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<b>Data Structures and Algorithms
with Object-Oriented Design Patterns in C++</b><br>
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<H1><A NAME="SECTION0018600000000000000000">Exceptions</A></H1>
<A NAME="seccppexceptions"> </A>
<P>
Sometimes unexpected situations arise during the execution of a program.
Careful programmers write code that detects errors and
deals with them appropriately.
However, a simple algorithm can become unintelligible
when error-checking is added because the error-checking code
can obscure the normal operation of the algorithm.
<P>
<em>Exceptions</em><A NAME=58008> </A> provide a clean way to detect
and handle unexpected situations.
When a program detects an error,
it <em>throws</em><A NAME=58010> </A> an exception.
When an exception is thrown,
control is transfered to the appropriate
<em>exception handler</em><A NAME=58012> </A>.
By defining a function that <em>catches</em> the exception,
the programmer can write the code to handle the error.
<P>
In C++, an exception is an object.
Typically exceptions are derived (directly or indirectly)
from the base class called <tt>exception</tt>
which is defined in the C++ standard library
header file called <tt><exception></tt>.
<P>
A function throws an exception by using the <tt>throw</tt> statement:
<PRE>class domain_error : public exception {};
void f ()
{
// ...
throw domain_error ();
}</PRE>
The <tt>throw</tt> statement is similar to a <tt>return</tt> statement.
A <tt>return</tt> statement represents the normal termination
of a function and the object returned matches the return value of the function.
A <tt>throw</tt> statement represents the abnormal termination of a function
and the object thrown represents the type of error encountered.
<P>
Exception handlers are defined using a <tt>try</tt> block:
<PRE>void g ()
{
try {
f ();
}
catch (domain_error) {
// exception handler
}
catch (range_error) {
// exception handler
}
}</PRE>
The body of the <tt>try</tt> block is executed
either until an exception is thrown or until it terminates normally.
<P>
One or more exception handlers follow a <tt>try</tt> block.
Each exception handler consists of a <tt>catch</tt> clause
which specifies the exceptions to be caught,
and a block of code,
which is executed when the exception occurs.
When the body of the <tt>try</tt> block throws an exception
for which an exception is defined,
control is transfered to the body of the exception handler.
<P>
In this example, the exception was thrown by the function <tt>f()</tt>
and caught by the function <tt>g()</tt>.
In general when an exception is thrown,
the chain of functions called is searched in reverse
(from caller to callee)
to find the closest matching <tt>catch</tt> statement.
As the stack of called functions unwinds,
the destructors for the local variables declared in those functions
are automatically executed.
When a program throws an exception that is not caught,
the program terminates.
<P>
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