1. Exceptions
for safe programming

2. Why Exceptions
exceptions are used to signal that something unusual has happened during program execution
when signal is sent, the control is transferred to specific place in a program designated to handle this exception
throwing the exception – the event of sending a signal
catching the exception – handling it
exception mechanism is for handling extraordinary situations: division by zero, array out of range
do not use it for ordinary transfer of control – bad style, cryptic program

3. Syntax
two constructs
try block – defines the program area where an exception may be thrown
try {
// some code
throw ARGUMENT; // throwing exception
// more code }
catch block (exception handler) – processes the exception
catch (TYPE PARAMETER){
// exception handling code
can be multiple catch blocks, distinguished by exception type
default exception catches all un-caught exceptions
catch (...){

4. Operation
when the exception is thrown, control is transferred to the first catch block outside the try-block that matches the type of the thrown exception
after catch-block finishes, control is passed to the first statement past the last catch-block

5. Objects as Exceptions classes can be used as exception types
can give exception a descriptive name
can be used to pass details about exception
example
empty class to give descriptive name
class divide_by_zero {};
more details to be passed
class wrongNumber{
...
private:
int offendingNumber; }

6. Catching Outside Functions
may be useful to handle the exception outside the function where it is thrown
function is said to throw the exception
by default function may throw any exception
function prototype/head may list exceptions thrown by function, if throws anything else – special unexpected () function is invoked (by default terminates program)
syntax
returnVal functionName(parameterList) throw (exceptionType);
example
double safe_divide(int n, int d) throw (divide_by_zero){
if (d == 0)
throw divide_by_zero();
return n/double(d); }

7. Catching Standard Exceptions
some pre-defined functions throw standard exceptions
need stdexcept header
at() – throws out_of_range
new – throws bad_alloc – unsuccessful allocation, possibly out of memory
example handling
try{
string s=”hello”;
s.at(5)=’!’; // throws out_of_range }
catch(out_of_range){
cout << ”string too short!”;

8. Questions on Exceptions
what are exceptions why are they needed?
what are try-block? catch-block? What does it mean to throw or catch an exception?
what is the type of an exception?
what is the default exception? how is it caught?
can an object be used to throw an exception? why is that useful?
how can an exception be thrown inside a function and caught outside it?
what are standard exceptions? What standard exceptions have we studied?