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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
<!-- $Id$ -->
<html> <head>
<title>Using ACE Exception Handling Macros to Enhance CORBA Portability</title>
</head>
<body text="#000000" link="#0000ff" vlink="#cc0000" bgcolor="#ffffff">
<Hr><P>
<h3>Using ACE Exception Handling Macros to Enhance CORBA Portability</h3>
<P>CORBA <CODE>Environment</CODE> arguments provide a way to handle
exceptions when native c++ exception handling is unavailable or
undesirable. However, writing portable code using both native C++
exception handling and <CODE>CORBA::Environment</CODE> objects is very
hard. If you plan to write portable code that must run on platforms
that do not have native C++ exceptions, therefore, we recommend you
use the ACE exception macros. This document explains how these macros
can help alleviate much of the accidental complexity. However, keep
in mind macros cannot solve all problems perfectly. </P>
<P>Before reading the rest of this document, we recommend you check
out pages 307 through to 322 in the book, <A
HREF="http://cseng.aw.com/bookdetail.qry?ISBN=0-201-37927-9&ptype=0">Advanced
Corba Programming with C++</A> by <A
HREF="http://www.triodia.com/staff/michi-henning.html">Michi
Henning</A> & <A HREF="http://www.iona.com/hyplan/vinoski/">Steve
Vinoski</A>. Likewise, we recommend that you read the Error Handling chapter from the
<A HREF="http://www.theaceorb.com/product/">TAO Developer's Guide</A>.
<P><HR><P>
<h3>Table of Contents</h3>
<ul>
<li><a href="#nutshell">ACE Try Macros in a Nutshell</a>
<li><a href="#examples">Examples</a>
<li><a href="#general">General Guidelines for Exception Handling</a>
<li><a href="#transition">Transition from ACE_TRY_ENV usage to the
ACE_ENV_ARG macros</a>
<li><a href="#caveats">Some Caveats</a>
</ul>
<HR><P>
<h3><a name="nutshell">ACE Exception Macros in a Nutshell</h3>
<P>This section explains some simple rules of writing programs for
platforms with and without native exception support using ACE's
exception macros.
</P>
<P>ACE exception macros are modelled like C++ language exceptions and
can be used like them, but with a small difference. These macros
rely on the CORBA::Environment variable to handle exceptions
on platforms that do not support exception
handling. (Please note that native exceptions can be turned on
or off at COMPILE time as an option to your make)
The exception macros have been modelled with some extra rules to
ensure this works even on
platforms without native exception support. See some <a
href="#examples">quick examples</a> on how to use ACE exception
macros.
</P>
<ol>
<li><P><em>Declaration of CORBA::Environment Parameter at Methods</em><br>
On platforms lacking native exceptions, all CORBA methods take an
extra parameter added at the end of their argument list. This
last parameter holds the CORBA::Environment variable.
However, on systems with native exceptions, no such extra method
parameter is added.<br>
In order for both configurations to work with the same source code,
following macros are defined. In native exception configurations,
they all resolve to <em>empty</em>.</p>
<ul>
<li><p><em>ACE_ENV_ARG_DECL</em></p>
in the pseudo exception configuration resolves to
<pre>
, CORBA::Environment ACE_TRY_ENV
</pre>
It is used for declaring the extra Environment parameter
at CORBA methods that take one or more regular parameters.
The fact that the comma is part of the macro substitution
caters for the hiding of this added argument when native
exceptions are used. However, by the same virtue its
usage is a bit peculiar -<br>
Example: a CORBA IDL defined method
<pre>
void mymethod (in boolean b);
</pre>
may be declared as follows in C++:
<pre>
void mymethod (CORBA::Boolean b ACE_ENV_ARG_DECL);
</pre>
Notice the missing comma before the ACE_ENV_ARG_DECL.
This notation is necessary because when using native exceptions,
also the presence of the comma before the (non-existent)
CORBA::Environment parameter must be hidden.</p>
</li>
<li><p><em>ACE_ENV_SINGLE_ARG_DECL</em></p>
is similar to <code>ACE_ENV_ARG_DECL</code>, but is used when
the method takes no other parameters. It is necessary as a
separate macro because it does not contain the leading comma
that <code>ACE_ENV_ARG_DECL</code> does.
