Pointers

1
Pointers

• Class 12 Classes and Object Oriented
  Programming

2
Structs

• Weve already seen we can create User-Defined
  aggregates of Data Types in structs.
• We can use this feature to model real world
  objects.
• Struct Date
• int month
• int year
• int day

3
Classes

• So what is a class?
• First, think again about structs they can be
  thought of as Objects that have different
  attributes to them.
• A Class is an Object that has BOTH attributes
  (data members like ints, floats, even other
  structs and classes) AND behaviors, or
  operations, (member functions functions that
  operate on the data inside the class)

4
Object Oriented Thinking

• A builder can build a home out of a blueprint.
• He can also reuse this blueprint many times to
  make many homes.
• Classes work in kind of the same way.
• Think of classes as cookie cutters with one
  class, you can stamp out, or create, many
  objects of that class.

5
Using Classes

• Youve actually (maybe) used classes before, and
  you just didnt know it.
• From the C style strings from several lectures
  ago, string itself is just a class.
• You can use classes other people created by
  including them, and you can also write your own
  classes, which we will do shortly.
• You can use a class very much like a struct..
• Consider a class with the following class
  declaration

6
Using classes

• class simpleExample
• public
• void setNumber(int)
• void print()
• int holdNumber
• Now lets look at for a minute. It looks exactly
  like a struct, except
• It says class instead of struct (this is obvious)
• It has this public thing. This is called an
  access modifier. More on these soon. public
  means it will allow anyone to do anything.
• Instead of just having data types, it also has
  functions.
• So lets see how we would use this thing..

7
Using our simple, useless class..

• We declare it like we do structs
• simpleExample se1 //instantiate object se1 of
  class simpleExample
• simpleExample se2 //instantiate object se2 of
  class simpleExample
• se1.holdNumber 5 //set holdNumber to 5
• se1.setNumber(84) //calls simpleExamples
  setNumber function
• se1.print() //calls
  simpleExamples print function
• So what does setNumber(int) and print() actually
  do? Where are they defined? Inside the Class
  Implementation

8
Class Definitions

• class simpleExample
• public
• void setNumber(int)
• void print()
• int holdNumber
• simpleExamplesetNumber(int anumber)
• holdNumber anumber
• simpleExampleprint()
• cout ltltholdNumber

9
Splitting up Files

• Having both the Declaration and Implementation in
  one file is not correct.
• With C, we want to break these up, with the
  Declaration into the header (.h) file and the
  Implementation in the source (.cc) file.

10
Splitting Up Files

• Inside the .cc File called
• simpleExample.cc
• include simpleExample.h
• simpleExamplesetNumber(int anumber)
• holdNumber anumber
• simpleExampleprint()
• cout ltltholdNumber

• Inside the .h Filecalled simpleExample.h
• class simpleExample
• public
• void setNumber(int)
• void print()
• int holdNumber

11
Compiling

• Now, if we compile simpleExample.cc, what will
  happen? why?
• Yes, it will fail. Because what does each
  program need?
• A starting point a main()
• So, when we want to use our custom class inside
  one of our programs, we have to do two things.

12
MyProgram

• In a program, called myProgram.cc, we want to use
  our simpleExample class.
• We need to include the header file
• include ltiostreamgt
• using namespace std
• include simpleExample.h
• int main()
• simpleExample se1
• se1.setNumber(4)

13
MyProgram

• Were still not there yet.. we have our program,
  and the header included, but not the
  implementation.
• We need to link it at runtime. There are a few
  ways to do this. The easiest is to just compile
  all the ccs at the same time.
• CC simpleExample.cc myProgram.cc

14
MyProgram

• This isnt necessary always (and can get
  annoying). What the compiler is actually looking
  for is an object file for (.o) for your class.
• You can make the compiler do this by specifying
  the c option with the compiler
• CC c simpleExample.cc
• This will produce a file called myProgram.o,
  which the compiler will look for if you just
  compile myProgram.cc

15
Preventing multiple inclusion

• When we include something, it copies the code
  for that file into your program.
• If we copy the same information in twice, it will
  cause problems. The second time its included,
  it will try to define things that are already
  defined, resulting in a compile time error.
• So, now that were making header files, we have
  to do our part in preventing this from happening.
• Fortunately, its very easy.
• Unfortunately, its a hack. But its a hack that
  works.

16
Preventing multiple inclusion

• ifndef _SIMPLEEXAMPLE_H_
• define _SIMPLEEXAMPLE_H_
• class simpleExample
• public
• void setNumber(int)
• void print()
• int holdNumber
• endif
• This will prevent the multiple inclusion of
  header files, thanks to the preprocessor.

17
Access Modifiers

• So, back to this public thing. Its an access
  modifier.
• Data Members AND Methods can either be public or
  private (or protected, but for now well just
  talk public and private)
• public means that anyone can directly access that
  data member or call that method.
• private means that ONLY methods WITHIN the class
  can access that data member of function

18
Access Modifiers

• So, lets redefine our example class correctly
• class simpleExample
• public
• void setNumber(int)
• void print()
• private
• int holdNumber

19
Access Modifiers

• So now holdNumber is private. If we try to
  access it like so..
• simpleExample se1
• se1.holdNumber5 //COMPILE TIME ERROR
• We now control access to the private int
  holdNumber though public access method,
  setMethod. setMethod then actually changes
  holdNumber.

20
Static members in Classes

• Remember the static keyword? This causes a
  variable (usually in a function) to remain in
  scope even after the function is finished (and
  all auto variables would get reclaimed)
• Well, what if we defined a static data member
  inside of a class?
• class simpleExample
• public
• void setNumber(int)
• void print()
• private
• static int holdNumber

21
Static members in Classes

• This will cause all instances of that class to
  share that variable. (Think of the cookie cutter
  example. Everything created by a specific cookie
  cutter shares the variable).
• So, lets look at an example in this example,
  we also define a public method called getNumber,
  that returns holdNumber.

22
Static members in Classes.

• class simpleExample
• public
• int getNumber()
• void setNumber(int)
• private
• static int holdNumber
• simpleExample se1
• simpleExample se2
• simpleExample se3
• se1.setNumber(4) //now holdNumber for ALL 3 is
  4
• se3.setNumber(se2.getNumber()1) //now
  holdNumber for ALL 3 is 5

23
Static members

• Im teaching these because your knowledge of C
  would be incomplete without it.
• That being said, you should try to never use
  static data members unless you have no other
  choice.
• They are confusing, sometimes difficult to debug,
  and can cause problems on large software systems
  (something called the static initialization
  order fiasco, among others)