On Sat, 08 Jun 2019 11:46:10 +0200 Arne Babenhauserheide <arne_...@web.de> wrote: > Chris Vine <vine35792...@gmail.com> writes: > > On Sat, 08 Jun 2019 10:07:45 +0200 > > Arne Babenhauserheide <arne_...@web.de> wrote: > > [snip] > >> Wow, I didn’t know that you could do that. > >> > >> However: "The details of that allocation are implementation-defined, and > >> it's undefined behavior to read from the member of the union that wasn't > >> most recently written." https://en.cppreference.com/w/cpp/language/union > >> > >> Can you guarantee that this works? > > > > This is C and not C++ and the provision to which you refer does not > > apply. > > > > Reading from a member of a union other than the one last written to is > > implementation defined in C89/90, and defined in C99 (with Technical > > Corrigendum 3) and above > > Thank you for the correction and explanation!
You have a good point though if visible type transformations were to appear in a header rather than a *.c file, because guile headers are (at the moment) intended to be in the common subset of C and C++ so that libguile.h can be included in a C++ programme. Having said that, gcc and clang support type punning through unions in C++ as well as C. I don't know if guile is supposed to compile with other compilers nowadays: but frankly it would be perverse for some other compiler which supports both C and C++ to invoke different behaviour for unions in such cases. Chris