Values, Variables, Types, Classes and Objects

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Values, Variables,Types, Classesand Objects
(c) Allan C. Milne School of Computing Creative
Technologies University of Abertay dundee
Last updated 2nd October 2005
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Agenda

• Values
• Variables
• Types
• Classes Objects

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Introduction

• There is often confusion between values,
  variables and types.
• Similarly for classes, objects and attributes (or
  fields).
• We will try to explain the concepts and clarify
  the terminology.
• This presentation is purely an outline, examples
  and explanations will be given in the lecture.

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Values

• Values are the bit sequences in memory that
  represent some quantity.
• Values might be
• primitive quantities (eg integers, reals ).
• structured quantities (eg objects, structs).
• Expressions return values.
• It is values that are passed as parameters and
  returned by methods.

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Variables

• Variables are the names of memory locations that
  contain values.
• Fundamentally, the variable name identifies the
  location of the value.
• However, the variable name is often used with an
  implicit dereference to access the value stored
  in the location.
• Variables might be local, fields, parameters.

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Types

• Types define the kind of values that are
• contained in a variable,
• passed as a parameter,
• returned by a method.
• User-defined types can be created.
• Types describe kinds of values, they do not have
  values themselves.
• Types form the basis of checking that actions are
  compatible or valid.

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Classes and Objects

• A class is a type
• it defines the kind of things that must be in
  values of the class.
• Classes define members that can be fields,
  methods, properties or events
• each member has its own type.
• An object (or instance) of a class is a value of
  the class type
• each member is a value of its defined type.