Module 13

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Module 13

• Class Interactions

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Today

• Were interested in how classes interact with
  each other and with the rest of the C universe
• Assignment operations and class members?
• Allowing other classes access to private data
  members?
• Defining your own operations?

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Note on Declaration/Instantiation

• When you declare an object belonging to a class,
  memory is set aside even if you havent
  instantiated the data members
• Thus some constructor is called in these
  instances (usually default constructor)

ComplexNumber n
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So suppose you want to change values after this

• You may want to assign an objects data members
  the same values from another object
• Your natural intuition says you might write
• This is called memberwise copying
• Your intuition is right! But where does it come
  from?

myRectangle yourRectangle
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Gang of Three

• When you build a class, you get these three
  different things for free (without you defining
  them)
• An assignment operator
• A copy constructor (allows instantiation from an
  existing object of the class)
• A destructor method
• Can you generate an example of using each?

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Problem!

• These three are grouped together because of what
  can happen when objects reference dynamic memory
  (example on board)
• The default copy constructor makes a shallow copy
  of its data.
• Usually this is sufficient, but you should be
  aware this is happening

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Static Data Members

• Remember our old friend static?
• If we declare a data member of a class static,
  then all objects of that class are referring to
  the same memory location
• Might be good for keeping track of how many
  Rectangles we create, for example

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Friend Functions

• Sometimes we want another class to have access to
  our (otherwise) private data
• Note we dont do this often!
• We can declare some functions to be friend
  functionsfunctions on a list of friends
  maintained inside the class
• Such a function has access to private data
  membersbut note that it lacks the reference of a
  particular data member!

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Redefining Operators

• I can, if I wish, decide to create a new operator
  for my classsay something like the operator
• An obvious one to define is the operation of the
  ltlt and gtgt operators
• Note that we had some lab problems with strings
  related to this

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Okay, lets build some

• Lets try to build some of these for a class to
  represent Rational numbers