CPSC 427: Object-Oriented Programming

Michael J. Fischer

Lecture 6
February 4, 2016

Improvements to already-submitted assignments

• main() does not require a return statement in c++11.
• Error messages from Fatal() are more useful if they are all different.
• Good to check age for reasonableness.
• To check validity of cin >> fname;, test cin.good() and/or cin.fail(), not stream.good().
• No need for stringstream in this assignment.
• The variable in which the birth year is stored should be a numeric type, not a string. Let >> do the conversion, don’t use stoi(). Why?

Improvements (cont.)

• To write integer age to output, just use cout << age; no need to use to_string().
• Write output directly to cout; don’t write to a string first. (This isn’t Java.)
• Style: Put main() definition before run() in main.cpp.
• Use conditional expression to make minor output changes: cout << ( age>1 ? "years" : "year") << " old";
• Don’t put an unnecessary else clause following a throw.
• Properly intent your code.
• Good style to make a function current_year() to compute the year. Why?

Improvements (cont.)

• No need to set struct tm pointer to NULL.
• In c++11, use nullptr instead of NULL.
• Remember to test your code with bad input as well as good!

Code demo The 06-BracketsCpp demo contains three interesting class declarations and illustrates the use of constructors, destructors, and dynamic memory management.

In the remainder of the lecture was spent going through the code line by line to see what it does and how the parts all fit together.

We briefly summarize below some of the points made during lecture.

Token class

Major points:

1.

enum is used to encode the bracket type (round, square, etc.) and the sense of the bracket (left, right).

2.

ch is the character representing the bracket, used for printing.

3.

classify() is a private function.

4.

The definitions of print() and operator<< follow our usual paradigms.

5.

The Token constructor initializes all data members.

Stack class

Major points:

1.

T is the element type of the stack. This code implements a stack of Token. (See typedef declaration.)

2.

Storage for stack is dynamically allocated in the constructor using new[] and deleted in the destructor using delete[].

3.

The empty square brackets are needed for both new and delete since the stack is an array.

4.

delete[] calls the destructor of each Token on the stack. Okay here because the token destructor is null.

5.

push() grows stack by creating new stack of twice the size, copying contents of the old stack into the new, and deleting the old stack.

Brackets class

Major points:

1.

Data member stk is dynamically allocated in the constructor and deleted in the destructor, but it is an object, not an array, and does not use the []-forms of new and delete.

2.

in.get(ch) reads the next character without skipping whitespace. There are other ways to do this as well.

3.

If read is !in.good(), we break from the loop and do further tests to find the cause.

4.

mismatch() uses the eofile argument to distinguish two different cases. Is this good design?

Main file

Major points:

1.

main() follows our usual pattern, except that it passes argc and argv on to the function run(), which handles the command line arguments.

2.

run() opens the input file and passes the stream in to analyze().

3.

The stream was not opened in a constructor, so there is no corresponding destructor for the matching close(). If an error is thrown, the stream will not be closed (except for the automatic cleanup that happens when a program exits).

4.

Question: Which is better, to pass the file name or an open stream? Why?