CPSC 427: Object-Oriented Programming

Michael J. Fischer

Lecture 9
September 28, 2016

Analysis of 08-BarGraph demo

• main.cpp
• graph.hpp
• graph.cpp
• row.hpp
• row.cpp
• rowNest.hpp
• item.hpp

main.cpp

Points to note:

• run() calls a static class method Graph::instructions() to print out usage information. It is called without an implicit parameter.
  By being static, the instructions can be printed before any Graph object is created.
• The file uses cin.getline() to safely read the file name into a char array fname.
  The simpler cin >> fname is unsafe. It should never be used.
  It would be okay if fname were a string.
• After the file has been opened, the work is done in two lines:
      Graph curve( infile ); // Declare and construct a Graph object.  
      cout << curve;         // Print the graph.

Design issues for main.cpp

1.

Should instructions be a static class method or a static constant?

2.

Should fname be a char[] or a string? If the latter, how does one prevent buffer overrun?

3.

Where should the file opening code go – in run() (where it is now), in Graph, or in a new controller class?

graph.hpp

Points to note:

• Class Graph aggregates 11 bars Row.
• The Row array is created by the constructor and deleted by the destructor.
• insert() is a private function. It creates an Item and inserts it into one of the Rows.
• instructions() is a static inline function. This shows how it is defined.
• instructions() could also be made out-of-line in the usual way, but the word static must not be given in the definition in the .cpp file; only in the declaration in the .hpp file.

graph.cpp

Points to note:

• The for-loop in the constructor does not properly handle error conditions and can get into an infinite loop.
  You should test yourself to be sure you know how to fix this problem.
• The constructor has an allocation loop. The destructor has a corresponding deallocation loop.
• bar[index]->insert( initials, score );

  shows the use of a subscript and a pointer dereferencing in the same statement.

• Why do we need the * in
  out << *bar[k] <<"\n";

Class discussion on bargraph design issues

Most of class time was spent discussing the design issues raised above in main.cpp, graph.hpp, and graph.cpp.

• Why is it useful for Graph to know the file name?
• If both infile and fname are passed as parameters to Graph(), the precondition that stream infile is opened on file fname cannot be checked. Why is this undesirable?
• What are the consequences of moving the file-opening code from run() to:
  • main.cpp, just after the call to banner()?
  • To the Graph constructor?
  • To a new controller class?
• Why is there a potential infinite loop in the Graph constructor? What should be done to fix it?