Classes, Member Functions Constructors and Destructors

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Classes, Member Functions Constructors and
Destructors

• A bullet list of the things to know

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Class

• class struct
• Except members in a class are private by
  default while members in a struct are public by
  default, but who cares
• a class is a type
• The design philosophy of C is that you can use
  any type just like you use int

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Member Functions

• A function declared inside the class definition
  is a member function
• Exception static member functions are not
  really member functions, more on that later
• Member functions have an implicit parameter
  (typically its the first parameter)
• The implicit parameter is handled automatically
  by the compiler (we dont declare it)
• The name of the parameter is this
• The type of the parameter is T where T is the
  name of the class

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Member Functions Cont.

• We pass the implicit argument to a member
  function using the x.doit() syntax
• x must be a variable of type T (i.e., an object
  of the appropraite class)
• the value of the implicit argument is x
• The implicit parameter is used for class
  scoping
• When the compiler is searching for a declaration
  for some variable that youre using it searches
  in this order
• Is there a local variable with that name? If so,
  use the local variable
• If not, is there a parameter with that name? If
  so, use the parameter
• If not, is there a data member with that name?
  If so, prepend (this). to the variable name
  and use the data member pointed to by this
• If not, is there a static data member with that
  name? If so, prepend T to the variable name
  and use the static data member
• If not, is there a global variable with that
  name? If so, use the global variable.
• What this means in practice is that you usually
  wont have to use this in your programs, it
  will be completely implicit. PLEASE DO NOT
  FORGET HOW IT REALLY WORKS!

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Constructors

• Constructors are functions provided to initialize
  the data members of objects.
• A class may have many constructors
• provided each constructors has different
  parameters
• If the class has one or more constructors
  defined, then anyone who creates a variable of
  that class will have to call the constructor
• i.e., the compiler enforces/ensures that class
  objects are always initialized properly. This is
  a really good thing.

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Constructors Cont

• The name of the constructor function must be the
  same as the name of the class
• The constructor function must be a member
  function
• And you can use this if you want
• We call the constructor whenever we create a
  variable of that type
• We put the arguments to the constructor inside
  parens after the variable name
• e.g., T x(1, 2) // declares variable x, 1 and 2
  are arguments to the constructor for T that has
  two int parameters
• If it matters (and it usually does not for well
  designed programs) then you can rely on the fact
  that variables are constructed in the order
  they are declared.
• Variables defined in different files are
  constructed in an arbitrary order!

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Destructors

• Used to provide clean up for classes
• This almost always means deallocate the chunks
  of memory pointed to by data members within this
  class.
• A class may have only one destructor
• The name of the destructor is TT
• i.e., the same as the class name but with in
  front
• and the destructor can have NO parameters
• The compiler will automatically call the
  destructor when the activation record containing
  the variable is destroyed (i.e, when the function
  returns)
• Global variables are destroyed when main ends
• If it matters (and it shouldnt for well-written
  programs) you can rely on the fact that variables
  are destructed in the opposite order that they
  are constructed (i.e., LIFO)