[TeMPOraL]

Wow, never realized the pointer cast version is undefined behaviour in C. Is C++ equivalent via reinterpret_cast undefined behaviour too?

Back in my C++ days, I would much prefer pointer-cast to union-struct, because the latter was subject to alignment/padding issues you mention, while for the former, I know of no desktop architecture that could mess up a int* -> float* cast.

I'm also doubly surprised because being able to do that stuff is what I'd consider the selling point of C/C++ over higher-level languages.

[jcranmer]

Pretty much all pointer casts where you would use reinterpret_cast in C++ are undefined behavior, in both C and C++. The underlying rule for strict aliasing is essentially the same in both languages (you can only access an lvalue via a pointer of its type, with possibly differing signedness, const, volatile, or via a char pointer, or via a field of a struct that has the same initial layout if you're in that same initial layout portion).

Yeah, C and C++ aren't the language most people think they are, and the strict aliasing rules are the most clear manifestation of that.

[tropo]

Trouble is, C was that language. Lots of people came to depend on this. The world needs a language with this ability.

The replacement is often "gcc with inline assembly" or "gcc with lots of __attribute__ extensions such as __may_alias__" or "gcc with optimizations disabled".

The need doesn't go away, despite the wishes of a language committee now dominated by compiler suppliers. From day 1, long before the language was standardized and even before the original (not ANSI) book was published, C was being used with aliasing. It was, after all, developed to support the creation of Unics (before that became UNIX).

[saagarjha]

reinterpret_cast in C++ has essentially the same rules, with exceptions made for const qualifiers and such, as well as void , char , and std::byte *. Use a memcpy instead.