4 ms·

What else can std visit generate? I’d imagine it does exactly that

• ynik 6y ago
  std::variant is implemented with variadic templates; but C++ doesn't have a "variadic switch" statement. So std::visit cannot directly use a switch statement. Instead it's usually implemented as a recursive function with the hope that the compiler will unroll the recursion and optimize it into a switch. This hope usually doesn't work out in practice.
  • steerablesafe 6y ago
    AFAIK the usual implementation is statically generating an array of function pointers and dispatching based on the type discriminator. This works even less for inlining.
    • sesuximo 6y ago
      On my phone so cannot check, but I thought this was mostly a solved problem? Inlining/folding a jump table should be easy enough especially since the table can be const
      • steerablesafe 6y ago
        https://godbolt.org/z/d4838K https://godbolt.org/z/d4838K Doesn't happen, apparently. edit: For a short while libc++ adopted mpark's variant visit, which AFAIK compiles down to switch case with some hand coding. AFAIK it had other problems so they rolled back to the constexpr array of function pointers. https://github.com/mpark/variant https://github.com/mpark/variant
        • sesuximo 6y ago
          If you call foo with an int or with a double, then it gets inlined. I don’t think they can inline foo as you’ve written it because the variant could be valueless by exception.
    • sesuximo 6y ago
      I just looked into this a bit more. For the simple examples I tried, clang and gcc inline + const prop calls to std::visit with O2.