CMSC 202

1
CMSC 202

• Lesson 26
• Miscellaneous Topics

2
Warmup

• Decide which of the following are legal
  statements
• int a 7
• const int b 6
• int const p1 a
• int const p2 b
• const int p3 a
• const int p4 b
• const int const p5 a
• const int const p6 a
• p1 a
• p3 a
• p5 a

3
Announcements

• Last regular class day
• Review on Tuesday
• Homework
• Bring Review Questions!
• Project 4
• Coming Soon!
• Project 5
• Regrades ONLY if ABSOLUTELY necessary
• ONLY if it will affect your course grade

4
Today

• Overload the dereferencing operator
• Weirdness of the operator!
• Bit-wise operators (important for 341)
• Binary representation
• Binary addition
• Bit-masking
• , , , ltlt and gtgt

5
Disclaimer

• The material I am about to present is an advanced
  concept from 341
• The 341 book (Weiss) actually has it WRONG!
• Short write-up with some good code
• Linked from the Slides webpage
• Topic
• Overloading Pointer Dereferencing
• Overloading Conversion Operator
• Oooo, Aaaah

6
Pointer Dereferencing

• Problem
• Imagine we want to create a templated Sort
• What if we have a collection of pointers?
• template lt class T gt
• void MySort( vectorltTgt collection )
• / code that sorts collection has something like
  /
• if (collection.at(i) lt collection.at(j))
• swap(collection.at(i), collection.at(j))
• // In main
• vectorltintgt vec
• srand(0)
• for (int i 0 i lt 1000 i)
• vec.push_back(new int(rand()))

What happens when we compare two items of type
int ?
7
Our PointerltTgt class

• template ltclass Tgt
• class Pointer
• public
• Pointer(T rhs NULL ) pointee(rhs)
• bool operatorlt( const Pointer rhs ) const
• return pointee lt rhs.pointee
• private
• T pointee

What if we want to print the pointee? What if we
want to change its value?
8
Overloading Pointer Dereferencing

• template ltclass Tgt
• class Pointer
• public
• Pointer(T rhs NULL ) pointee(rhs)
• bool operatorlt( const Pointer rhs ) const
• return pointee lt rhs.pointee
• // Pointer dereferencing operator
• const T operator () const
• return pointee
• private
• T pointee

9
Using the Pointer Dereference

• template ltclass Tgt
• ostream operator ltlt(ostream sout, PointerltTgt p)
• sout ltlt p ltlt endl
• return sout

Dereferencing a class that overloads the pointer
dereferencing operator calls that
method! Smart pointers
10
Overloading Conversion Operator

• template ltclass Tgt
• class Pointer
• public
• Pointer(T rhs NULL ) pointee(rhs)
• bool operatorlt( const Pointer rhs ) const
• return pointee lt rhs.pointee
• // Conversion operator
• operator const T () const
• return pointee
• private
• T pointee

This looks very similar Whats the
difference? Position of the word operator
Operator name is const T
11
Differences

• // Pointer dereferencing operator
• const T operator () const
• return pointee
• // Conversion operator
• operator const T () const
• return pointee

Dereferencing Returns an object of type T (after
dereferencing the data member!)
Conversion Converts something of type Pointer
into something of type const T (before
dereferencing the data member!)
12
Final Notes about

• If both dereferencing and conversion are
  overloaded
• Dereferencing operator takes precidence
• (put in some cout statements to verify this!)
• Conversion operator
• Can be used to convert between ANY two types!
  Cool!
• Good examples in below material
• Additional Resources
• ANSI/ISO C Professional Programmer's Handbook
• http//www-f9.ijs.si/matevz/docs/C/ansi_cpp_pro
  gr_handbook/ch03/ch03.htmHeading12
• C Annotations Version 6.1.2
• http//www.icce.rug.nl/documents/cplusplus/cpluspl
  us09.htmll144
• C/C Pointers
• http//uvsc.freshsources.com/Operator_Overloading.
  ppt

13
Decimal Numbers

• Humans
• Represent everything in decimal, 1 -gt 10
• Base 10 notation
• Each position is a power of 10
• 8 0 3 610

Base 10 count by 10s
8 103
0 102
3 101
6 100
800010 3010 610 803610
14
Binary Numbers

• Computers
• Represent everything in binary, 1s and 0s
• Base 2 notation
• Each position is a power of 2
• 1 0 1 12

Base 2 count by 2s
1 23
0 22
1 21
1 20
810 210 110 1110
15
Binary Numbers

• Usually represented in sets of 4 digits
• 4, 8, 16, 32, etc.
• Bit
• Binary digit
• Byte
• Collection 8-bits
• Integers stored in 4 bytes or 32 bits
• 32-bits
• Can represent up to 232-1 values
• Two other common programming formats
• Octal base 8
• Has digits 0-gt7
• Hexadecimal base 16
• Has digits 0-gt9 and A-gtF

16
Binary Representations

• What decimal equivalent are the following binary
  numbers?
• 0001
• 0100
• 1000
• 1001
• 1100
• 1111
• 0101

17
Binary Addition

• Just like decimal addition
• Except 12 12 102
• Carry a 1 when you add two or more 1s
• Lets try a simple one

In decimal?
1
1
1001
9
0011
3
0
0
1
1
12
We leave off base subscript if the context is
clear
18
Binary in C

• Why do we care?
• Binary describes size of data
• Integer stored in 32-bits, limited to 5 billion
  values (or 232-1)
• - 2.7 billion -gt 2.7 billion
• Thats great, but why do we REALLY care?
• Assume lots of boolean values
• Can use each bit to represent a separate value!
• Compress data, optimize data access
• Get at raw data
• Look for these again in Hash Tables!

19
Bit-wise Operators

• Operate on each bit individually.
• Bit-wise not
• 1 becomes 0
• 0 becomes 1
• Bit-wise logical and
• 1 if both 1
• 0 otherwise
• Bit-wise logical or
• 1 if either or both 1
• 0 otherwise

20
More Bit-wise Operators

• Bit-wise exclusive or
• 1 if either but not both 1
• 0 otherwise
• ltlt N
• Bit-wise left shift
• Moves all bits to the left N places
• Shifts on a zero on the right
• Left-most bit(s) discarded
• gtgt N
• Bit-wise right shift
• Moves all bits to the right N places
• Shifts on a zero on the left
• Right-most bit(s) discarded

21
Bit-wise Compound Assignment

• and assign
• and assign
• and assign
• ltlt
• ltlt and assign
• gtgt
• gtgt and assign

22
Bit Masking

• So, have a bunch of boolean values to store
• Often called flags
• Need two things
• Variable to store value in
• Bit-mask to retrieve or set the value
• Use characters unsigned value
• char flags 0 // binary 0000
• char flag4 8 // binary 1000
• char flag3 4 // binary 0100
• char flag2 2 // binary 0010
• char flag1 1 // binary 0001

23
Bit Masking Operations

• // Set flag1 to true
• flags flags flag1 // 0000 0001
• // Set flag1 to false
• flags flags flag1 // 0001 1110
• // Set several flags to true
• flags flags flag1 flag3 // 0000 0001
  0100
• // Set all flags to false
• flags flags flags // 0101 0101
• // Set to a specific value
• flags 11 // 1011
• // Set all flags to true
• flags flags flags // 1011 0100

24
Practice

• Convert the following decimal digits into binary
• 7, 5
• Add them together using binary addition
• Check your result using decimal
• What about these two numbers?
• 9, 13
• Use only 4-bits to represent these numbers
• What about negative numbers?

25
Challenge

• Use bit-wise operators to implement binary
  addition