c++ - Flatbuffers VectorIterator::operator- undefined behavior - Stack Overflow

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I was digging into code of flatbuffers and it does seem to me that there is undefined behavior in the core library. But I just can't believe it is there (and it seems to work in some usecases)...

I'd highly appreciate if somebody verifies my way of thinking/shows me that I missed something :)

.h#L80

  difference_type operator-(const VectorIterator &other) const {
    return (data_ - other.data_) / element_stride;
  }

My rationale, why I think it is UB:

difference_type == std::ptrdiff_t which is based on standard Signed ["something" >= 16b] ()

Let's expect that the first parenthesis end up negative (which should be valid usecase)

1. Now when I assume the ptrdiff_t is <= size_t:

   • We get to:

     • ptrdiff_t retval = (uint8_t* - uint8_t*)/size_t

   • pointer arithmetics also end up as ptrdiff_t. So we are at:

     • ptrdiff_t retval = ptrdiff_t/size_t

   • This leads me to:

     • Signed = Signed/Unsigned

   • And based on standard (8/11.5.3) we get the first operand of divison promoted to Unsigned: The standard snippet

     • the result will end up as "something positive and huge" as the negative number got promoted to huge unsigned one

2. Now lets assume ptrdiff_t is > size_t

   • pretty much same steps apply, except the last one:
     • based on standard (8/11.5.4), the second operand of division get promoted to signed number of bigger bitsize
     • this leads to return value to be (the intuitive) negative number that says distance between the two VectorIterators.

So I see there 2 possible outcomes of the operator-, both complying with the standard requirements, based on ptrdiff_t implementation by specific compiler.

Thanks!

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