In general, if you do things correctly, you don’t need to understand ODR violations.

If you do things incorrectly, it might be hard to figure out. But having header files mean different things in difference places is a long-standing issue that goes back to C and has always been hard to figure out.

Contracts add the new issue that you can do everything right and still get surprising behavior and it is even harder to figure out because your tools won’t help you.

John.

> On Oct 20, 2025, at 9:56 AM, Oliver Rosten <oliver.rosten_at_[hidden]> wrote:
>
> PS Most people I teach have no prior idea what an ODR violation is.
>
> On Mon, 20 Oct 2025 at 14:55, Oliver Rosten <oliver.rosten_at_[hidden] <mailto:oliver.rosten_at_[hidden]>> wrote:
>> That's a proof of existence but not of wide-spread usage.
>>
>> I am honestly ignorant here. As far as I know the C++ ecosystem as a whole is not making rigorous use of things like this. But I may be wrong...
>>
>> On Mon, 20 Oct 2025 at 14:53, Ville Voutilainen <ville.voutilainen_at_[hidden] <mailto:ville.voutilainen_at_[hidden]>> wrote:
>>> On Mon, 20 Oct 2025 at 16:46, Oliver Rosten
>>> <oliver.rosten_at_[hidden] <mailto:oliver.rosten_at_[hidden]>> wrote:
>>> >
>>> > Hi John,
>>> >
>>> > I'm not convinced by this:
>>> >
>>> >> No, it is not a pre-existing problem.
>>> >> Other than contracts, if you end up with different function definitions it is an ODR violation and your program is IFNDR and can be rejected by your tools.
>>> >
>>> >
>>> > There's a difference between "can be in principle" and "is in general practice". Is it not the case that, in most instances, for all practical purposes there is no difference between an ODR violation that's IFNDR and the contracts mixed-mode: you get what the linker gives you?
>>>
>>> See, for example,
>>> https://devblogs.microsoft.com/oldnewthing/20160803-00/?p=94015
>>> See also https://maskray.me/blog/2022-11-13-odr-violation-detection