Lecture 16 - Defining New Types - Defining New Types...

Defining New Types
Lecture 22
Hartmut Kaiser
[email protected]
http://www.cct.lsu.edu/˜ hkaiser/fall_2011/csc1254.html

Programming Principle of the Day
• Hide Implementation Details
▫ Hiding implementation details allows change to the
implementation of a code component while minimally
affecting any other modules that use that component.
▫ Encapsulation is key
 A language mechanism for restricting access to some of
the object's components
 A language construct that facilitates the bundling of data
with the methods (or other functions) operating on that
data
http://en.wikipedia.org/wiki/Information_Hiding
http://en.wikipedia.org/wiki/Encapsulation_(object-oriented_programming)
11/10/2011
Lecture 22
CSC 1254, Fall 2011,
Defining New Types
2

Defining New Types
• C++ has two kinds of types
▫ built-in types
 int, double, char, etc.
▫ class types (user defined types)
 string, vector, istream
 Rely entirely on built-in types and other class types
and generally available language facilities
• Design of C++ rests on the idea of creating user
defined types usable similar to built-in types
▫ Requires substantial language support
▫ Taste and judgment in class design
11/10/2011
Lecture 22
CSC 1254, Fall 2011,
Defining New Types
3

student_info Type Revisted
• We wrote the student_info type and related
functions
▫ However, these were not well suited for other
programmers to use
 Any newly created student_info object required first to
read data into it
 Otherwise the values would have been uninitialized
 No way to check whether held data was valid
 Well, without looking at member values
 Requires internal knowledge of student_info
 Assumption was that after grades were read they
wouldn’t change
 Interface to student_info was scattered (separate,
unrelated functions)
11/10/2011
Lecture 22
CSC 1254, Fall 2011,
Defining New Types
4

Class (User Defined) Types
• Class types allow to group several data items
into one entity
• That’s what we used:
struct student_info
{
std::string name;
double midterm, final;
std::vector< double > homework;
};
• Four data elements: data members
▫ A string, two double’s, and a vector<double>
11/10/2011
Lecture 22
CSC 1254, Fall 2011,
Defining New Types
5

Class (User Defined) Types
• Programmers may – and must – handle those
data items directly
▫ They may – because nothing is preventing them
▫ They must – because there is no other way
• It would be better to hide implementation
details of how things are stored
▫ Manipulate members through functions only
▫ This forms the interface of our class
11/10/2011
Lecture 22
CSC 1254, Fall 2011,
Defining New Types
6

Class (User Defined) Types
• Important general remarks
▫