v.end() ; it++) {
int val = *it;
}
}

Types of iterators
template<class InputIterator, class Type>
InputIterator
find( InputIterator _First,
InputIterator _Last,
const Type& _Val );
• Each container provides a
different type
input
forward
bidirectional
random access
output
Types

Allocators
• STL written for maximum flexibility
• Each container has an allocator
• Allocator is responsible for memory
management (new/delete)
template < class Type,
class Allocator = allocator<Type> >
class vector {
...
};
• Advice: Ignore allocators

Streams
• C
– flush, fprintf, fscanf, sprintf, sscanf
– fgets, getc
• C++
– cout, cin, cerr

Numerics
• valarray
– matrix and vector (not std::vector)
– slices and gslices
• complex
• random numbers

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