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[ 08.html ]
Preparation
Study this page and Chpater 10 on Exceptions in C++
and write down the questions,
doubts, and surprises that you have on a piece of paper.
Include your printed name!
Assigned Work Due
Hand in your questions, doubts and surprises.
Question: would anyone like the option of Emailing there
question... to me before class?
Quick Exercise
What does this program do?
[ 09t.cpp ]
More on the type_ids of exceptions
[ exceptions.cpp ]
Input
The best laid plans Gang Aft Agly
When in trouble -- raise an exception
And use the standard exceptions in the C++ Library.
Standard Exceptions
<exception>
class exception{
public: exception();
virtual string what();
}
<stdexcept>
UML inheritance diagram: all have what(). All provide
a constructor with a string argument.
logic_error("That is not logical, Captain.");
Put fig 10.1 on your cheat sheet!
[ exceptions.dia ]
(dia)
[ exceptions.png ]
(graphic)
Try....and catch the exceptions later
Notice the syntax: 'try' intruduces a block of code '{'....'}'
in which particular exceptions are handled in a special way. The
ways that exceptions are handled are listed at the end of the block with
'catch' functions:
try{
- block where catches apply
}
catch ( type of exception )
{
- what to do with it
}
You can have as many 'catch's as you want. Always list the
most specific first.
Put the catch(...){...} last.
Things you can do in a catch:
- fix the problem some other way
- throw something -- let some other part of the program do it.
- catch and thro it again: thow;
- Always catch standard exceptions using an &!
Do-It-Yourself Exceptions
You can throw ANYTHING.
Better to throw a special kind of exception -- derive your
exception from one of the stdeceptions and define a
constructor. Page 10 shows how.
Warning the programmer what can be thrown by a function
Inspired by the Java "throws" clause.
Quizzes to come
Note the material in this class will be tested in a
a quiz in class 12.
Questions
Do I have to memorize and distinguish all the different C++ exception types
Not in CSci202. You might be wise to draw the hierarchy on your cheat sheet.
You may need the diagram in the lab as well. However, I don't, for
example expect you to know how a runtime error differs from a logi error and
so on.
What happens if an exception is raised and is not caught or trapped
The program aborts with a generic error message. You client probably
has a heart attack.
Please give an example
Normally we catch errors in a function that calls the function that
throws the exception.
try {
function();
} catch (...) { cout << "Catch\n"; }
void function()
{
...
.AS_is throw 42;
...
}
What does a throw actually do
It undoes any function calls and jumps to the first catch that
can handle the object that is thrown.
How to create my own expction class
class mine: public standard {
{ public: mine(....) : .... {}
...
};
Should we remove exceptions when the program is debugged
No. Wen well done, exceptions become part of the excepted
and desirable behavior of your program. Th user never knows
what compilcated paths of "try and error" were needed to produce what
they wanted.
How does abort differ from throw
Abort teminates the program, but you can catch the 'throw' and keep
the program running.
Why do program still crash with all these techniques available
People still use the old languages.... and some are not well
trained in modern techniques.
Why has my browser started crashing.
On a Microsoft machine,
You've proably got a virus. Get your computer checked out and
cleaned... and take more care in the future.
On a Linux machine -- the code has a bug. Get an update... or
join the people who develop open source.
Exeptions vs assert
Use assrt as a testing tool. Use exceptions to
handle bad data in a function.
Is there a best way to handle exceptions
Not really.... there are some good patterns but you have to take
time to think up the best thing to do with every one.
Should we use exceptions in this course
Yes. In the next Lab!
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Quiz
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