ACE try macros will completely replace TAO try macros soon. If you are still using TAO try macros in your code, you should replace them with ACE try macros to avoid possible breakage when we remove TAO try macros.<\p>
It would be useful if the reader can go through the book, "Advanced Corba Programming with C++" by Michi & Steve, from pages 307 through to 322.
CORBA::Environment provides a way to handle exceptions when native c++ exception handling is unavailable or undesirable. However, writing portable code for both (with and without) native exception handling capability is very hairy.
ACE provides a set of macros to help dealing with the chaos, but please keep in mind that no amount of macros is going to solve the problem perfectly.
This section explains some simple rules of writing programs for platforms with and without native exception support using ACE's try macros.
ACE try 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 it works even on platforms without native exception support. See some quick examples on how to use ACE try macros.
Name of CORBA::Environment variable
      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 try macros
      assumes that the variable is named ACE_TRY_ENV.
      ACE_TRY_ENV itself is also a macro which can be
      redefined.
      
      
      You can redefine the name of the variable to
      something else to avoid name clashing.  Alternatively, there's
      another macro (ACE_ADOPT_CORBA_ENV) that allow you
      to use another variable name as the default CORBA::Environment
      within a function.
      
Definition of the CORBA::Environment variable 
      If you are using TAO for writing application programs and you
      are interested in using exceptions (which is a pretty neat way
      to go about), the environmnet variable can be brought in to your
      scope by the adding a statement ACE_DECLARE_NEW_CORBA_ENV;
      You can then invoke the methods on the servant from the client
      side as
      
      object_reference->func_name (x, y, ACE_TRY_ENV);
      
      Even if you are interested in making calls within the client
      side, you can define your method like this
      
      int AN_OBJ::foobar (int a, int b, CORBA_Environment &ACE_TRY_ENV);
      
  Throwing exceptions:
      Use ACE_THROW and ACE_THROW_RETURN to
      throw exceptions.  They should never be used within a try
      block; please use ACE_TRY_THROW instead.
      
Propagating exceptions:
      To simulate native exceptions on platforms without native
      exception handling, every  function call that may
      throw exceptions must be followed by ACE_CHECK or
      ACE_CHECK_RETURN.
Exception-throwing functions include the following categories:
Any function that takes a
            CORBA_Environment argument.
ACE_NEW_THROW_EX.  Notice that you
            should not use ACE_NEW_THROW,
            ACE_NEW_THROW_RETURN,
            ACE_NEW_TRY_THROW anymore because they don't
            work right with ACE try macros.  Instead, use
            ACE_NEW_THROW with appropriate ACE_CHECK*
            macros.
ACE_GUARD_THROW_EX,
            ACE_READ_GURAD_THROW_EX, and
            ACE_WRITE_THROW_EX.
        
ACE_TRY blocks.  Follow every
            ACE_ENDTRY with appropriate ACE_CHECK*
            macros.
You should pass ACE_TRY_ENV to these
      functions.
      
Be very careful not to combine exception throwing functions in one statement like this:
          x = obj1->callme (ACE_TRY_ENV) + obj2->dare_me (ACE_TRY_ENV);
          ACE_CHECK;
      
      This example may work differently when native exception handling is enabled/disabled.
Catching exceptions:
      Use ACE_TRY to catch exceptions if there's an
      ACE_TRY_ENV available.  Otherwise, you should use
      ACE_DECLARE_NEW_CORBA_ENV to create one at
      proper scope.  The use of
      ACE_TRY_NEW_ENV is considered depricated because it
      can't deal with the case when you have multiple TRY
      blocks in the scope of ACE_TRY_NEW_ENV.  If there are
      more than one try blocks in a function, use ACE_TRY_EX
      for all subsequence try blocks to avoid name clashing of labels.
      
Within a ACE_TRY block, use the variable
            ACE_TRY_ENV to pass down the
            CORBA_Environment (see this example.)
            
Follow every exception throwing function with
            ACE_TRY_CHECK. If you are using a TRY block
            within another try block add a ACE_TRY_CHECK
            at the end of this TRY block ie. after
            ACE_ENDTRY.
            
Use ACE_CATCH to catch exceptions of certain
            type.
            
ACE_CATCHANY catches any exceptions
            of type CORBA_Exception.  The caught
            exception is stored in a variable call
            ACE_ANY_EXCEPTION.
            
ACE_CATCHALL emulate the catch
            (...) c++ statement.  It is identical to
            ACE_CATCHANY on platforms without native
            exception support.  You can not access the caught
            exception within the ACE_CATCHALL block.
Use ACE_RETHROW to rethrow the same exception
            within a ACE_CATCH or
            ACE_CATCHANY block.
            
A ACE_TRY block must be terminated with
            a ACE_ENDTRY statement.
            
Throw an exception within a ACE_TRY
            block or ACE_CATCH block using
            ACE_TRY_THROW.
            
Printing out exceptions.  Use ACE_PRINT_EXCEPTION
      (EX,INFO) to print out an exception.  The macro takes two
      arguments, a reference to an exception (EX) and a char
      * 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.)  You should always print out
      the exception itself, not the CORBA_Environment that carries the
      exception.
$ACE_ROOT/ace/CORBA_macros.h for complete definitions of
macros discussed here.
      
