4 ms·

This makes me wonder how protobufs manages to implement zero-copy structs.

• tsimionescu 5y ago
  Well, in practice memcpy and std::bit_cast will not normally copy, if you're careful with how you use the variables and so on, so it's not completely impossible to do this. There are also compilers (g++ for example, if I understand correctly) that define type punning through unions to work like in C (allowing you to read and write through different members of the union, which the C++ standard prohibits).
• marcosdumay 5y ago
  I bet they use UB in more than one place. It's just some instance of UB that compilers aren't exploiting with crazy interpretations that lead to faster non-working programs. The people saying that it's technically possible to create that code without UB say that with a certainty that isn't viable. You don't need to just read the standard from beginning to end to determine if your code has UB, you need to also remember it all. Some part of it not applying to your case doesn't stop the UB on that part from appearing in your code, and there isn't a usable list of UB you can quickly check against (the closest that exists is the set of UB that compilers are using on their crazy extrapolations, what is not the same thing). There is just no way to know that you avoided it all.