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Introduction

The main intent of this paper is to provide a user level introduction to the C++ exception mechanism with a basic introduction to the way it is used in aips++. This should allow developers to use the mechanism.

C++ provides a standard method to handle exceptional conditions. When such conditions arise, control is transferred to a code block which can handle the exception, if one exists. If no handler exists which is capable of handling the exception, the program is aborted. This package handles cleaning up the call stack which may have been created between when the handler was installed and when the exception was thrown. In addition, any objects which were created on the stack during this period will be deleted.

This provides a mechanism for transferring control from the point where an exceptional condition occurs to the code to handle the exception without memory leaks. Some of the advantages of this approach are:

• The code to display the error message can be moved out to the appropriate level. Otherwise, the code to display an error message might have to be deep within an application in a place which could otherwise be independent of the graphical user interface. Without exceptions error messages must be carefully propagated out via return values until the appropriate level for their display is reached.
• The exception mechanism allows for an incremental graceful exit. The same exception can be thrown a number of times so that each layer of the program can have a chance to respond to the exception as appropriate. This can be important for maintaining the integrity of portions of the program, e.g. the user interface or database portions.
• Using exceptions a program can continue after an error even though the error may have been fatal at the point where it occurs. In a more traditional approach, this would require careful checks of return codes.

So while everything which can be accomplished with exceptions can be accomplished with returned error codes or other traditional approaches, the use of exceptions is cleaner.