CIS 250 - Introduction to classes and structs

Objectives

• use classes and structs
• describe the differences between structs and classes

structs and classes

These topics are covered well in the text and structs should be familiar to you from previous C/C++ programming. If you are familiar with classes, but not structs, then all you have to know is that classes and structs are the same thing, except that the members of a class are private by default, while the members of a struct are public by default. Structs are generally used only when only data is contained. The topics listed below are just a brief outline of what will be discussed in class.

• assume that statements about structs below also apply to classes
• declaring a struct: struct structName { members };
• structs usually just have explicit data members, but may also have function members
• declaring a variable using a struct type: structName structVar;
• accessing members of a struct: structVar.memberName
• struct variables can be passed to functions either by value or by reference
• see demostruct1.cpp example
• you can create an array of structs
• structs can be nested
• see demostruct2.cpp example
• classes are just like structs except for the default access permission
• functions can also be added as members of a struct or class
• public access: all other code can see
• private access: only seen within the struct itself
• protected access: seen within the struct itself and derived classes (subclasses)
• see democlass1.cpp example
• good practice to separate specification (header file) from implementation
• convention is to only use struct when treating as C-style records and not including functions as members
• built-in operators: member access, assignment (shallow copy only)
• automatically provided functions
  • default constructor
  • destructor
  • copy assignment operator
  • copy constructor
• assignment works with objects that do not dynamically allocate memory, but most other operators do not unless code is specifically written to implement/overload them
• a const function has the keyword const at the end of its signature; that specifies that the function may not modify the values of its data members
• accessor methods allow public access to private data member values
• mutator methods allow public access to modify private data member values
• predicate methods return true or false
• constructors
  • a default constructor is automatically provided
  • a default constructor has no parameters
  • if you define any constructor, then the default constructor is not automatically provided
  • constructors are often overloaded
  • a copy constructor is automatically provided, but it only does a shallow copy which is not suitable for objects that dynamically allocate memory
  • constructors have the same name as the class
  • constructors have no return data type
  • constructors are automatically invoked when objects are created, but can not be explicitly called
  • constructors are usually used to initialize "instance" data members
  • invoking a default constructor: className varName;
  • invoking a non-default constructor: className varName(param1, param2);
  • creating an array of objects: className arrayName[sizeOfArray];
  • constructors may have default parameter values
• destructor
  • has same name as class, but with a leading tilde (~)
  • there is only one destructor function per class
  • a destructor has no return data type
  • a destructor has no parameters
  • usually used to deallocate resources such as memory that the object has allocated dynamically
  • destructors are automatically invoked when objects go out of scope
• all members of a class are either static or instance
• static members are specified using the keyword static
• static functions do not have access to instance data members and may be called directly using the class name even before any objects of the class have been created
• there are individual copies of each instance data member for each object created
• no instance data members are created before an object has been created from a class
• static data members are associated with the class rather than instances (objects) of the class; for that reason, there is only a single copy of each static data member which is shared by the class and all objects created from the class
• static data members exist before any objects have been created from a class