Our program should receive from its caller a long-distanc

1
Parameter Passing Mechanisms
Reference Parameters
2
Problem

• Using OCD, design and implement a function that,
  given a string containing a long-distance
  telephone number, decomposes that number into its
  area code, exchange, and local number.

3
Preliminary Analysis

• Our function can receive the long-distance phone
  number through a string parameter.
• This problem requires that our function somehow
  communicate three values (area code, exchange,
  local number) to its caller.
• A function cannot return multiple values -- the
  return statement only returns one value
• return Expression

4
Behavior

• Our program should receive from its caller a
  long-distance phone number (a string). It
  should check that the number is a valid long-
  distance number. If so, it should compute and
  pass back its area code, exchange, and local
  number.

5
Objects

• Description Type Movement Name

long distance string in ldNumber
number
area code string out areaCode
exchange string out exchange
local number string out localNum
6
Operations

• Description Predefined? Library?
  Name

receive a string yes built-in --
select part of yes string substr a
string
pass back 3 strings yes built-in ??
7
Algorithm

• 0. Receive ldNumber from caller, plus empty
  variables areaCode, exchange and localNum.
• 1. Check that ldNumber is a long-distance number.
• 2. Fill areaCode with appropriate substring of
  ldNumber.
• 3. Fill exchange with appropriate substring of
  ldNumber.
• 4. Fill localNum with appropriate substring of
  ldNumber.

8
Discussion

• Since a function cannot return 3 strings, we
  will instead require the caller to pass us three
  string variables, which our function will then
  fill in with the appropriate values.
• Normal parameters are called value parameters and
  are built as copies of their arguments.
• Changing a value parameter changes the copy, not
  its corresponding argument.

9
Solution

• Reference parameters are parameters declared with
  an ampersand () following the parameters type
  (and before its name).
• A reference parameter is an alias (i.e., another
  name for) its corresponding argument.
• Changing the value of a reference parameter
  changes the value of its corresponding argument.

10
Coding

• include ltstringgt // string
  class
• include ltcctypegt // isdigit()
• using namespace std
• void ChopLDPhoneNumber(string ldNumber, //
  value IN
• string areaCode, //
  reference OUT
• string exchange, //
  reference OUT
• string localNum) //
  reference OUT
• for (int i 0 i lt ldNumber.size() i)
  // check all
• assert(isdigit(ldNumberi))
  // digits
• assert(ldNumber0 1 // check for
  leading 1
• ldNumber.size() 11) // check
  number of digits
• areaCode ldNumber.substr(1, 3)
• exchange ldNumber.substr(4, 3)
• localNum ldNumber.substr(7, 4)

11
Testing

• The caller must now supply a variable for each
  reference parameter, to be filled in by the
  function.
• cout ltlt Enter a L-D phone number
• string original, part1, part2, part3
• cin gtgt original
• ChopLDNumber(original, part1, part2, part3)
• cout ltlt \nArea code ltlt part1
• ltlt \nExchange ltlt part2
• ltlt \nLocal number ltlt part3
• ltlt endl

12
Notes

• When function ChopLDNumber() is called
• a copy of argument original is made to create
  parameter ldNumber,
• an alias of argument part1 is made to create
  parameter areaCode,
• an alias of argument part2 is made to create
  parameter exchange,
• an alias of argument part3 is made to create
  parameter localNum.

13
0. Before the function call
original
16165551234
part1
part2
part3
Memory
14
1. ldNumber is createdas a copy of original
original
16165551234
part1
part2
part3
ldNumber
16165551234
Memory
15
2. areaCode is createdas an alias for part1
original
16165551234
part1
areaCode
part2
part3
ldNumber
16165551234
Memory
16
3. exchange is createdas an alias for part2
original
16165551234
part1
areaCode
part2
exchange
part3
ldNumber
16165551234
Memory
17
3. localNum is createdas an alias for part3
original
16165551234
part1
areaCode
part2
exchange
part3
localNum
ldNumber
16165551234
Memory
18
4. The function checks ldNumber for validity
original
16165551234
part1
areaCode
part2
exchange
part3
localNum
ldNumber
16165551234
Memory
19
5. The function computesareaCode, changing part1
original
16165551234
part1
areaCode
616
part2
exchange
part3
localNum
ldNumber
16165551234
Memory
20
6. The function computesexchange, changing part2
original
16165551234
part1
areaCode
616
part2
exchange
555
part3
localNum
ldNumber
16165551234
Memory
21
7. The function computeslocalNum, changing part3
original
16165551234
part1
areaCode
616
part2
exchange
555
part3
localNum
1234
ldNumber
16165551234
Memory
22
8. The function returns, destroying all parameters
original
16165551234
part1
616
part2
555
part3
1234
Memory
23
9. part1, part2, and part3now contain the
information!
original
16165551234
part1
616
part2
555
part3
1234
Memory
24
Notes

• By default, parameters are value parameters.
• Reference parameters are specified by placing an
  ampersand after the parameters type.
• Reference parameters must be specified in both a
  functions prototype and its definition, or a
  linking error will occur.
• Variables must be passed as arguments for
  reference parameters to fill, or a compiler error
  will occur.

25
Consider

• Copying argument original consumes time.
• Creating an alias for an argument takes almost no
  time.
• We could speed up calls to our function by making
  parameter ldNumber a reference parameter.
• However, we then run the risk of changing
  original if we mistakenly change ldNumber.

26
Solution

• Constant reference parameters are reference
  parameters whose declaration is preceded by the
  keyword const.

void ChopLDPhoneNumber(const string ldNumber,
// IN string areaCode,
// OUT string
exchange, // OUT
string localNum) // OUT // ...
Const reference parameters are read-only
reference parameters -- aliases of their
arguments -- but they cannot be changed.
27
0. Before the function call
original
16165551234
part1
part2
part3
Memory
28
1. ldNumber is created as aconst reference of
original
original
16165551234
ldNumber
part1
part2
part3
Memory
29
Conclusion

• The rest of the function proceeds as before,
  except that all accesses to ldNumber now access
  original instead of the copy.
• Any attempt to change ldNumber will generate a
  compiler error (which makes sense, since its
  movement is IN, not OUT).

30
Discussion

• Copying time is not significant for simple types
  (e.g., int, char, double, ...), but it is
  significant for class types (e.g., string,
  RandomInt, ...).
• Use reference parameters for arguments whose
  movement is OUT.
• Use const reference parameters to store class
  arguments whose movement is IN.
• Use value parameters to store simple type
  arguments whose movement is IN.

31
Summary

• C provides 3 parameter mechanisms
• value, used for IN parameters whose arguments are
  simple types.
• const reference, for IN parameters whose
  arguments are class types.
• reference, for all OUT parameters.

32
Announcement

• Lab Test Open books, Open notes.
• Wed. 2 hours.
• 3 programs.
• You need prepare your money on your card.
• You need to print all the results out in the lab.
• You need in hand in the test sheet.