Ch 12. Features Found Only in C

1
Ch 12. Features Found Only in C

• Timothy Budd
• Oregon State University

2
Introduction

• C includes a number of features that have no
  equivalent in Java.
• Table 12.1 summarizes these features.

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Table 12.1 Features Found in C but not in Java
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Global Variables

• Global variable a variable that is declared
  outside the scope of any class or function.
• int top 0 // global variable, top of
  stackint data100 // data area for stackint
  push(int val) datatop val int pop()
  return datatop--
• Can be declared static and accessible only within
  the file in which they are declared.

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Global Variables

• A global variable used in two or more files must
  be declared as extern in all but one of the
  files.
• A value declared as extern cannot at the same
  time be initialized
• extern int top
• // top is declared and initialized in another
  file
• Whenever possible, avoid the use of global
  variables.

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The Preprocessor

• Initial step prior to analysis by the compiler.
• The text of a C program is scanned and
  directives that begin with a hash mark are
  processed.
• Major categories are
• include ltfilenamegt define name value
  undef name if test ifdef name ifndef
  name else endif

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The Preprocessor

• Include files are used to incorporate header
  files that are used by many files within a
  project or by many different projects.
• include ltcstdiogt // include standard I/O
  library include "rational.h" // include my own
  header file
• The include directive differs from the import
  statement
• Not a statement and does not end with a
  semicolon.
• Performs a textual inclusion, which is not true
  for Java

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The Preprocessor

• A define directive creates a symbolic constant.
  Associating the given text with the name
• // limit of twelve elements define MAX 12
• Do not use the preprocessor to define symbolic
  constants.
• const int max 12 // limit of twelve elements

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The Preprocessor

• A common idiom is found in header files to
  prevent the contents of file from being scanned
  by the compiler twice, should the file happen to
  be included more than one time.
• // see if file has already been scanned
  ifndef FOOCLASSDEFINITION // if not, then
  define symbol now, so later // inclusions will
  skip this file define FOOCLASSDEFINITION ...
  // contents of file endif

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typedef Statement

• Use typedef to give a contextually meaningful
  name to an existing data type.
• typedef double fahrenheit // define alias for
  double fahrenheit currentTemp // declare a new
  variable
• Two types that expand to the same base are
  considered the same
• currentTemp 2.3 // can combine with
  doublestypedef double celsius // define new
  aliascelsius newTemp 42 // declare another
  variablecurrentTemp newTemp // can combine,
  both doubles

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typedef Statement

• typedef statements are often used to eliminate
  confusing syntax.
• int (fun)(char )10
• typedef int ( funPtr)(char ) // funPtr -
  pointer to a functionfunPtr fun10 // declare
  array of pointers to functions

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typedef Statement

• typedef statements are also frequently used to
  shorten compound type names.
• map lt const char , map lt const char , unsigned
  int, charCompare gt, charCompare gt aGraph
• typedef map lt const char , unsigned int,
• charCompare gt cityInfotypedef map ltconst char
  , cityInfo, charComparegt graphgraph aGraph
  // declare a new graph value

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The const Keyword

• Used to define a quantity that does not change.
• Applied to variables, it provides a way of
  creating symbolic constants
• const int maxCharCount 80 // upper limit on
  character
• Applied to an object value, it indicates that the
  object will not changed in any fashion stronger
  than the Java notion of final, only the object
  will not be reassigned.

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The const Keyword

• Applied to arguments, particularly argument
  by-reference arguments, it indicates that the
  value will not be modified inside the function.
• int foo (const box aBox) // contents of
  aBox cannot be changed ... aBox.value 17
  // ERROR! compiler will issue error
  message ...

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The const Keyword

• Applied to a member function, the keyword
  indicates that the function does not make a
  change to the receiver class string
  public ... // computing length does not
  alter string int length() const ...
• Only member functions that have been declared as
  can be used with objects that are themselves
  declared const.

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The const Keyword

• When applied to a pointer, the const keyword
  either indicates that the pointer itself is
  constant, or that the value it references is
  constant, depending on the placement
• int i 7 const int p i // pointer to
  a constant int const q i // constant
  pointer p 8 // not allowed, p points to a
  const q 8 // allowed, q is pointing to non
  const p q // allowed, p itself is not
  constant q p // not allowed q is
  constant const int const r i // both
  aspects are constant

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The const Keyword

• Casting away const is using a cast operator to
  remove the const restriction.
• const char name "Fred" char p
  static_castltchar gt(name)
• // throw away the const part
• p0 'D' // name is now Dred
• The use of cast operation such as static_cast and
  dynamic_cast is preferable to the traditional
  cast syntax.