Example: a CORBA IDL defined method
<pre>
void mymethod ();
</pre>
may look like this in C++:
<pre>
void mymethod (ACE_ENV_SINGLE_ARG_DECL);
</pre>
<li><p><em>ACE_ENV_ARG_DECL_WITH_DEFAULTS</em>,
<li><em>ACE_ENV_SINGLE_ARG_DECL_WITH_DEFAULTS</em></p>
are similar to the above two macros but add a default value
to the parameter declaration. In case of TAO, the default value
is <code>TAO_default_environment()</code>.<p>
Notice that these macros are only used at the declaration
of methods (usually in header files), not at their definition
(usually in implementation files.) At the definition,
instead use the corresponding macro without "_WITH_DEFAULTS".
Example: the CORBA IDL defined method
<pre>
void mymethod ();
</pre>
in the C++ header file looks like this:
<pre>
void mymethod (ACE_ENV_SINGLE_ARG_DECL_WITH_DEFAULTS);
</pre>
and in the C++ implementation file may look something like:
<pre>
void mymethod (ACE_ENV_SINGLE_ARG_DECL)
{
// ...
}
</pre>
</ul>
<br>
</li>
<li><P><em>Passing the CORBA::Environment Parameter into Method Calls</em><br>
Now that we've seen how to declare methods with Environment
parameters, let's see how to invoke such methods.</p>
<ul>
<li><p><em>ACE_ENV_ARG_PARAMETER</em></p>
in the pseudo exception configuration resolves to
<pre>
, ACE_TRY_ENV
</pre>
and is written as the last parameter of a method call that takes
one or more regular parameters. Again we need to omit the
comma that would normally precede this last parameter, as the
comma is already part of the macro definition. For example,
the CORBA IDL method
<pre>
void mymethod (in boolean b);
</pre>
would be invoked as follows:
<pre>
some_var.mymethod (bparam ACE_ENV_ARG_PARAMETER);
</pre>
</ul>
<ul>
<li><p><em>ACE_ENV_SINGLE_ARG_PARAMETER</em></p>
is similar to <code>ACE_ENV_ARG_PARAMETER</code> but is used
for calling methods that don't take any regular parameters.
Our example of a CORBA IDL method
<pre>
void mymethod ();
</pre>
we would invoke as follows:
<pre>
some_var.mymethod (ACE_ENV_SINGLE_ARG_PARAMETER);
</pre>
</ul>
<br>
</li>
<li><P><em>Definition of the CORBA::Environment variable </em><br>
We have seen how to declare methods with the CORBA::Environment
parameter, and how to invoke such methods. However, where does
the variable to pass into methods as the CORBA::Environment
parameter come from in the first place?</p>
<P>An environment variable can be defined in the needed scope
(for example, in the main program, or in a more local scope)
by the statement</p>
<pre>
ACE_DECLARE_NEW_CORBA_ENV;
</pre>
<P>You can then invoke the methods on the servant from the client
side as
<pre>
object_reference->func_name (x, y ACE_ENV_ARG_PARAMETER);
</pre>
Even if you are interested in making calls within the client
side, you can define your method like this
<pre>
int AN_OBJ::foobar (int a, int b ACE_ENV_ARG_DECL);
</pre>
<li><P><em>Throwing exceptions:</em><br>
Use <code>ACE_THROW</code> and <code>ACE_THROW_RETURN</code> to
throw exceptions. They should never be used within a try
block; please use <code>ACE_TRY_THROW</code> instead.
</P>
</LI>
<li><P><em>Propagating exceptions:</em><br>
To simulate native exceptions on platforms without native
exception handling, <em>every</em> function call that may
throw exceptions must be followed by <code>ACE_CHECK</code> or
<code>ACE_CHECK_RETURN</code>.</p>
<p><a name="exc-func">Exception-throwing functions include the
following categories:</p>
<ol>
<li><p>Any function that takes a
<code>CORBA_Environment</code> argument.</p>
</li>
<li><p><code>ACE_NEW_THROW_EX</code>. Notice that you
<em>should not</em> use <code>ACE_NEW_THROW</code>,
<code>ACE_NEW_THROW_RETURN</code>,
<code>ACE_NEW_TRY_THROW</code> anymore because they don't
work right with ACE try macros. Instead, use
<code>ACE_NEW_THROW</code> with appropriate ACE_CHECK*
macros.</p>
</li>
<li><P><code>ACE_GUARD_THROW_EX</code>,
<code>ACE_READ_GURAD_THROW_EX</code>, and
<code>ACE_WRITE_THROW_EX</code>.
<li><p><code>ACE_TRY</code> blocks. Follow every
<code>ACE_ENDTRY</code> with appropriate ACE_CHECK*
macros.</p>
<li>
</ol>
<P>You should pass <code>ACE_TRY_ENV</code> to these
functions.
