Lecture 17
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Friday, February 18
CS 215 Fundamentals of Programming II - Lecture 17
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Outline
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Note: most of the material in this lecture is not
in the textbook
Error handling
Exceptions and exception handling (some of
this material is in Appendix L of the textbook)
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throw operator
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try-catch block
Managing complex projects
Friday, February 18
CS 215 Fundamentals of Programming II - Lecture 17
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Error Handling
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All functions have a pre-condition that says
under what assumptions are made about the
arguments passed the function in order for the
function to compute a valid result. What should
a function do when the arguments to not meet
the precondition?
Could return an invalid result. I.e., user is
solely responsible for using the function
correctly.
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CS 215 Fundamentals of Programming II - Lecture 17
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Error Handling
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Could cause the program to exit. E.g., the
Rational constructor from Project 2 uses assert.
Good for errors that cannot be remedied.
Could set an error flag or return an error code.
E.g., Rational operator<< sets the output
stream to a failed state that can be tested. Or
Document SetCurrent returns false if used
incorrectly. Works but the user can ignore the
error.
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CS 215 Fundamentals of Programming II - Lecture 17
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Exceptions
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Want a system that forces a user to know there
was an error and also allow the user to remedy
the problem. See file: except-example.cpp
An exception is an object that contains
information and is transmitted back to a
function caller without using a return.
The C++ <stdexcept> library defines a number
of classes used for system exceptions that
programmers also can use. For this example,
we will use the out_of_range class.
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CS 215 Fundamentals of Programming II - Lecture 17
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Throwing an Exception
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An exception is thrown when an error occurs.
In C++, exceptions are thrown using the throw
operator. Usually the exception is a newly
constructed object.
In our example, if the function PickNumber is
not called with a positive integer, it throws an
out_of_range exception that is constructed
with a string error message.
// check if called with valid argument
// if not, construct and throw exception
if (upper <= 0) throw out_of_range("Bound is not > 0.");
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CS 215 Fundamentals of Programming II - Lecture 17
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Throwing an Exception
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When an exception is thrown, the function
terminates immediately and control transfers
back to the caller like a return.
Unlike a return, instead of going back to the
point of the call, the system searches for an
exception handler (code to handle the
exception).
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CS 215 Fundamentals of Programming II - Lecture 17
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Handling an Exception
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An exception is handled by catching it and
providing code to handle the error.
In C++, this is done by putting the call to a
function that may throw an exception in a try
block with a catch block that receives the
thrown exception.
try {
// call a function that throws
}
catch (const <exception type> & e){
// code to handle the exception
}
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CS 215 Fundamentals of Programming II - Lecture 17
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Handling an Exception
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If the function does not throw an exception, the
catch block is skipped.
If the function throws an exception and a catch
block parameter matches the thrown exception
type, the handler code is executed, then
execution continues on after the catch block.
There may be more than one catch block
attached to a try block. This allows a try block
to contain multiple function calls that may throw
different types of exceptions.
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CS 215 Fundamentals of Programming II - Lecture 17
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Handling an Exception
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If there is no catch block with a parameter that
matches the thrown exception, the calling
function is terminated and the exception is
rethrown to its caller. This is repeated until a
handler is found.
If the main program is reached and no handler
is found for the exception, the program
terminates with an unhandled exception error.
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CS 215 Fundamentals of Programming II - Lecture 17
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Handling an Exception
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The example program shows one typical use
for exceptions. The try-catch block is placed
inside a while-loop that asks the user for input
until the input is valid.
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CS 215 Fundamentals of Programming II - Lecture 17
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Managing Complex Projects
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Handout on strategies for managing complex
projects and how to use gdb symbolic
debugger.
0. First and foremost, spend serious time doing
analysis and design. Analyses are given in this
class, but think about what you want each
operation to do before writing any code.
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CS 215 Fundamentals of Programming II - Lecture 17
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Managing Complex Projects
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1. Implement a piece at a time. Start with an
empty class definition with just the attributes.
Write the first operation prototype in the class
definition, then the implementation for that
operation, then a test for that operation in a
driver program.
Start with a basic constructor and an output
operation. Don't worry about complex output
formatting until later. Then do the accessors,
then the mutators. Don't worry about special
cases or error checking at first.
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CS 215 Fundamentals of Programming II - Lecture 17
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Managing Complex Projects
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2. Debugging output can be controlled using
debug functions. Examine files: rational.h,
rational.cpp, rationaltest.cpp. (Skeleton files
from Project 2.)
void PrintDebug (bool debug, const string & message){
if (debug)
cerr << message << endl;
}
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Use boolean constants to control output
const bool DEBUG1 = true;
PrintDebug (DEBUG1, "Entering constructor");
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Managing Complex Projects
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3. Use simple test cases first, then think about
boundary cases.
4. Sometimes it is better to start over. Usually
you have a much better idea of where you want
to go, so you get there faster.
5. Last, but not least, use a symbolic debugger
to trace code. For Linux/UNIX, use gdb. There
are several GUI front-ends, but the commandline version works well and can be used in any
terminal. Compile files with -g option.
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