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And there s more! Another bene t of assertions is the con dence that your code is behaving correctly, and that you are not ignoring defects which may later manifest themselves where they are hard to track down, for example intermittently or through behavior seemingly unrelated to the code that contains the error Say you write some library code containing no assertions and then create some code to test it, which runs and returns no errors; are you con dent that everything behaves as you expect and the test code checks every boundary condition Are you sure Certain Adding assertions to test fundamental assumptions in your code will show you immediately if it is swallowing or masking defects and it should give you more con dence that the test results are valid Sure, you might hit some unpleasant surprises as the test code runs for the rst time, but once you ve ironed out any failures, you can be more con dent about its overall quality What s more, the addition of assertions also protects your code against any regressions that may be introduced during maintenance and refactoring but which would not otherwise be picked up by your test code What more could you ask The cases so far could be considered as self defense , in that I ve discussed using assertions to catch bugs in your code Let s move on to consider defensive programming in general Defensive programming is not about retorting It works OK on my machine after being informed that your code doesn t work as expected It s based on defending your code against irresponsible use or downright abuse by code that calls it Defensive code protects functions against invalid input, by inspecting data passed in and rejecting corrupt or otherwise awed parameters, such as strings that are too long or out-of-range numerical values You ll need to consider how to handle bad parameters depending on how your code is called; for example, you may want to assert that the data is good, terminating with a panic if it is not Alternatively, you may decide to continue the ow of execution, so instead of assertions, you ll check each incoming parameter (eg using if statements) and return to the caller if invalid data is detected either with an error value or a leave code Another method would be to check incoming data and, if a parameter is invalid, substitute it with a default parameter or continue with the closest legal value What you don t want to do is ignore invalid input and carry on regardless, since this could lead to problems later on, such as data corruption Whatever method you use to handle illegal input, it should be consistent throughout your code Your clients should be testing with debug versions of your libraries and thus you could use __ASSERT_DEBUG statements to alert them of invalid input, or other misuse, so they can correct it
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__ASSERT_DEBUG assertions can be added early in the development process to highlight programming errors and can be left in to validate the code as it is maintained and refactored acting as a means to design by contract
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You still need to be defensive by checking for illegal usage in release builds too, but the case for using __ASSERT_ALWAYS isn t clear cut Remember, your assertions will terminate the ow of execution and panic the library, displaying a nasty program closed dialog to the user, which is generally best avoided where possible Additionally, you should consider the impact on the speed and size of your code if you apply assertion statements liberally in release builds If you decide not to use __ASSERT_ALWAYS to check incoming values, you should use another defensive technique to guard against illegal input, such as a set of if statements to check values and return error codes or leave when data is unusable You could use these in combination with a set of __ASSERT_DEBUG statements to alert the client programmer to invalid use in debug builds, but often it is preferable to keep the ow of execution the same in both debug and release builds In such cases, I suggest you don t use debug assertions to check input, but instead use if statement checking in both modes, and document each expected return value for your functions Client programmers should understand their responsibility to interpret the return value and act accordingly I ll illustrate this with an example later in this chapter To determine whether you should use __ASSERT_ALWAYS or another, less terminal, defense, I recommend that you consider whether the calling code may be able to take a different action if you do return an error Invalid input is a bug from the perspective of your code, but may be caused by an exceptional condition in the calling code which can be handled A simplistic example would be a call to your code to open and write to a le, where the caller passes in the full le name and path, as well as the data to be written to the le If the le does not exist, it is probably more appropriate to return this information to the caller through a returned error code or leave value than to assert in a release build Client code can then anticipate this and deal with it, without the need for your library to panic and alarm the user accordingly
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