</p>
<P>Be very careful not to combine exception throwing functions
in one statement like this:
</P>
<pre>
x = obj1->callme (ACE_ENV_SINGLE_ARG_PARAMETER)
+ obj2->dare_me (ACE_ENV_SINGLE_ARG_PARAMETER);
ACE_CHECK;
</pre>
<P>This example may work differently when native exception
handling is enabled/disabled.
</p>
</LI>
<li><P><em>Catching exceptions:</em><br>
Use <code>ACE_TRY</code> to catch exceptions if there's an
<code>ACE_TRY_ENV</code> available. Otherwise, you should use
<code>ACE_DECLARE_NEW_CORBA_ENV</code> to create one at
<em>proper</em> scope. The use of
<code>ACE_TRY_NEW_ENV</code> is considered depricated because it
can't deal with the case when you have multiple <code>TRY</code>
blocks in the scope of <code>ACE_TRY_NEW_ENV</code>. If there are
more than one try blocks in a function, use <code>ACE_TRY_EX</code>
for all subsequence try blocks to avoid name clashing of labels.
</p>
<ul>
<li><P>Within a <code>ACE_TRY</code> block, use the variable
<code>ACE_TRY_ENV</code> to pass down the
<code>CORBA_Environment</code> (see <a
href="#try_env">this</a> example.)
</p>
</LI>
<li><P>Follow <em>every</em> exception throwing function with
<code>ACE_TRY_CHECK</code>. If you are using a TRY block
within another try block add a <code>ACE_TRY_CHECK</code>
at the end of this TRY block ie. after
<code>ACE_ENDTRY</code>.
</p>
</LI>
<li><P>Use <code>ACE_CATCH</code> to catch exceptions of certain
type.
</p>
</LI>
<li><P><code>ACE_CATCHANY</code> catches <em>any</em> exceptions
of type <code>CORBA_Exception</code>. The caught
exception is stored in a variable call
<code>ACE_ANY_EXCEPTION</code>.
</p>
</LI>
<li><p><code>ACE_CATCHALL</code> emulate the <code>catch
(...)</code> c++ statement. It is identical to
<code>ACE_CATCHANY</code> on platforms without native
exception support. You can not access the caught
exception within the <code>ACE_CATCHALL</code> block.</p>
<li><P>Use <code>ACE_RE_THROW</code> to rethrow the same exception
within a <code>ACE_CATCH</code> or
<code>ACE_CATCHANY</code> block.
</p>
</LI>
<li><P>A <code>ACE_TRY</code> block must be terminated with
a <code>ACE_ENDTRY</code> statement.
</p>
</LI>
<li><P>Throw an exception within a <code>ACE_TRY</code>
block or <code>ACE_CATCH</code> block using
<a href="#ace_try"><code>ACE_TRY_THROW</code></a>.
</p>
</LI>
</ul>
</li>
<li><p><em>Printing out exceptions.</em> Use <code>ACE_PRINT_EXCEPTION
(EX,INFO)</code> to print out an exception. The macro takes two
arguments, a reference to an exception (EX) and a <code>char
*</code> string (INFO) which provides more information on the
exception. Since there's no portable way to print out
exceptions, you can redefine ACE_PRINT_EXCEPTION to fit your
need (or define it to null.) <em>You should always print out
the exception itself, not the CORBA_Environment that carries the
exception.</em></p>
</li>
<li><P><em>Name of CORBA::Environment variable</em><br>
A function that may throw a CORBA::Exception needs a
CORBA::Environment variable to pass up exceptions (to throw in
the C++ sense) and to gather (catch () in the C++ sense)
exceptions from functions it called. By default, ACE exception
macros assume that the variable is named <code>ACE_TRY_ENV</code>.
<code>ACE_TRY_ENV</code> itself is also a macro which can be
redefined.
</pre>
<P>
You can redefine the name of the variable to
something else to avoid name clashing. Alternatively, there's
another macro (<code>ACE_ADOPT_CORBA_ENV</code>) that allow you
to use another variable name as the default CORBA::Environment
<em>within</em> a function.
</P>
</LI>
</ol>
<HR><P>
<h3>Examples</h3><a name="examples">
Refer to <a href="../ace/CORBA_macros.h"><code>
$ACE_ROOT/ace/CORBA_macros.h</code></a> for complete definitions of
macros discussed here.
<ul>Examples on using ACE try macros:
<li><p>
<pre>
<a name="try_env">
ACE_TRY // Use ACE_DECLARE_NEW_CORBA_ENV to create ACE_TRY_ENV
// if you got undefined symbol warning here.
