C for Java Programmers

1
C for Java Programmers

• Lecture 8

2
That was I/O

• Yesterday we had a reasonably detailed look at
• Streams
• C console I/O revisited
• This is object orientated
• Formatting flags
• File I/O
• Customisation
• Coursework ONE

3
Tidying up some loose ends

• Today well be steaming through
• C specifics
• Namespaces
• Classes in C
• How good are you really with pointers?

4
2 Different Types of C

• C underwent an extensive revolutionary process
  during its development and standardisation.
• As a result there are actually 2 versions of C
• traditional C
• Standard C
• Standard C was defined by the ANSI/ISO
  standardisation committee.
• Standard C is essentially a superset of
  traditional C.

5
Old Compilers

• You have to be careful if you are using an older
  compiler.
• During the process of standardisation, the
  ANSI/ISO committee added many new features to the
  language since this occurred over a period of
  years an older compiler might not support them.
• This is important because 2 recent additions will
  affect every program that you write

6
Namespaces

• The key differences between old and modern C
  style lie in new-style headers and the namespace
  statement.

include ltiostream.hgt int main () cout ltlt
this is old style return 0
include ltiostreamgt using namespace std int
main () cout ltlt this is new style return
0
traditional C
modern C
7
Headers

• We know that if you want to use a library
  function in your code.
• you must include its header file, using the
  include statement (e.g. include ltstdio.hgt) and
  that file literally is included in your program.
• This is essentially C-style.
• Standard C created a new kind of header that
  doesnt use specific filenames instead they
  specify standard identifiers that can be mapped
  to files by the compiler.

8
Header Comparison

• C headers are an abstraction that simply
  guarantees that the appropriate prototypes and
  definitions required by the C library have been
  declared.
• Because these new-style headers are not filenames
  they dont have the .h extension. For example
• C still supports the old C-style but be aware
  that the comparision isnt always direct
• iostream.h ltiostreamgt
• math.h ltcmathgt
• string.h ltcstringgt

9
Namespace Analysis

• Contents of new-style headers are contained in
  the std namespace.
• The purpose of namespaces is to localise the
  names of identifiers to avoid name collisions
  elements declared in one namespace are completely
  separate from elements declared in another.
• Originally this wasnt used an all C library
  functions were in the global namespace now they
  are in std.
• So we use the following statement to bring the
  std namespace into visibility
• using namespace std

10
Why do all of this?

• C saw an explosion of variable, function and
  class names prior to the namespace statement
  all of his competed for slots in the global
  namespace.
• If you wrote a function called abs() for example
  it too would be stored in the global namespace,
  and hence there would be a conflict with the one
  already there.
• This got even worse as there was increased use of
  3rd Party libraries, which might use the same
  names for classes.

11
Classes in C
BJARNEY

• C began as an expanded version of C Started
  in 1979 it was originally called C-with-classes
  (taking OOP from Simula67).
• C is an excellent language but it has its
  pragmatic limits once code exceeds from 25,000
  to 100,000 lines it can become so complex that it
  is impossible to grasp in its totality.
• C was created to allow this barrier to be
  broken.
• I believe classes should be there for a purpose
  to make your code easier to grasp, to speed up
  your coding, to allow other people to use it and
  so on.

12
Structs and Classes

• It is easier to understand classes in general if
  you view them as an extension of structs
• Structs encapsulate data to make your program
  easier to use. You can define it once then use it
  throughout your code.
• To access its fields all you need to do is to use
  the dot operator.
• So essentially Classes are Structures with member
  functions. In fact structs and classes in C
  have evolved to be almost identical.

13
Anatomy of a Class

• Classes have the following features
• Declaration
• Access Control
• Constructor
• Destructor
• Variables
• Methods
• A Special Pointer called this.

