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System Response After a Class-Based Exception

A class-based exception can be raised in a statement block for one of the following reasons:

In both cases, the occurrence of a class-based exception interrupts the sequential processing of the current processing block, and the system responds as follows:

  • If the exception is raised when a TRY block of a TRY control structure is executed, an appropriate CATCH block is searched as a handler. Execution of each TRY block opens a context, also called a protected area, into which the execution of other TRY blocks can be embedded. Usually, it is embedded by means of the call of procedures, less frequently by means of nesting TRY blocks in the source text. Starting at the position where the exception is raised, the system scans the TRY control structures of the participating TRY blocks from the inside to the outside for the first CATCH block, in which the exception class or one of its superclasses appears. If it finds a CATCH block, there are two possible cases:
  • If the addition BEFORE UNWIND is not declared in the statement CATCH, the context in which the exception was raised is first deleted, including all called procedures and their local data. The CATCH block is then executed.
  • If the addition BEFORE UNWIND is declared in the statement CATCH, the CATCH block is executed immediately. If the CATCH block is exited using the statement RESUME in a resumable exception, the program resumes after the place where the exception was raised. In all other cases, the system deletes the context in which the exception was raised, after the CATCH block is exited.
If the CATCH block is not exited with a statement such as RESUME, RETRY, or any other processing block exits, processing continues after the block's TRY control structure.
  • If no handlers are found in any of the participating TRY control structures of a protected area, or if the exception is not raised during processing of a TRY block of a TRY control structure, a runtime error occurs at the point where the exception occurred. The short dump of the runtime error contains the name of the exception class and the exception text.

Note the following special features:

  • If the user leaves the procedure context during the handler search, the procedure's interface will be checked. Only exceptions declared there can be propagated from the procedure. Exceptions of the categories CX_STATIC_CHECK and CX_DYNAMIC_CHECK must be declared explicitly, while exceptions of category CX_NO_CHECK are always declared implicitly. If the exception is not declared in the interface, the exception of the predefined class CX_SY_NO_HANDLER is raised at the call position of the procedure, in whose attribute PREVIOUS a reference to the original exception is stored. This is done at the call position of the procedure,
  • If a handler is found, the CLEANUP blocks of all TRY control structures that have thus far been scanned unsuccessfully are executed from the inside to the outside, directly before their context is deleted. This means that, if BEFORE UNWIND is declared for the CATCH block, the CLEANUP blocks are executed when the handler is exited; otherwise they are executed before being handled. Exceptions that are raised within a CLEANUP block cannot exit the block; they must be handled there.

    No CLEANUP blocks are executed in the following cases:
  • TRY control structures whose exception is raised in a CATCH block (a CATCH block is not part of the protected range).

See also:

Other versions: 7.31 | 7.40 | 7.54


Class-Based Exceptions in Procedures

Class-Based Exceptions in Event Handlers