{
some_operation (arg1, arg2 ACE_ENV_ARG_PARAMETER);
ACE_TRY_CHECK;
.
.
if (whatever)
ACE_TRY_THROW (CORBA::BAD_PARAM ());
some_other_operation (arg1, arg2, arg3 ACE_ENV_ARG_PARAMETER);
ACE_TRY_CHECK;
}
<a name="ace_try">
ACE_CATCH (CORBA_some_exception, ex)
{
// error handling.
if (still_has_error)
ACE_TRY_THROW (CORBA::NOWAY ());
}
ACE_CATCHANY
{
// error handling.
// then rethow the exception.
ACE_RE_THROW;
}
ACE_ENDTRY;
ACE_CHECK;
</pre>
</p>
</li>
<li><p><code>ACE_TRY</code> and also declares a label for internal
use. To avoid defining the same label multiple times within a
function, use <code>ACE_TRY_EX</code> with different labels for
different try blocks instead. For example,<br>
<pre>
ACE_TRY_EX (block_1)
{
some_operation (arg1, arg2 ACE_ENV_ARG_PARAMETER);
ACE_TRY_CHECK_EX (block_1);
some_other_operation (arg1, arg2, arg3 ACE_ENV_ARG_PARAMETER);
ACE_TRY_CHECK_EX (block_1);
}
ACE_CATCH (CORBA_some_exception, ex)
{
// error handling.
}
ACE_CATCHANY
{
// error handling.
}
ACE_ENDTRY;
ACE_CHECK_RETURN (-1);
// Some other operation here
// .
// .
// .
// .
ACE_TRY_EX (block_2)
{
foo (arg ACE_ENV_ARG_PARAMETER);
ACE_TRY_CHECK_EX (block_2);
bar (arg1, arg2 ACE_ENV_ARG_PARAMETER);
ACE_TRY_CHECK_EX (block_2);
}
ACE_CATCH (CORBA_some_exception, ex)
{
// error handling.
}
ACE_CATCHANY
{
// error handling.
}
ACE_ENDTRY;
ACE_CHECK_RETURN (-1);
</pre>
</p>
<li><p>You may want to make a different series of calls after you
encounter/catch an exception. Here is what we recommend.
<pre>
ACE_TRY
{
// Calls that can raise an exception
some_call1 (arg1, arg2 ACE_ENV_ARG_PARAMETER);
ACE_TRY_CHECK;
.
.
.
ACE_TRY_CHECK;
}
ACE_CATCH (CORBA_some_exception, ex)
{
// Caught an exception, so we need to make some other calls
// to continue..
ACE_TRY_EX (block1) // basically a label
{
some_other_call1 (arg1,.. ACE_ENV_ARG_PARAMETER);
ACE_TRY_CHECK_EX (block1);
}
ACE_CATCH (CORBA_some_other_exception, ex1)
{
// Handle the exception here..
}
ACE_ENDTRY;
ACE_CHECK_RETURN (-1); // Needed to catch uncaught exceptions
}
ACE_ENDTRY;
ACE_CHECK_RETURN (-1);
</pre>
</p>
<li><p>Be <em>VERY</em> wary of <code>ACE_CATCHALL</code>. It catches
exceptions of any type. If your program depends on it, then,
more often than not, there're something wrong with it.
</P>
</li>
<li><p>Instead of depending on <code>ACE_CATCHALL</code>, use
<code>auto_ptr</code> style mechanism to prevent memory leaks
from exceptions.
</p>
</li>
</ul>
<HR><P>
<h3><a name="general">General Guidelines for Exception Handling</h3>
<ul>
<li><p>Don't catch an exception just to rethrow it. Exceptions cost
you performance.
</p>
</li>
<li><p>When exceptions occur, make sure an object's is still in
a valid state or change to a state that can be safely
destructed.
</p>
</li>
<li><p>Watch out for side effect in the expression which may cause
exceptions. In the following example, what should
<code>i</code> be if an exception does occur?<br>
<pre>
ACE_TRY
{
obj[i++] = foo_bar_method (a, b ACE_ENV_ARG_PARAMETER);
}
</pre></p>
</li>
<li><p>Make sure an exception doesn't cause resource leak (memory,
socket, ...) (hint: Use auto_ptr to avoid memory leak,
and ACE_Guard for locks.)
