Question

Why is this a common practice to have assert macro do something useful only in debug configuration? If it exists to test invariants and detect coding bugs, then would not it be easier to go ahead and do the same big boom in production software?

I have some S60 background and there exist __ASSERT_ALWAYS and __ASSERT_DEBUG, where the latter is equivalent to assert.

Answer 1

Checking for assertion costs. You have extra operations that you may not want to exist in the final product. If assertions were always going to work, then people would start using them less "to not kill performance". And believe me, there are a lot of people out there who consider the extra checks a performance kill and would avoid it. These are the same people who actually have to use assert more!

A more important reason is that, assert, if failed, will just abort your program. There is no usefulness in that whatsoever for the end user (except perhaps for security, to avoid running code with unexpected data, but that’s arguably better handled by actual error checking). If you want your program to actually terminate with a message or do something useful, you would have to write your own assert. In that case, you can of course choose to keep it in release mode also.

Finally, assertion helps you find bugs, especially hidden bugs, but in the execution of the software, they may actually not happen. Imagine the following code:

struct X
{
    // other stuff
    int stage;
};

X x;
... do some stuff
assert(x.stage == STAGE_2);
x.stage = STAGE_3; // go to next stage
... do more stuff

In such an example, your logic says x should be in STAGE_2. If it is not, it’s a bug. However, if you remove the assert, fix x.stage and move on, there is hope that the bug is not so severe. In such a case, the end-user can actually continue working without noticing this. If you had kept assert in release mode too, you would force the application to exit over a bug that didn’t have any visible effect.

In reality, you get updates all the times for your software, in which they claim they have fixed bugs. Some of those, are indeed bugs that assert would have caught. However, you as the end-user didn’t have any problem and were actually happy that you weren’t interrupted due to those asserts, weren’t you?