### Using ACE try Macros to Enhance CORBA Portability

CORBA `Environment` 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 `CORBA::Environment` 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.

Before reading the rest of this document, we recommend you check out pages 307 through to 322 in the book, Advanced Corba Programming with C++ by Michi Henning & Steve Vinoski. Likewise, we recommend that you read the Error Handling chapter from the TAO Developer's Guide.

### Table of Contents

- ACE Try Macros in a Nutshell
- Examples
- General Guidelines for Exception Handling
- Transition from TAO try macros to ACE try macros
- Some Caveats

### ACE Try Macros in a Nutshell

This section explains some simple rules of writing programs for platforms with and without native exception support using ACE's try macros.

1. 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.

2. 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.

3. 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);

4. 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.

5. 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:

   1. Any function that takes a `CORBA_Environment` argument.

   2. `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.

   3. `ACE_GUARD_THROW_EX`, `ACE_READ_GURAD_THROW_EX`, and `ACE_WRITE_THROW_EX`.

   4. `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.

6. 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`.

7. 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.

### Examples

Refer to `$ACE_ROOT/ace/CORBA_macros.h` for complete definitions of macros discussed here.

Examples on using ACE try macros:

-           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::BAD_PARAM ());

                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.

### General Guidelines for Exception Handling

- 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`.

### Transition from TAO try macros to ACE try macros

This list tries to give a comprehensive list of mapping between TAO try macros and ACE try macros. It's sole purpose is to provide hints in the converting the use of TAO try macros to ACE try macros and is by no mean complete.

1. Rename all `CORBA_Environment` variables to `ACE_TRY_ENV`.

2. Replace `TAO_TRY` `TAO_TRY_VAR` with `ACE_TRY`. Added `ACE_DECLARE_NEW_CORBA_ENV` if necessary.

3. Replace `TAO_TRY_EX` `TAO_TRY_VAR_EX` with `ACE_TRY_EX`.

4. 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.

5. Replace `TAO_THROW`, `TAO_THROW_ENV`, `TAO_THROW_RETURN`, `TAO_THROW_ENV_RETURN` with `ACE_THROW` and `ACE_THROW_RETURN`.

6. Replace `ACE_NEW_THROW`, `ACE_NEW_THROW_RETURN`, `ACE_TRY_NEW_THROW` with `ACE_NEW_THROW_EX` and appropriate `ACE_CHECK*` macros. aformention

7. Replace `TAO_CHECK_ENV` and `TAO_CHECK_ENV_EX` with `ACE_TRY_CHECK` and `ACE_TRY_CHECK_EX`.

8. 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.

9. Replace `TAO_RETHROW`, `TAO_RETHROW_RETURN`, `TAO_RETHROW_RETURN_VOID` with `ACE_RETHROW`.

10. Replace `TAO_CATCH`, `TAO_CATCHANY`, and `TAO_CATCHALL` with `ACE_CATCH`, `ACE_CATCHANY` and `ACE_CATCHALL` respectively.

11. 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`.)

### Caveats

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;
          }

Back to the ACE documentation page.
Back to ACE Documentation Home.