c++_throw.html
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<TITLE>Data Structures and Algorithms: C++ exceptions</TITLE>
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PLDS210 University of Western Australia">
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exceptions, error handling, C++">
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<FONT FACE=helvetica SIZE=+1><I>Data Structures and Algorithms</I></FONT>
</TD></TR>
<TR><TD><FONT FACE=helvetica SIZE=+2><B>C++ Exceptions</B></FONT>
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<P>
C++ defines a mechanism that is somewhat similar to
Ada's exception mechanism.
<P>
C++ allows you to execute a <TT><FONT COLOR=green>throw</FONT></TT>
statement when an error is detected.
When a <TT><FONT COLOR=green>throw</FONT></TT> is invoked,
control jumps immediately to a <TT><FONT COLOR=green>catch</FONT></TT>
routine.
<P>
First you define a <TT><FONT COLOR=green>try</FONT></TT> block which
encloses the "normal" (no exception) code.
The <TT><FONT COLOR=green>try</FONT></TT> block is followed by a number of
<TT><FONT COLOR=green>catch</FONT></TT> blocks:
if code in the <TT><FONT COLOR=green>try</FONT></TT> block throws an exception,
it is caught by one of the <TT><FONT COLOR=green>catch</FONT></TT> blocks.
<FONT COLOR=green>
<PRE>try {
classX x;
x.mangle( 2 );
x.straighten( -2 );
}
catch( const char *string ) { ..... }
catch( RangeErr &re ) { .... }
catch( ... ) {
// catches any other error
....
}
</PRE></FONT>
<TT><FONT COLOR=green>classX</FONT></TT>'s constructor, <TT><FONT COLOR=green>mangle</FONT></TT> and <TT><FONT COLOR=green>straighten</FONT></TT>
methods contain <TT><FONT COLOR=green>throw</FONT></TT> statements when
problems are encountered:
<FONT COLOR=green>
<PRE>classX::mangle( int degree ) {
if ( degree > MAX_MANGLE )
throw "Can't mangle this much!";
.... // Normal code for mangle
}
</PRE></FONT>
The <TT><FONT COLOR=green>throw</FONT></TT> causes control to jump straight out of
the <TT><FONT COLOR=green>mangle</FONT></TT> method to the <TT><FONT COLOR=green>catch</FONT></TT> block,
skipping all subsequent statements in the <TT><FONT COLOR=green>try</FONT></TT>
block.
<P>
However, like much of C++, the rules
which are used to associate a <TT><FONT COLOR=green>throw</FONT></TT>
with the correct <TT><FONT COLOR=green>catch</FONT></TT> are too complex
to contemplate.
Stroustroup attempted to make the <TT><FONT COLOR=green>throw</FONT></TT>
like a general method invocation with parameters,
overloading, <I>etc, etc</I>.
<P>
Historical note:
many early C++ compilers did not implement
the <TT><FONT COLOR=green>throw/catch</FONT></TT> pair,
(possibly because of the complexity alluded to above!)
and some textbooks avoid it -
or relegate it to the last few pages!
<P>
Basically, like a lot of C++ features,
the <TT><FONT COLOR=green>throw/catch</FONT></TT>
mechanism is best used in a very simple way,
<I>eg</I> by providing a single <TT><FONT COLOR=green>catch</FONT></TT>
block with a single <TT><FONT COLOR=green>int</FONT></TT> or
<TT><FONT COLOR=green>enum</FONT></TT>
parameter!
Ada's simple and clean mechanism may lack some
power (an exception handler can't be passed a
parameter), but it's a lot easier to
understand and use!
<P>
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Continue on to <A HREF="javascript:if(confirm('http://www.ee.uwa.edu.au/~plsd210/ds/java_throw.html \n\nThis file was not retrieved by Teleport Pro, because it is linked too far away from its Starting Address. If you increase the in-domain depth setting for the Starting Address, this file will be queued for retrieval. \n\nDo you want to open it from the server?'))window.location='http://www.ee.uwa.edu.au/~plsd210/ds/java_throw.html'" tppabs="http://www.ee.uwa.edu.au/~plsd210/ds/java_throw.html">Java Exceptions</A><BR>
Back to the <A HREF="ds_ToC.html" tppabs="http://www.ee.uwa.edu.au/~plsd210/ds/ds_ToC.html">Table of Contents</A>
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© <A HREF=mailto:morris@ee.uwa.edu.au>John Morris</A>, 1998
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