Variable Scope and Lifetime

• A local variable has its scope within a function or block, while a global variable has its scope throughout an entire file.
• Scope of a variable

  • Extent to which different parts of the program have access to the variable
  • Where the variable is visible

  
• Lifetime of a variable


  • How long the variable persists in memory
  • Period from the point when the variable's storage is allocated to the point when it is deallocated
• External variable


  • Variable defined outside of any function
  • Scope: entire source code file that contains this variable's definition
  • Lifetime: from its definition until program termination

  
• Local variable


  • Variable defined within a function
  • Scope: limited to the function in which they are defined
• The main() function has no external variables
• The print_value() function has no external variables
• Automatic variable


  • Local variables are automatic by default
  • Created anew each time the function is called
  • Destroyed when execution leaves the function
• The output of the program with automatic variables is "First display. X: 1 Second display. X: 1"
• Static variable


  Local variable that retains its value between calls to the function
• The output of the program with static variables is "First display. X: 1 Second display. X: 2"
• Learning Objectives


  • Determine the scope and lifetime of types of variables – external and local, static and automatic
  • Properly demonstrate passing argument/s to functions using pass-by-value method and pass-by-reference method
  • Demonstrate how to pass arrays to functions
• Variable Scope


  The extent to which different parts of the program have access to the variable. It refers to where the variable is visible.
• Variable Lifetime


  How long the variable persists in memory. It is a period from the point when the variable's storage is allocated to the point when it is deallocated.
• Sample Program 1


  1. Declare variable x outside of any function

  2. Access x in main() function and print_value() function

  3. x persists until program termination
• Sample Program 2


  1. Declare variable x inside main() function

  2. Cannot access x in print_value() function

  3. x still exists but cannot be accessed
• Sample Program 3


  1. Declare variable x at global scope (Line 3)

  2. Declare variable x at local scope (Line 15)

  3. Local variable x takes priority when accessed in print_value() function

  4. Global variable x has longer lifetime than local variable x
• Sample Program 4


  1. Declare variable x in main() function

  2. Declare variable x in print_value() function

  3. Scopes of the two x variables do not overlap

  4. Lifetime of x in main() is longer than x in print_value()
• External Variable


  Variable declared outside of any function. Its scope is the entire source code file that contains this variable's definition. Its lifetime is from its definition until program termination.
• Local Variable


  Variable defined within a function. The scope of local variables are limited to the function in which they are defined.
• Automatic Variable


  Local variables that are created anew each time the function is called, and they are destroyed when execution leaves the function.
• Static Variable


  Local variable that retains its value between calls to the function in which it is defined.
• Sample Program 6


  1. Declare automatic variable x in display() function

  2. x is destroyed after each call to display()

  3. A new x is declared on each call
• Sample Program 7


  1. Declare static variable x in display() function

  2. x retains its value between calls to display()

  3. Output shows x incrementing on each call
• Functions, Arguments and Parameters
• We already know what parameters and arguments are and how they work
• Static variable


  A variable that retains its value between function calls
• Sample Program 7


  1. Declare static int x

  2. Print "First display."

  3. Call display()

  4. Print "Second display."

  5. Call display()
• Sample Program 7 output


  • First display.
  • X: 1
  • Second display.
  • X: 2
• Formal parameter


  Parameter found in the function header, placeholder for data passed to the function
• Actual parameter (argument)


  Parameter specified in the function call, the data that needs to be passed to the function
• Pass by value


  • The function is passed a copy of the variable's value, the function has no access to the original variable and cannot modify it
• Pass by reference


  • The function is passed a reference to (address of) the original variable, the function has access to the original variable and can modify it
• Passing by reference


  Allows the function to modify the argument variable
• Passing by value


  Does not allow the function to modify the argument variable
• Sample Program 9


  1. Declare x=2, y=4

  2. Call by_value(x, y)

  3. Call by_reference(&x, &y)
• Sample Program 9 output

  • Before by_value(), x = 2, y = 4.
  • After by_value(), x = 2, y = 4.
  • Before by_reference(), x = 2, y = 4.
  • After by_reference(), x = 0, y = 0.
• Passing 1D arrays to functions

  • Can use pointer notation, array notation without size, or array notation with size
• Passing 2D arrays to functions

  • Can specify all sizes (subscript values) or only the leftmost size (subscript value)
• The name of an array is a pointer to the array's first element