Type Punning in C and C++

(blog.pwkf.org)

34 points | by ingve 2 hours ago

8 comments

  • eqvinox 47 minutes ago
    The example included below "Why C and C++ Differ" is UB in both C and C++ (and says nothing about "why" C & C++ differ). The article isn't overall wrong but that bit is just...

    (Feels a bit like LLM junk, but honestly not sure.)

    • LoganDark 40 minutes ago
      > A pointer cast worked fine at -O0 and silently broke at -O2.

      Feels like LLM to me. I looked at some of the rest of the article and... pretty much certain now.

  • scoopr 35 minutes ago
    I just watched video[0] that argues if you can consteval something, then it must be free of UB, which seems compelling, at least for these lower level helpers.

    Unfortunately, it seems memcpy is not constexpr, so cannot be used like this, but std::bit_cast actually works[1] as constexpr, so I think in C++ that would now be the most preferred use. It won't allow the union either.

    [0] https://www.youtube.com/watch?v=-LAXqqqX274 [1] https://godbolt.org/z/xGzjTMGvv

  • LelouBil 15 minutes ago
    I don't understand the example in "Why C and C++ Differ".

    Shouldn't the code return 3 in the base case and 4 if the check for 2 was assumed to always hold ?

  • StilesCrisis 21 minutes ago
    The conclusion is simply wrong. `union` should not be used for type-punning in C++, or in C code which might be compiled by a C++ compiler.

    I can't downvote posts yet, but if you have the ability, consider it. This is clearly LLM slop without human review.

  • antiloper 1 hour ago
    > The C vs C++ distinction here is genuinely treacherous

    What's so genuine about this?

    • tialaramex 1 hour ago
      Despite the fact they're two different languages, people are often taught as though it's basically just a subset relationship and then they carry this through into the actual software they write.

      Because neither of these are memory safe languages, you are required to ensure you've made no mistakes or else anything might happen.

    • ndr 56 minutes ago
      It continues with

      > and most blog posts on the topic get it wrong.

      This smells like Claudism/LLMish.

  • pipe01 43 minutes ago
    Is it necessary to use the lowest possible font weight?
  • K0IN 1 hour ago
    Does reinterpret_cast works in c++ for this?
  • jefftk 49 minutes ago
    [flagged]