14
Class Structure
Header
Constructor
Destructor
Member Functions
Main Program
15
Class Header
A Valid C Name
class Keyword

• class ClassName
• public
• //Prototypes for constructors
• //and public member functions
• //and declarations for public
• //data attributes go here
• ...
• ...
• private
• //Prototypes for private data
• //members and declarations for
• //private data attributes go here
• ...
• ...

public Keyword
private Keyword
Never forget this semi-colon at the end of the
header
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Example Header

• class rectangle
• public
• int height
• int width
• void initialise(int w, int h)
• int getArea()
• void display()
• private
• void drawRow()
• void drawColumn()

Instance Variables
Public methods
Only available to functions inside the class in
this case display will use them
17
Public and Private

• Set something as private and it can only be
  accessed by functions within the class itself.
• This allows you to protect everything inside your
  class from outside interference.
• You can set public first or private first its up
  to you I recommend public section first as this
  provides the interface to the class.
• There is also protected section well look at
  these in more detail later on when we cover
  inheritance.

18
Class Member Functions

• Unlike Java class member functions are not
  contained within the class block of code but are
  written after it.
• Because of this you need to indicate which class
  a function is associated with.
• type classNamefunctionName
• //function body
• For example
• int rectanglegetArea()
• return heightwidth

General Form
19
Constructor

• It is very common for some part of an object to
  require initialisation before it is used. Hence
  there is a special function that allow us to do
  this the constructor.
• The constructor class is a function that has the
  same name as the class
• classNameclassName(parameters)
• //Initialise Variables by copying
• //the parameters given to our
• //classes instance variables

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Full Example

• class rectangle
• public
• int height
• int width
• rectangle()
• rectangle()
• void initialise(int w, int h)
• int getArea()
• void display()
• private
• void drawRow()
• void drawColumn()

21
Full example - Constructor
rectanglerectangle() height 5 width
5 cout ltlt rectangle has been
constructed\n rectanglerectangle() cout
ltlt rectangle has been destroyed\n void
rectangleinitialise(int h, int w) height
h width w
Constructor
Destructor
Initialise Function
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Full Example Member Functions
int rectanglegetArea() return heightwidth
void rectangledisplay() drawRow() for
(int i0 iltheight-2 i) drawColumn()
drawRow() void rectangledrawRow() for
(int i0 iltwidth i) cout ltlt cout ltlt
endl void rectangledrawColumnRow() cout
ltlt for (int i0 iltwidth-2 i) cout ltlt
cout ltlt \n
Private Functions
23
What can C classes do

• that java cant?
• Function Overloading
• Operator Overloading
• Destructors
• Separate Class Header from its methods
• Classes dont need the new statement

24
Combining Everything

• Were soon going to be using pointers, classes,
  DMA all combined together, so its important to
  know at this point
• How much do you really know
• about pointers?

..Surprise 30 minute Test on Pointers ?
25
Exercise of the day (really)

• What does the program do?

include ltiostreamgt using namespace std int
main() int x 10 int px x cout
ltlt px ltlt endl cout ltlt x ltlt ' ' ltlt (px)
ltlt ' ' ltlt x ltlt endl return 0
26
Exercise of the day (really)

• What does the program do?

include ltiostreamgt using namespace std int
main() int x 1,2,3,4,5,6,7,8,9 int
px int i cout ltlt x0 for (i0, px
xi lt sizeof(x)/sizeof(int)-1 px,i) cout
ltlt ", " ltlt px ltlt "-" ltlt px1 cout ltlt endl
return 0
27
Exercise of the day (really)

• What does the program do?

int main(int argc, char argv) for (char
pc argvpc lt argv argcpc) cout ltlt
pc ltlt ", " cout ltlt "\b\b " ltlt endl
return 0
28
Exercise of the day (really)

• What does the program do?

int main() int x 2,4,5,9,2,0,3,9,4,38,2
82,77 const int dimx sizeof(x)/sizeof(int)
int px, qx int endx x dimx for
(pxxpxltendx-1px) for (qxxqxltendx-1qx
) if (qx0 lt qx1) int h qxqx
(qx1)(qx1) h for (px xpx lt
endxpx) cout ltlt px ltlt " " cout ltlt
endl return 0