But if you spell that `assert(false)` instead of as a comment, the intent is equally clear, but the behavior when you're wrong is well-defined.

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JohnFen 13 hours ago | parent | next [-]

I agree that including that assert along with the comment is much better. But the comment alone is better than nothing, so isn't without value.

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eterm 13 hours ago | parent | prev | next [-]

Better yet, `assert(false, message)`, with the message what you would have written in the comment.

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andrewf an hour ago | parent | next [-]

Swap the parameters around for C++ and similar langs where `assert(a, b)` evaluates the same as `(void) a; assert(b)`.

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addaon 13 hours ago | parent | prev [-]

`assert(false)` is pronounced "this can never happen." It's reasonable to add a comment with /why/ this can never happen, but if that's all the comment would have said, a message adds no value.

	▲	eterm 12 hours ago \| parent \| next [-]
		Oh I agree, literally `assert(false, "This cannot happen")` is useless, but ensuring message is always there encourages something more like, `assert(false, "This implies the Foo is Barred, but we have the Qux to make sure it never is")`. Ensuring a message encourages people to state the assumptions that are violated, rather than just asserting that their assumptions (which?) don't hold.
	▲	breatheoften 8 hours ago \| parent \| prev [-]
		what language are we talking about? If it's cpp then the pronounciation depends on compiler flags (perhaps inferred from CMAKE_BUILD_TYPE)

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zffr 11 hours ago | parent | prev | next [-]

At least on iOS, asserts become no-ops on release builds

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josephg 10 hours ago | parent | next [-]

It really depends on the language you use. Personally I like the way rust does this:

- assert!() (always checked),

- debug_assert!() (only run in debug builds)

- unreachable!() (panics)

- unsafe unreachable_unchecked() (tells the compiler it can optimise assuming this is actually unreachable)

- if cfg!(debug_assertions) { … } (Turns into if(0){…} in release mode. There’s also a macro variant if you need debug code to be compiled out.)

This way you can decide on a case by case basis when your asserts are worth keeping in release mode.

And it’s worth noting, sometimes a well placed assert before the start of a loop can improve performance thanks to llvm.

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addaon 9 hours ago | parent [-]

> debug_assert!() (only run in debug builds)

debug_assert!() (and it's equivalent in other languages, like C's assert with NDEBUG) is cursed. It states that you believe something to be true, but will take no automatic action if it is false; so you must implement the fallback behavior if your assumption is false manually (even if that fallback is just fallthrough). But you can't /test/ that fallback behavior in debug builds, which means you now need to run your test suite(s) in both debug and release build versions. While this is arguably a good habit anyway (although not as good a habit as just not having separate debug and release builds), deliberately diverging behavior between the two, and having tests that only work on one or the other, is pretty awful.

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josephg 7 hours ago | parent [-]

I hear you, but sometimes this is what I want.

For example, I’m pretty sure some complex invariant holds. Checking it is expensive, and I don’t want to actually check the invariant every time this function runs in the final build. However, if that invariant were false, I’d certainly like to know that when I run my unit tests.

Using debug_assert is a way to do this. It also communicates to anyone reading the code what the invariants are.

If all I had was assert(), there’s a bunch of assertions I’d leave out of my code because they’re too expensive. debug_assert lets me put them in without paying the cost.

And yes, you should run unit tests in release mode too.

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addaon 4 hours ago | parent [-]

But how do you test the recovery path if the invariant is violated in production code? You literally can’t write a test for that code path…

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josephg 3 hours ago | parent [-]

There is no recovery. When an invariant is violated, the system is in a corrupted state. Usually the only sensible thing to do is crash.

If there's a known bug in a program, you can try and write recovery code to work around it. But its almost always better to just fix the bug. Small, simple, correct programs are better than large, complex, buggy programs.

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addaon 3 hours ago | parent [-]

> Usually the only sensible thing to do is crash.

Correct. But how are you testing that you successfully crash in this case, instead of corrupting on-disk data stores or propagating bad data? That needs a test.

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josephg an hour ago | parent [-]

> Correct. But how are you testing that you successfully crash

In a language like rust, failed assertions panic. And panics generally aren't "caught".

> instead of corrupting on-disk data stores

If your code interacts with the filesystem or the network, you never know when a network cable will be cut or power will go out anyway. You're always going to need testing for inconvenient crashes.

IMO, the best way to do this is by stubbing out the filesystem and then using randomised testing to verify that no matter what the program does, it can still successfully open any written (or partially written) data. Its not easy to write tests like that, but if you actually want a reliable system they're worth their weight in gold.

	▲	addaon an hour ago \| parent [-]
		> In a language like rust, failed assertions panic. And panics generally aren't "caught". This thread was discussing debug_assert, where the assertions are compiled out in release code.

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addaon 11 hours ago | parent | prev [-]

You can (and probably should) undef NDEBUG even for release builds.

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fwip 9 hours ago | parent | prev [-]

I think you might have missed that they threw an exception right under the comment.