write_json(msg, buffer): number of chars written?

[akm76]

I'm trying to figure out if there is a way to know how many characters write_json(msg, buffer)
has written to an oversized buffer shared with network stack.
I know I could do write_json to a string and ask for it's size(), but that would be inefficient (several extra copies). Also calculating "expected" size of msg isn't great, many variable length strings, arrays and fp numbers.

So what would be the right way to get serialized char count out?

[stephenberry]

Under the hood of glz::write calls it is doing this:

size_t ix = 0; // overwrite index
to<Opts.format, std::remove_cvref_t<T>>::template op<Opts>(std::forward<T>(value), ctx, buffer, ix);
if constexpr (resizable<Buffer>) {
   buffer.resize(ix);
}

So, you can use this API yourself to figure out how many bytes have been written with ix

But, if you pass it an oversized raw buffer there is already an API that hands you back the number of bytes written:

template <auto Opts, class T, raw_buffer Buffer>
  requires write_supported<T, Opts.format>
[[nodiscard]] glz::expected<size_t, error_ctx> write(T&& value, Buffer&& buffer, is_context auto&& ctx)
{
  size_t ix = 0;
  to<Opts.format, std::remove_cvref_t<T>>::template op<Opts>(std::forward<T>(value), ctx, buffer, ix);
  if (bool(ctx.error)) [[unlikely]] {
     return glz::unexpected(error_ctx{ctx.error});
  }
  return {ix};
}