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Old EKA1 architecture
Instead of C++ exceptions, which were not part of the C++ standard when Symbian OS was designed, the operating system uses a lightweight exception-handling mechanism, called a leave. Leaves may occur as a result of an error condition or abnormal event, such as insufficient memory or disk space to complete a request. The leave propagates the error to a point in the calling code where it can be handled, called a TRAP harness, unwinding the call stack as it does so.
However, because of the jump, any local resources, such as memory allocated on the heap, will be orphaned, potentially leading to memory or resource handle leaks. Developers working on Symbian OS use the Cleanup stack to keep track of resources to which the only pointer is an automatic variable. In the event of a leave, the Cleanup stack will destroy each of the resources placed upon it.
There are some important places in code which should never leave – namely C++ constructors and destructors. Symbian OS classes typically use two-phase construction to avoid leaves occurring in construction code.
EKA2 architecture ( S60 3rd Edition onwards )
The aforementioned restrictions do not apply anymore starting from the new EKA2 kernel architecture. The Leaves and TRAP harnesses are still there, but they are implemented in terms of normal C++ exceptions and try-catch blocks. This means that the Symbian code still looks the same, but under the hood Leave is an exception and TRAP is a try-catch block. The call stack is not merely unwound anymore. C++ takes care of calling the destructors for all stack objects so the possibility of a resource leak is gone.
Symbian coding convention has not changed even though the biggest reason behind no longer exists.
- T-classes still shouldn't own data even though they would work perfectly fine if they did.
- Two-phased construction is still there even though normal C++ style construction would be enough.
- CleanupStack is still used even though it is not strictly necessary.
In the future Symbian will probably move to more standard C++ so the conventions need to be re-evaluated.