### Using ACE Exception Handling 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 ACE_TRY_ENV usage to the ACE_ENV_ARG macros
- Some Caveats

### ACE Exception Macros in a Nutshell

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

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 quick examples on how to use ACE exception macros.

1. Declaration of CORBA::Environment Parameter at Methods
   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.
   In order for both configurations to work with the same source code, following macros are defined. In native exception configurations, they all resolve to empty.

   - ACE_ENV_ARG_DECL

     in the pseudo exception configuration resolves to

                 , CORBA::Environment ACE_TRY_ENV

     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 -
     Example: a CORBA IDL defined method

                 void mymethod (in boolean b);

     may be declared as follows in C++:

                 void mymethod (CORBA::Boolean b  ACE_ENV_ARG_DECL);

     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.

   - ACE_ENV_SINGLE_ARG_DECL

     is similar to `ACE_ENV_ARG_DECL`, 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 `ACE_ENV_ARG_DECL` does. Example: a CORBA IDL defined method

                 void mymethod ();

     may look like this in C++:

                 void mymethod (ACE_ENV_SINGLE_ARG_DECL);

   - ACE_ENV_ARG_DECL_WITH_DEFAULTS,

   - ACE_ENV_SINGLE_ARG_DECL_WITH_DEFAULTS are similar to the above two macros but add a default value to the parameter declaration. In case of TAO, the default value is `TAO_default_environment()`.

     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

                 void mymethod ();

     in the C++ header file looks like this:

                 void mymethod (ACE_ENV_SINGLE_ARG_DECL_WITH_DEFAULTS);

     and in the C++ implementation file may look something like:

                 void mymethod (ACE_ENV_SINGLE_ARG_DECL)
                 {
                   // ...
                 }

2. Passing the CORBA::Environment Parameter into Method Calls
   Now that we've seen how to declare methods with Environment parameters, let's see how to invoke such methods.

   - ACE_ENV_ARG_PARAMETER

     in the pseudo exception configuration resolves to

                 , ACE_TRY_ENV

     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

                 void mymethod (in boolean b);

     would be invoked as follows:

                 some_var.mymethod (bparam  ACE_ENV_ARG_PARAMETER);

   - ACE_ENV_SINGLE_ARG_PARAMETER

     is similar to `ACE_ENV_ARG_PARAMETER` but is used for calling methods that don't take any regular parameters. Our example of a CORBA IDL method

                 void mymethod ();

     we would invoke as follows:

                 some_var.mymethod (ACE_ENV_SINGLE_ARG_PARAMETER);

3. Definition of the CORBA::Environment variable
   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?

   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

             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_ENV_ARG_PARAMETER);

   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  ACE_ENV_ARG_DECL);

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_ENV_SINGLE_ARG_PARAMETER)
                   + obj2->dare_me (ACE_ENV_SINGLE_ARG_PARAMETER);
                 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_RE_THROW` 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.

8. 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 exception macros assume 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.

### 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_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;
              }

            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;

- `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_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);

- 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_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);

- 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_ENV_ARG_PARAMETER);
              }

- 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 ACE_TRY_ENV usage to the ACE_ENV_ARG macros

Before TAO version 1.2.2, IDL defined methods were declared using direct mentions of `CORBA::Environment ACE_TRY_ENV`. 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.

CORBA applications that do not need support for emulated exceptions can use direct C++ exception handling and omit the CORBA::Environment parameter entirely.
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.
The script `$ACE_ROOT/bin/subst_env.pl` 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:

            void noargs ();
            void withargs (in boolean b);

At each example, first the old usage is given, then its `subsitution`.

#### Method declaration

- void noargs (CORBA::Environment &);

  `void noargs (ACE_ENV_SINGLE_ARG_DECL_NOT_USED);`

- void noargs (CORBA::Environment &ACE_TRY_ENV);

  `void noargs (ACE_ENV_SINGLE_ARG_DECL);`

- void noargs (CORBA::Environment &ACE_TRY_ENV = TAO_default_environment ());

  `void noargs (ACE_ENV_SINGLE_ARG_DECL_WITH_DEFAULTS);`

- void withargs (CORBA::Boolean b, CORBA::Environment &);

  `void withargs (CORBA::Boolean b ACE_ENV_ARG_DECL_NOT_USED);`

- void withargs (CORBA::Boolean b, CORBA::Environment &ACE_TRY_ENV);

  `void withargs (CORBA::Boolean b ACE_ENV_ARG_DECL);`

- void withargs (CORBA::Boolean b, CORBA::Environment & ACE_TRY_ENV = TAO_default_environment ());

  `void withargs (CORBA::Boolean b ACE_ENV_ARG_DECL_WITH_DEFAULTS);`

#### Method invocation

- noargs (ACE_TRY_ENV);

  `noargs (ACE_ENV_SINGLE_ARG_PARAMETER);`

- withargs (bparam, ACE_TRY_ENV);

  `withargs (bparam ACE_ENV_ARG_PARAMETER);`

### 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;
          }

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