Oops!… I violated ODR again


Often when you’re trying to debug a piece of code, the debugger steps into a block you didn’t expect it to. Most of the time this is because your code has a logic error, and it wasn’t doing what your mental model thought it would.

Other times it’s because you accidentally compiled with optimizations on, and the compiler did some magic to make the outcome the same even if the code was different.

This post talks about a scenario where it’s neither of those things. Instead it’s something much more dangerous and difficult to detect — a violation of the One Definition Rule (ODR).

In this post I’ll talk a little about C++’s One Definition Rule, and then discuss how one manifested itself in a project I was working on. I’ll talk about how to detect them, and how I resolved mine.
(I talk a lot about violating ODR in the context of a tool I use (SWIG), but the manner in which I violated ODR is applicable to any C++ library that links to another.)

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