Functions with Reference Parameters

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Chapter 9
Functions with Reference Parameters
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Outline

• Reference parameters in input functions
• General passing by reference
• const reference parameters
• Review of global scope
• Hand Tracing

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Value parameters passing by value

• What were doing so far is passing by value
• A copy of that arguments value is passed to the
  corresponding parameter of the called function
• If a function changes the value of one of its
  parameters, that change will stay local to the
  function
• it will NOT affect the arguments value back at
  the point of the call

void inc(int x) int main () int a 5
inc(a) cout ltlt a return 0 void inc (int
x) x
void inc(int a) int main () int a 5
inc(a) cout ltlt a return 0 void inc (int
a) a
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Reference parameters Passing by Reference

• A reference parameter allows a function call to
  give a value to or update the current value of a
  variable argument appearing in the call.
• The ampersand, , between the parameters data
  type and its name, says the parameter is a
  reference parameter

void inc(int x) int main () int a 5
inc(a) cout ltlt a return 0 void inc (int
x) x
The following notations are equivalent int
x int x int x
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Reference parameters Passing by Reference

• void input(double dWeight, char chConfirm)
• int main ()
• double weight
• char confirm
• input(weight, confirm)
• cout ltlt weight ltlt ltlt confirm
• return 0
• void input(double dWeight, char chConfirm)
• cout ltlt "Please enter the weight "
• cin gtgt dWeight
• cout ltlt Delivery confirmation (y or n)? "

1. Use an ampersand between the type name and
the variable name ...
2. In the parameter list of both the function
declaration and the header of the functions
definition
3. NO (and data type) in function call.
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Reference parameters Passing by Reference
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Reference parameters Passing by Reference
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Reference parameters Passing by Reference
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Argument-to-Parameter Matching

• Value Parameters
• Argument can be an expression, variable, or
  literal
• C will do its best to match data types
• Reference Parameters
• Argument must be a variable
• Not a literal literals values cant be changed
• Argument and parameter must be of same data type
• One exception, for file types chapter 11
• If function is to reach back to the variable at
  point of call, it must know that variables data
  type exactly
• As before, argument/parameter matching is by
  order names are totally irrelevant

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Argument-to-Parameter Matching
Argument must be a variable
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Argument-to-Parameter Matching
Argument and parameter must be of same data type
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Use Reference Parameters in Functions that
calculate more than one value

• Value-returning function usually only return one
  value

• So lets return the results in Reference
  Parameters

Area
Circum

• A value-returning function should be used only
  when the sole purpose is to calculate and return
  a single value.
• A value-returning function can be used with a
  reference parameter

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One more example swapping two values
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One more example swapping two values
if (w2 gt w3) swap(w2, w3)
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const Reference Parameters
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Constant Parameters

• The const modifier can also be applied
  to reference parameters
• const says that the function can not modify
  the parameters value, even though its by
  reference
• Sample call in main program
• int Num1
• Get_Int ("Enter integer number ", Num1)
• function definition
• void Get_Int (const string Prompt, int Nbr)
  
• cout ltlt Prompt
• cin gtgt Nbr
• Prompt "Got it"
• cout ltlt Prompt

Illegal assignment
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Constant Parameters

• When do we use them?
• When we need to pass a variable (object) by
  reference, but do not want to let the called
  function modify it
• Question
• Then why not just pass the variable (object) by
  value?
• Answer
• Yes, usually we should pass it by value
• However, for large, complex objects, making a
  copy of the whole object can be very inefficient,
  in terms ofstorage space and/or time
• Array, by default, is always passed by reference

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A Tale of Two Functions

• This code will produce a compiler error
• void Func (string S)
• . . . . . . . . . . . . . . .
• int main()
• Func ("This is a sample string.")
• return 0
• . . . . . . . . . . . . . . .
• void Func (string S)
• cout ltlt S ltlt endl

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A Tale of Two Functions
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A Tale of Two Functions, contd.

• But this code will compile just fine
• void Func (const string S)
• . . . . . . . . . . . . . . .
• int main()
• Func ("This is a sample string.")
• return 0
• . . . . . . . . . . . . . . .
• void Func (const string S)
• cout ltlt S ltlt endl

(Constant argument requires a const reference
parameter)
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Global Scope
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Global Scope

• An constant or variable is global if it
  is available to all functions of the program
• Defined before and outside any function
• Example
• // Joes second program 04/07/1998
• include ltiostreamgt
• using namespace std
• const double PI 3.1415926535
• bool Debugging false
• int main ()
• ...

Global items
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Global Scope

• Global Constants
• DO use them
• Two main situations
• Physical or mathematical constants (PI)
• Data structure sizes used several places in the
  code
• Global Variables
• Do NOT use them
• Except in really exceptional circumstances
• Puts you in danger of unexpected side-effects
• Function does its job, but also changes a global
  variable
• Some other function doesnt expect that variable
  to change