      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_TRY_ENV);
          ACE_TRY_CHECK;
          .
          .
          if (whatever)
            ACE_TRY_THROW (CORBA::BadParam);
          some_other_operation (arg1, arg2, arg3, ACE_TRY_ENV);
          ACE_TRY_CHECK;
        }
      
      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_RETHROW;
        }
      ACE_ENDTRY;
      ACE_CHECK;
      
      
  ACE_TRY and also declares a label for internal
      use.  To avoid defining the same label multiple times within a
      function, use ACE_TRY_EX with different labels for
      different try blocks instead.  For example,
      
      ACE_TRY_EX (block_1)
        {
          some_operation (arg1, arg2, ACE_TRY_ENV);
          ACE_TRY_CHECK_EX (block_1);
          some_other_operation (arg1, arg2, arg3, ACE_TRY_ENV);
          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_TRY_ENV);
          ACE_TRY_CHECK_EX (block_2);
          bar (arg1, arg2, ACE_TRY_ENV);
          ACE_TRY_CHECK_EX (block_2);
        }
      ACE_CATCH (CORBA_some_exception, ex)
        {
          // error handling.
        }
      ACE_CATCHANY
        {
          // error handling.
        }
      ACE_ENDTRY;
      ACE_CHECK_RETURN (-1);
      
      
  You may want to make a different series of calls after you encounter/catch an exception. Here is what we recommend.
      ACE_TRY
        {
          // Calls that can raise an exception
          some_call1 (arg1, arg2, ACE_TRY_ENV);
          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_TRY_ENV);
              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);
      
      
  Be VERY wary of ACE_CATCHALL.  It catches
      exceptions of any type.  If your program depends on it, then,
      more often than not, there're something wrong with it.
      
Instead of depending on ACE_CATCHALL, use
      auto_ptr style mechanism to prevent memory leaks
      from exceptions.
      
Don't catch an exception just to rethrow it. Exceptions cost you performance.
When exceptions occur, make sure an object's is still in a valid state or change to a state that can be safely destructed.
Watch out for side effect in the expression which may cause
      exceptions.  In the following example, what should
      i be if an exception does occur?
      ACE_TRY
        {
            obj[i++] = foo_bar_method (a, b, ACE_TRY_ENV);
        }
  Make sure an exception doesn't cause resource leak (memory, socket, ...) (hint: Use auto_ptr to avoid memory leak, and ACE_Guard for locks.)
Don't catch any exception that you don't know how to handle.
Never throw an exception from destructor (unless you know what it implies.)
Use exceptions to provide more information about the error.
Rethrow a different exception only to provide more
      information.  Do not catch an exception just to rethrow, say,
      unknow_exception.
Rename all CORBA_Environment variables to
      ACE_TRY_ENV. 
Replace TAO_TRY TAO_TRY_VAR with
      ACE_TRY.  Added
      ACE_DECLARE_NEW_CORBA_ENV if necessary.
Replace TAO_TRY_EX TAO_TRY_VAR_EX
      with ACE_TRY_EX.
Replace TAO_CHECK_RETURN and
      TAO_CHECK_RETURN_VOID with ACE_CHECK_RETURN
      and ACE_CHECK. These macros are used
      outside of TRY/CATCH blocks.
Replace TAO_THROW, TAO_THROW_ENV,
      TAO_THROW_RETURN, TAO_THROW_ENV_RETURN
      with ACE_THROW and
      ACE_THROW_RETURN. 
Replace ACE_NEW_THROW,
      ACE_NEW_THROW_RETURN,
      ACE_TRY_NEW_THROW with
      ACE_NEW_THROW_EX and appropriate
      ACE_CHECK* macros. aformention
Replace TAO_CHECK_ENV and
      TAO_CHECK_ENV_EX with ACE_TRY_CHECK
      and ACE_TRY_CHECK_EX.
Replace TAO_TRY_THOW and
      TAO_TRY_THROW_EX with ACE_TRY_THROW
      and ACE_TRY_THROW_EX.  Notice that you can also use
      ACE_TRY_THROW* within CATCH blocks.
Replace TAO_RETHROW,
      TAO_RETHROW_RETURN,
      TAO_RETHROW_RETURN_VOID with
      ACE_RETHROW.
Replace TAO_CATCH, TAO_CATCHANY,
      and TAO_CATCHALL with ACE_CATCH,
      ACE_CATCHANY and ACE_CATCHALL
      respectively. 
Replace TAO_ENDTRY with ACE_ENDTRY
      followed by an appropriate CHECK macro (ACE_CHECK,
      ACE_CHECK_RETURN, ACE_TRY_CHECK, or
      ACE_TRY_CHECK_EX.)
As we already mentioned no set of macros can cover all cases
and preserve the semantics between native C++ exceptions and the
CORBA::Environment based mapping.
Some of the problems that our macros are described below:
Using the macros in loops can produce problems with
      break and continue statements, for
      example:
      
    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;
    }