assert() Versus Exceptions in C++
By: Daniel Malcolm
It is important to note that assert() is not intended to handle runtime error conditions such as bad data, out-of-memory conditions, unable to open file, and so forth. assert() is created to catch programming errors only. That is, if an assert() "fires," you know you have a bug in your code.
This is critical, because when you ship your code to your customers, instances of assert() will be removed. You can't depend on an assert() to handle a runtime problem, because the assert() won't be there.
It is a common mistake to use assert() to test the return value from a memory assignment:
Animal *pCat = new Cat; Assert(pCat); // bad use of assert pCat->SomeFunction();
This is a classic programming error; every time the programmer runs the program, there is enough memory and the assert() never fires. After all, the programmer is running with lots of extra RAM to speed up the compiler, debugger, and so forth. The programmer then ships the executable, and the poor user, who has less memory, reaches this part of the program and the call to new fails and returns NULL. The assert(), however, is no longer in the code and there is nothing to indicate that the pointer points to NULL. As soon as the statement pCat->SomeFunction() is reached, the program crashes.
Getting NULL back from a memory assignment is not a programming error, although it is an exceptional situation. Your program must be able to recover from this condition, if only by throwing an exception. Remember: The entire assert() statement is gone when DEBUG is undefined.
It is not uncommon to find that a bug appears only after the instances of assert() are removed. This is almost always due to the program unintentionally depending on side effects of things done in assert() and other debug-only code. For example, if you write
ASSERT (x = 5)
when you mean to test whether x == 5, you will create a particularly nasty bug.
Let's say that just prior to this assert() you called a function that set x equal to 0. With this assert() you think you are testing whether x is equal to 5; in fact, you are setting x equal to 5. The test returns TRUE, because x = 5 not only sets x to 5, but returns the value 5, and because 5 is non-zero it evaluates as TRUE.
Once you pass the assert() statement, x really is equal to 5 (you just set it!). Your program runs just fine. You're ready to ship it, so you turn off debugging. Now the assert() disappears, and you are no longer setting x to 5. Because x was set to 0 just before this, it remains at 0 and your program breaks.
In frustration, you turn debugging back on, but hey! Presto! The bug is gone. Once again, this is rather funny to watch, but not to live through, so be very careful about side effects in debugging code. If you see a bug that only appears when debugging is turned off, take a look at your debugging code with an eye out for nasty side effects.
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