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Default Arguments

• The use of default arguments can greatly reduce
  the amount of code in a class.
• class box public box (int v 7) val(v)
  void test (int v 3, double d) val v
  (int) d private int valbox aBox(3)
  // explicit argumentbox bBox // uses default
  value of 7aBox.text(2, 3.14) // both arguments
  specifiedbBox.test(2.3) // uses implicit
  definition for first argument

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Default Arguments

• Java programmers sometimes achieve the same
  effect by having one procedure simply invoke
  another of the same name
• class box public box () box(7) // invoke
  myself public box (int v) val v public
  void test (int v, double d) val v (int)
  d public void test (double d) // use more
  general method test(3, d) private int
  val

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Friends

• A friend is a function or class that is permitted
  to access the internal state of another class.
• class box public box (int v) value(v)
  private int value // declare plus
  function as friend friend int plus(const box ,
  const box )

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Friends

• int plus (const box left, const box right)
  // access to private data field value // is
  permitted, since plus is declare // as
  friend return left.value right.valueint
  minus (const box left, const box right) //
  ERROR will produce an error message // since
  access to private data field // is not
  permitted return left.value - right.valuebox
  a(3), b(4)int i plus(a, b)

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Friends

• Following program illustrates how to define a
  class that is guaranteed to have only a single
  instance
• class singletonBox
• public int value private singletonBox
  () value(0) friend theBox() const
  singletonBox theBox() // declare the one and
  only one box static singletonBox actualBox //
  return this singleton object return actualBox

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Name Spaces

• A programmer can explicitly declare that all the
  functions and definitions in one part of a
  program are defined within a certain name space
• // declare everything in name space
  stacknamespace StackADT int height class
  Stack ...

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Name Spaces

• Names defined inside a namespace can be used
  outside the namespace description.
• Use a fully qualified name, similar to a fully
  qualified method name // read the value of the
  height variable int max StackADTheight
• Alternatively, all the names defined in a name
  space can be included in a program using a single
  statement using StackADT
• // include all the names defined in the
  namespace

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Multiple Inheritance

• In C, a class can inherit from two or more
  parent classes.
• Avoid the use of multiple inheritance.

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Example of Multiple Inheritance

• class GraphicalDeck
• public CardDeck, public GraphicalObject
  public void draw (Window w) // draw image on
  window GraphicalObjectdraw(w) Card
  draw () // draw card from deck return
  CardDeckdraw() ...

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Example of Multiple Inheritance

• class GraphicalDeck
• public CardDeck, public GraphicalObject
  public void draw () // draw image on
  window GraphicalObjectdraw() Card
  getCard () // draw card from deck return
  CardDeckdraw() ...

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Example of Multiple Inheritance

• class A ...private int dataField
• class B public A ...
• class C public A ...
• class D public B, public C ...// question
  how many dataFields does this have?

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Example of Multiple Inheritance

• class A ...private int dataField
• class B virtual public A ...
• class C virtual public A ...
• class D public B, public C ...

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Example of Multiple Inheritance

• class A public // constructor A (int d)
  dataField(d) ... private int dataField
  
• class B virtual public A public B (int b)
  A(7) // initialize A with 7 ...
• class C virtual public A public C (int c)
  A(11) // or, initialize A with 11 ...
• class D public B, public C public // Nope,
  neither, initialize A with 42 D () A(42),
  B(12), C(22) ...

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goto Statement

• Avoid using a goto statement, except to break out
  of nested loops.
• int test (int i) // example of bad code
• i i 2 tst if (i gt 10) ret return i
  2 i i - 3 if (i lt 1) goto ret i i
  5 goto tst

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goto Statement

• One of the most common uses for the goto
  statement is to break out of multiple levels of
  looping, something that in Java is performed
  using a break
• static final int gold 1static final int
  dross 0int field100100...searchForGold
  // a Java search of a doubly nested loopfor
  (int i 0 i lt 100 i) for (int j 0 j lt
  100 j) if (fieldij GOLD) break
  searchForGold // ... execution comes here on
  break

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goto Statement

• The same search could be written in C as
  follows
• // a C search of a doubly nested loopfor (int
  i 0 i lt 100 i)
• for (j 0 j lt 100 j) if (fieldij
  GOLD) goto FoundGoldfoundGold // ...
  execution comes here on found value