</p>
</li>
<li><p>Don't catch any exception that you don't know how to handle.</p>
</li>
<li><p>Never throw an exception from destructor (unless you know what
it implies.)</p>
</li>
<li><p>Use exceptions to provide more information about the error.</p>
</li>
<li><p>Rethrow a different exception only to provide <em>more</em>
information. Do not catch an exception just to rethrow, say,
<code>unknow_exception</code>.</p>
</li>
</ul>
<HR><P>
<H3><a name="transition">Transition from ACE_TRY_ENV usage
to the ACE_ENV_ARG macros</h3>
<P>Before TAO version 1.2.2, IDL defined methods were declared using
direct mentions of <code>CORBA::Environment ACE_TRY_ENV</code>.
The problem with this approach was that the ACE_TRY_ENV had
to be passed into ORB core method calls even when native exceptions
are supported. The TAO internal usage of the ACE_ENV_ARG family of
macros fixes this.</p>
<p>For people who want to continue to use their old code that uses the
old <code>ACE_TRY_ENV</code> macros, they can define
<CODE>ACE_ENV_BKWD_COMPAT</CODE> in their <code>config.h</code> file.
<P>CORBA applications that do not need support for emulated exceptions
can use direct C++ exception handling and omit the CORBA::Environment
parameter entirely.<BR>
On the other hand, applications that shall support environments without
C++ exceptions (such as all applications that are part of to TAO itself)
should use the ACE_ENV_ARG macros.<BR>
The script <code>$ACE_ROOT/bin/subst_env.pl</code> can assist in the
conversion from the direct ACE_TRY_ENV usage to the ACE_ENV_ARG macros.
Here is a list of the substitutions that the script does. For context,
two sample IDL methods are used:
<PRE>
void noargs ();
void withargs (in boolean b);
</pre>
At each example, first the <em>old usage</em> is given, then its
<code>subsitution</code>.
</p>
<H4>Method declaration</h4>
<UL>
<li><P><em>void noargs (CORBA::Environment &);</em></P>
<P><code>void noargs (ACE_ENV_SINGLE_ARG_DECL_NOT_USED);</code></P>
</li>
<li><P><em>void noargs (CORBA::Environment &ACE_TRY_ENV);</em></P>
<P><code>void noargs (ACE_ENV_SINGLE_ARG_DECL);</code></P>
</li>
<li><P><em>void noargs (CORBA::Environment &ACE_TRY_ENV = TAO_default_environment ());</em></P>
<P><code>void noargs (ACE_ENV_SINGLE_ARG_DECL_WITH_DEFAULTS);</code></P>
</li>
<li><P><em>void withargs (CORBA::Boolean b, CORBA::Environment &);</em></P>
<P><code>void withargs (CORBA::Boolean b ACE_ENV_ARG_DECL_NOT_USED);</code></P>
</li>
<li><P><em>void withargs (CORBA::Boolean b, CORBA::Environment &ACE_TRY_ENV);</em></P>
<P><code>void withargs (CORBA::Boolean b ACE_ENV_ARG_DECL);</code></P>
</li>
<li><P><em>void withargs (CORBA::Boolean b, CORBA::Environment &
ACE_TRY_ENV = TAO_default_environment ());</em></P>
<P><code>void withargs (CORBA::Boolean b ACE_ENV_ARG_DECL_WITH_DEFAULTS);</code></P>
</li>
</ul>
<H4>Method invocation</h4>
<UL>
<li><P><em>noargs (ACE_TRY_ENV);</em></P>
<P><code>noargs (ACE_ENV_SINGLE_ARG_PARAMETER);</code></P>
</li>
<li><P><em>withargs (bparam, ACE_TRY_ENV);</em></P>
<P><code>withargs (bparam ACE_ENV_ARG_PARAMETER);</code></P>
</li>
</ul>
<HR><P>
<H3><a name="caveats">Caveats</H3>
<P>As we already mentioned no set of macros can cover all cases
and preserve the semantics between native C++ exceptions and the
<CODE>CORBA::Environment</CODE> based mapping.
Some of the problems that our macros are described below:
<P>
<UL>
<LI><P>Using the macros in loops can produce problems with
<CODE>break</CODE> and <CODE>continue</CODE> statements, for
example:
</P>
<PRE>
for (int i = 0; i < 10; ++i)
{
ACE_TRY
{
if (x[i] == 0)
continue; // will *not* work
if (x[i] == -1)
break; // will *not* work either
}
ACE_CATCH (CORBA::Exception, ex)
{
}
ACE_ENDTRY;
ACE_CHECK;
}
</PRE>
</LI>
</UL>
<P><HR><P>
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