Reference parameters (6.2.3)

1
Reference parameters (6.2.3)

• We might need a function to return more than one
  value
• Find roots of a quadratic
• 2 return values. What are they?
• Get first and last name of a user and return them
• 2 return values
• Functions are limited to one return value using
  return
• When you need to return several values back from
  a function
• Use reference parameters
• The idea is that when the value of the parameter
  is modified, function modifies the value of the
  corresponding argument as well

2
Value Parameters - Pass by value

• The parameters we have seen so far are value
  parameters
• their arguments are passed by value
• if you change the value of a parameter in
  function, corresponding argument does NOT change
  in the caller function
• Example see passbyvalue.cpp (not in book)
• parameter a changes in function average, but
  corresponding argument, num1, does not change in
  main
• Enter two integers 10 15
• in main before calling average num1 10, num2
  15
• beginning of function average a 10, b 15
• end of function average a 25, b 15
• average is 12.5
• in main after calling average num1 10, num2
  15

3
Reference Parameters Pass by Reference

• Reference parameters are used to change the value
  of the argument in the caller function as well
• ampersand () between type and name
• void myfunction (int num)
• if you change the value of a reference parameter
  in function, corresponding argument changes in
  the caller function
• Argument of a reference parameter must be a
  variable
• This is a reasonable rule, because otherwise its
  value wont change
• See passbyreference.cpp (not in book)
• parameter a changes in function average
  corresponding argument, num1, changes in main as
  well
• Enter two integers 10 15
• in main before calling average num1 10, num2
  15
• beginning of function average a 10, b 15
• end of function average a 25, b 15
• average is 12.5
• in main after calling average num1 25, num2
  15

4
Underlying Mechanisms

• For value parameters, the arguments values are
  copied into parameters
• arguments and parameters have different memory
  locations
• double average (int a, int b)
• average(num1, num2)

10
15
copy value
copy value
main function
10
15
5
Underlying Mechanisms

• For reference parameters, the parameter and the
  argument share the same memory location
• parameter is an alias of argument
• double average (int a, int b)
• average(num1, num2)

15
refers to the same memory location
copy value
main function
10
15
6
Example Swap

• Write a function that swaps the values of its two
  integer parameters
• void swap(int a, int b)
• int temp
• temp a
• a b
• b temp
• How do we use this in main?
• int a5, b8
• swap(a,b) // a becomes 8, b becomes 5
• swap(a,5) // syntax error, arguments must be
  variables

7
Examples

• Whats prototype for a void function that takes a
  string as parameter and returns the number of
  vowels and consonants in it?
• void letters(string s, int vowels, int
  consonants)
• Whats prototype for a void function that returns
  the number of hours, minutes in N seconds. Where
  N is a parameter?
• void TimeConversion(int N, int hours, int
  minutes)

8
Reference Parameters are not only to return
multiple values

• Even if you have a single value to return, you
  may prefer to return it as a reference parameter,
  not as the return value of the function.
• ToLower function, defined in strutils, changes
  its argument to all lowercase. It is actually a
  void function, i.e. does not return anything as
  the functions return value
• Prototype is
• void ToLower(string s)
• Example use
• string s "HeLLo"
• ToLower(s) // s becomes hello

9
Example (See roots.cpp)

• Roots of a quadratic equation
• ax2 bx c 0
• what could be a prototype?
• void roots(double a, double b, double c, double
  r1, double r2)
• What happens if
• one root?
• no roots?
• how will you inform the caller function about the
  number of roots?
• necessary in order to let the caller function to
  interpret arguments for r1 and r2
• Solution let the function return (as the return
  value) an integer value for the number of roots
• So, the prototype becomes
• int roots(double a, double b, double c, double
  r1, double r2)

10
Parameter Passing const-reference

• Pass by value (value parameters) has overheads
• copying the value
• memory allocation for both parameter and argument
• Sometimes we want to avoid the overhead of making
  the copy, but we dont want to allow the argument
  to be changed.
• const-reference parameters avoid copies, but
  cannot be changed in the function
• trying to change a const-reference parameter is a
  syntax error
• defined with const before a reference parameter
• void demo (const int num, const string s)

11
Example

• Count number of occurrences of a letter in a
  string
• Write a function for it
• Look at every character in the string
• int LetterCount(const string s, const string
  letter)
• // post return number of occurrences of letter
  in s
• int k, count 0, len s.length()
• for(k0 k lt len k)
• if (s.substr(k,1) letter)
• count
• return count
• Calls below are legal
• int ec LetterCount("elephant", "e")
• string s "hello"

12
General rules for Parameters

• Const-reference parameters allow constants and
  literals to be passed
• For example, elephant cannot be passed as an
  argument for a reference parameter, but it is ok
  with const-reference
• Some good-programming tips
• Built-in types (int, double, bool, char) - pass
  by value unless you change it and return from the
  function
• All other types - pass by const-reference unless
  you change it and return from the function
• When you change and want to return the changed
  value, use reference parameters
• What about using classes as the parameter type?
• use reference parameters if you are changing the
  state of the parameter object
• that is why we used reference parameters for
  Robot objects
• use const reference if you are not changing the
  state of the parameter object