Function Parameters and Scope

1
Function Parameters and Scope

• Computer Programming I
• Dr. Tim Margush
• Mathematics, Science and Technology Department
• Farquhar Center - Nova Southeastern University

2
What is a Function?

• A function is a program unit representing the
  abstraction of some task
• When designing a function, you must clearly
  identify this task and
• Identify "input" required
• Plan the processing to be performed
• Return a result (if required)

3
Determining Required Parameters

• What objects must the function have access to in
  order to accomplish its task?
• Member functions automatically have access to the
  objects declared within the class definition
• In general, any objects needed by the function
  are specified in the parameter list and passed to
  the function as an argument

4
Function Prototype

• The types of parameters are specified in the
  function prototype (as is the return type)
• void showPages(void)
• double areaOfRectangle(double, double)
• char middleInitial(string name)
• Naming of parameters is not required in the
  prototype, but is a form of documentation

5
Function Heading

• Functions are defined by specifying a function
  heading followed by a program block
• double areaOfRectangle(double x, double y)
• return xy
• The parameter names are required here
• The return type and number and type of parameters
  must match the prototype

6
Scope

• The scope of an identifier is that part of the
  program in which the identifier can be used (has
  meaning)
• Scope (or visibility) is determined by the
  placement of the identifier's declaration
• There are four kinds of scope
• Block, File,Function, and Function Prototype

7
Block Scope

• Identifiers declared within a block or in a
  parameter list of a function definition have
  block scope
• Block scope extends from the point of the
  identifier's declaration to the end of the block
  in which it is defined
• A parameters' scope includes the entire function
  body that follows the parameter list
• blocks are sections of executable statements,
  beginning with , and ending with a matching

8
File Scope

• Identifiers declared outside all blocks and
  parameter lists have File scope
• File scope extends from the point of declaration
  to the end of the source file
• Function names all have file scope
• Identifiers declared with file scope are
  sometimes called global identifiers
• Identifiers with block scope are called local
  identifiers

9
Function Prototype Scope

• An identifier appearing in a function prototype
  parameter list has function prototype scope
• This scope extends from the point of declaration
  to the end of the parameter list.
• Such identifiers serve no purpose other than to
  document the intended use of a parameter

10
Function Scope

• Statement labels are the only identifiers with
  function scope
• A statement label is declared inside a function
  by placing the identifier followed by a colon in
  front of a statement
• Statement labels have meaning everywhere in the
  function body in which they are declared

11
Obscured Identifiers

• If an existing identifier is redeclared within
  its scope, then the original identifier is
  obscured for the scope of the redeclaration
• The redeclaration must be at block scope and may
  not be in the same block of the original
  identifier

12
An Obscure Example

• function x has file scope
• x has function prototype scope
• x has block scope
• x has block scope
• x has block scope

• int x(double x)
• void fun(void)
• cout ltlt x()
• string x 2
• double x 2.0
• cout ltlt x
• int x(double x)
• return (int) x

13
Value Parameters

• So far, arguments to functions have been passed
  by value
• This means the parameter receives a copy of the
  argument's value
• A value parameter can correspond to any argument
  which is an expression representing a value of an
  appropriate type

14
Reference Parameters

• When a parameter becomes a reference to an object
  represented by the argument, pass-by-reference is
  being used
• In pass-by-reference, no copy of the argument is
  made
• The parameter is an alias for the object
  represented by the argument

15
Changing an Argument's State

• An object passed by reference can have it's state
  changed
• Operations on the parameter are applied to the
  original object
• Methods applied to the parameter are applied to
  the original object
• Object's passed by value are protected
• Changes to the parameter affect only the copy

16
Specifying Pass-By-Reference

• The character is used after the type in the
  parameter list to indicate pass-by-reference
• Each parameter may be specified independently as
  pass-by-value or pass-by-reference
• int joy(float, string , double)
• by-value, by-reference, by-value
• void fun(int x, double y, string z)
• by-reference, by-value, by-reference
• Make a conscious choice!

17
An Example to Reference
radius receives a copy of r

• void CA(int, float , float )
• int main()
• float r, a, c
• cin gtgt r
• CA(r, c, a)
• cout ltlt c ltlt " " ltlt a
• return 0

• void CA(int radius,
• float circ, float area)
• circ 2PIradius
• area PIradiusradius

objects a and c are modified by the function AC
circ becomes an alias for c and area for a during
execution of the function
18
Constant References

• Pass-by-reference is more efficient that
  pass-by-value
• The object need not be duplicated
• Saves time and memory
• To protect an object passed by reference from
  changes, use const
• void fun(const int x, const string y)
• x and y cannot be changed, so the actual
  arguments are protected, even though aliased to x
  and y

19
Principle of Least Privilege

• Allow functions only the minimum access to
  information required to accomplish their task
• Make appropriate use of pass-by-value and
  pass-by-reference
• Make appropriate use of the const modifier