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ugly code termination, I advise you to consider carefully before using them against invalid input data which may have arisen from an exception that the caller can handle more gracefully This chapter illustrated some aspects of defensive programming besides the use of assertion statements, but added this note of caution You should consider carefully how much defensive code to use, and whether it varies between debug and release builds It can create additional complexity in your code, leaving it open to its own set of bugs, and, if you check parameter data for every possible error, it can also make your code slow and bloated You should take care to use defensive techniques where they are most effective, and document them clearly so the client can build their own bug catching and exception handling around them The paradox is that you want problems to be noticeable so they are agged up during development, but you want them to be inconspicuous in your production code You should consider your approach to defensive code and assertions appropriately for each project you work on In all cases, it s best to keep it consistent and consider error handling as an important issue to be determined and de ned at an architectural level The next chapter discusses useful debug macros and test classes on Symbian OS for tracking down programming errors such as memory leaks and invalid internal state You can nd more information about handling leaves (Symbian OS exceptions) in 2
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If you have built castles in the air, your work need not be lost; that is where they should be Now put foundations under them Henry David Thoreau (Walden)
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Memory management is an important issue on Symbian OS, which has a number of mechanisms to assist you in writing memory leak-proof code This chapter starts by discussing a set of debug macros that allow you to check that your code is managing heap memory correctly (you will nd them documented in your SDK under Memory Allocation ) It will also discuss other useful debug macros and the RTest and RDebug classes
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There are macros which can be used to check that you are not leaking any memory under normal conditions and others that can be used to simulate out-of-memory (OOM) conditions It is advisable to check your code using each macro type, to ensure both that it is leave-safe and that it can handle OOM gracefully (that is, to ensure that heap memory is not orphaned by a leave occurring under OOM as I discussed in 2) This kind of testing is known as Alloc Heaven Testing Here are the macros, as de ned in e32defh They are only compiled into debug builds, and are ignored by release builds, so they can be left in your production code without having any impact on the code size or speed
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#define __UHEAP_MARK User::__DbgMarkStart(RHeap::EUser) #define __UHEAP_MARKEND User::__DbgMarkEnd(RHeap::EUser,0) #define __UHEAP_MARKENDC(aCount) User::__DbgMarkEnd(RHeap::EUser,aCount) #define __UHEAP_FAILNEXT(aCount) User::__DbgSetAllocFail(RHeap::EUser,RHeap::EFailNext,aCount) #define __UHEAP_SETFAIL(aType,aValue) User::__DbgSetAllocFail(RHeap::EUser,aType,aValue) #define __UHEAP_RESET User::__DbgSetAllocFail(RHeap::EUser,RHeap::ENone,1)
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The __UHEAP_MARK and __UHEAP_MARKEND macros verify that the default user heap is consistent The check is started by using __UHEAP_MARK and a following call to __UHEAP_MARKEND performs the veri cation If any heap cells have been allocated since __UHEAP_MARK and not freed back to the heap, __UHEAP_MARKEND will cause a panic1 These macros can be nested inside each other and used anywhere in your code You must take care to match the pairs, because a __UHEAP_MARKEND without a prior __UHEAP_MARK causes a panic __UHEAP_FAILNEXT simulates a heap failure (ie out of memory) The macro takes a TInt parameter which speci es which memory allocation call will fail (if the parameter is 1, the rst heap allocation fails, if it s 2, the second allocation fails, and so on) The __UHEAP_RESET macro cancels the failure checking Consider the following example, where a method CTest::SomethingL() makes several memory allocations internally To test the leave safety of the method, it is necessary to check that failure of each of these allocations is handled correctly The best way to do this is to call SomethingL() inside a loop as follows: 1 2 3 4 5 Set the __UHEAP_FAILNEXT parameter Use __UHEAP_MARK to mark the heap Call CTest::SomethingL() inside a TRAP macro Call __UHEAP_MARKEND to check that no memory has been leaked Check the error from the TRAP; if it s KErrNoMemory, you should go round the loop again, incrementing the number of allocations which succeed before the failure point If it s KErrNone, all the allocation points inside SomethingL() have been tested and the test is complete
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void TestLeaveSafetyL() { // extra check to demonstrate that these macros can be nested __UHEAP_MARK; CTest* test = CTest::NewLC(); for (TInt index=1;;index++) { __UHEAP_FAILNEXT(index); __UHEAP_MARK; // This is the function we are OOM testing
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1 The call to User::__DbgMarkEnd() will raise a panic if the heap is inconsistent The panic raised is ALLOC nnnnnnnn, where nnnnnnnn is a hexadecimal pointer to the rst orphaned heap cell I ll discuss how to track down what is causing a panic of this type shortly
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