[midjji]

This is also known as the most common invocation of undefined behaviour in game programming. If you do this, write to y, then read from [1]. You are invoking undefined behaviour, and compilers doing different things here between windows, linux mac, and different compiler versions is a common cause of "why isnt my game working right on XXX, it works fine on YYY questions.

[genocidicbunny]

Type punning is not undefined, it's implementation defined in C. In practice, every major C compiler will be fine with type punning, though it may disable some optimizations.

The story is different in C++, but in practice many compilers support it the same as in C. Especially for games, where VC++ (PC, Xbox) and Clang (PS4/PS5) are the most commonly used compilers, it also works as expected. The trick is to only use type punning for trivial structs that don't invoke complications like con/de-structors or operators. The GP's example of a Vec3 struct that puns float x,y,z with float[3] is a very common one in games.

[midjji]

Something being very common and a very common source of portability issues isn't exactly contradictory. Its a bad idea, and it is outright being taught in modern game programming courses that its a bad idea, but common in older guides, specifically because it caused so many problems. Im pissed at this specific construct because I got it handed to me in a huge game library and had to spent a long time figuring out why it wasn't working in rare, but important cases.

[genocidicbunny]

But my point is that on the platforms that matter, it's not really a source of portability issues, and not a problem. For gamedev, anything outside of VC++ and Clang are niche and thus largely ignored.

[saagarjha]

I don’t see the undefined behavior here?

[shultays]

Op probably means cpp, where it is indeed undefined behavior. not sure about c. I doubt that if this would cause a "my game does not work on XXX" though. Is there really a compiler out there that will handle such abuse differently?

[midjji]

yes its undefined behaviour in both C and C++. Yes, a number of compilers treat this differently, its also poorly supported on custom hardware using standard compilers like gcc. So compiling for some mobile device with slightly custom ... good luck.

[bruce343434]

It is not undefined behavior in C. And the OP was about C.

[bruce343434]

I don't mean cpp