CS 192

1
CS 192

• Lecture 19
• Winter 2003
• February 09-10, 2004
• Dr. Shafay Shamail

2
Unions

• Look just like structures and have same syntax
• Use union keyword instead of struct
• Like a struct, can hold different data types, BUT
  only one type at a time i.e. members share
  storage
• Like Sayeed Saigol (auditorium) can hold
  different classes, but only one at a time
• Also, Sayeed Saigols size is equal to the
  largest class size
• But better than building a different room for
  each class saves space
• Likewise, a union is automatically allocated
  memory as large as the largest data type it
  contains as its member
• Saves memory

3
Unions

• include ltiostream.hgt
• union int_or_double
• int x
• double y
• void main()
• int_or_double number/only 8 bytes for a
  double allocated/
• number.x 100/store as int/
• cout ltlt number.xltltendlltltnumber.yltltendl
• number.y 100.0 /store as double. int value
  lost/
• coutltltnumber.xltltendlltltnumber.yltltendl
• Output 100 -9.25596e061 0 100
  //2nd and 3rd values are junk
• This output is system dependent. System tries to
  read the number at the memory location in the
  format you apply. Succeeds for the correct type,
  junk for the wrong type
• Try with int x and float y. Same storage space
  required for both, but still garbage values.
• Programmer is responsible for storing and
  interpreting properly

4
Unions

• include ltiostream.hgt
• union int_or_char
• int x
• char y
• void main()
• int_or_char value
• value.x 97//store as int
• cout ltlt value.xltltendlltltvalue.yltltendl
• value.y 'b'//store as char
• coutltltvalue.xltltendlltltvalue.yltltendl
• Output 97 a 98 b
• Can do all the things that we can do with a
  struct
• Union can have a struct as a member, and vice
  versa

5
Unions

• First initializer stored as the data type of the
  first member
• include ltiostream.hgt
• union int_or_double
• int x
• double y
• void main()
• int_or_double number
• number.x 100.3
• cout ltlt number.xltltendlltltnumber.yltltendl
• number.y 100
• coutltltnumber.xltltendlltltnumber.yltltendl
• Output 100 junk junk 100
• Apart from memory, unions also useful where you
  know a value will be either one of two types e.g.
  you can have a union for items that have either a
  char name20 or an int serial_no

6
Bit Fields

• An int or unsigned member of a structure or union
  can be declared to consist of a specified no. of
  bits
• Such a member is called a bit field, and the no.
  of associated bits is called its width
• struct card
• unsigned pips 4//cant do so outside
  struct/union
• unsigned suit 2
• How many values can we store in 4 and 2 bits?
• 2n values (0 thru 2n-1) in n bits
• Conserves memory how many things storable in 6
  bits or 1 byte? Only a character
• Here, 64 1-bit variables using bit fields
• The width is at most the no. of bits in a machine
  word

7

• An example that uses bit fields for cards and
  outputs them. Also note how array of struct
  passed as usual array
• include ltiostream.hgt
• include ltiomanip.hgt
• struct BitCard
• unsigned face 4
• unsigned suit 2
• void fillDeck(BitCard )
• void deal(BitCard )
• main()
• BitCard deck52
• fillDeck(deck)
• deal(deck)
• return 0

• void fillDeck(BitCard wDeck)
• for (int i 0 i lt 51 i)
• wDecki.face i 13
• wDecki.suit i / 13
• / Output cards in two column format. Cards 0
  25 subscripted with k1 in column 1. Cards 26-51
  subscripted with k2 in column 2 /
• void deal(BitCard wDeck)
• for (int k1 0, k2 k1 26 k1 lt 25
  k1, k2)
• cout ltlt "Card" ltlt setw(3) ltlt
  wDeckk1.face ltlt " Suit"ltlt setw(2) ltlt
  wDeckk1.suit ltlt " "
• cout ltlt "Card" ltlt setw(3) ltlt
  wDeckk2.face ltlt " Suit"ltlt setw(2) ltlt
  wDeckk2.suit ltlt endl