How funny this should sound to a Symbian C++ programmer? Till now, I do not have a clear idea on Leave and Trap mechanism of Symbian OS. Anyways, better let than never!! Here are some informations that I found essential to know about Leave - Trap mechanism of Symbian C++.
BASICS
1. Standard C++ exception handling mechanism adds substantially to the size of compiled code and RAM overheads (irrespective of whether the exceptions are thrown or not). That is why Symbian OS introduced the Leave - Trap mechanism. Before Symbian OS version 9 , the standars C++ exception handling were disabled in compilers so that any call to try catch or throw would result in a compilation error.But in Symbian OS 9, the standars C++ is supported (Hurrah!!!!).
2. The Leave - Trap mechanism is an alternative to standard C++ exception and conventional error checking. In other words, Leave- Trap constitutes to lightweight error handling of Symbian OS.
3. a "leave" (i.e. User::Leave() or User::LeaveifError()) suspends the code execution at the point where the leave occurs and resumes the execution at the point where the "leave" is "Trap"ped. The trap harness in Symbian is actually a "TRAP" macro. The "leave" sets the stack pointer to the context of the "TRAP" and jumps to that location.
4. User::Leave() or User::LeaveIfError() ---- Throw
TRAP ------------------------------------- try + catch
5. Critical Information on Leave:
Unlike C++ throw, a Leave will simply deallocate objects on the stack - it does not call the descriptor of that object. So, if a stack object owns a resource which must be deallocated or released as a part of destruction, that resource is not released if a leave occurs. That is why we have a class naming convention of Symbian OS which clearly states which kinds of classes can be instantiated and used safely on the stack (T classes). So, only a stack based T class will be cleaned up correctly if a Leave occurs (because there is nothing of an object of T class in any place other than stack).
6.R classes may also be created on the stack, but they must be made ‘‘leave safe’’, if used in functions that may leave, by using the cleanup stack.
new(ELeave)
1. The overloaded new operator which takes ELeave as a parameter guarantees that the pointer return value will be valid if a Leave is not occured in the constructor. Use of this overload allows the pointer to be used without further test that the allcation was successful(The allocation would leave if it were not successful).
BASICS
1. Standard C++ exception handling mechanism adds substantially to the size of compiled code and RAM overheads (irrespective of whether the exceptions are thrown or not). That is why Symbian OS introduced the Leave - Trap mechanism. Before Symbian OS version 9 , the standars C++ exception handling were disabled in compilers so that any call to try catch or throw would result in a compilation error.But in Symbian OS 9, the standars C++ is supported (Hurrah!!!!).
2. The Leave - Trap mechanism is an alternative to standard C++ exception and conventional error checking. In other words, Leave- Trap constitutes to lightweight error handling of Symbian OS.
3. a "leave" (i.e. User::Leave() or User::LeaveifError()) suspends the code execution at the point where the leave occurs and resumes the execution at the point where the "leave" is "Trap"ped. The trap harness in Symbian is actually a "TRAP" macro. The "leave" sets the stack pointer to the context of the "TRAP" and jumps to that location.
4. User::Leave() or User::LeaveIfError() ---- Throw
TRAP ------------------------------------- try + catch
5. Critical Information on Leave:
Unlike C++ throw, a Leave will simply deallocate objects on the stack - it does not call the descriptor of that object. So, if a stack object owns a resource which must be deallocated or released as a part of destruction, that resource is not released if a leave occurs. That is why we have a class naming convention of Symbian OS which clearly states which kinds of classes can be instantiated and used safely on the stack (T classes). So, only a stack based T class will be cleaned up correctly if a Leave occurs (because there is nothing of an object of T class in any place other than stack).
6.R classes may also be created on the stack, but they must be made ‘‘leave safe’’, if used in functions that may leave, by using the cleanup stack.
new(ELeave)
1. The overloaded new operator which takes ELeave as a parameter guarantees that the pointer return value will be valid if a Leave is not occured in the constructor. Use of this overload allows the pointer to be used without further test that the allcation was successful(The allocation would leave if it were not successful).
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