On Fri, 30 Jan 2026 at 15:25, Patrick Palka <[email protected]> wrote: > > On Thu, 29 Jan 2026, Jonathan Wakely wrote: > > > > I wondered about replacing _M_bytes[7] with a union like: > > > > union { > > unsigned char _M_bytes[7]; > > struct { // Used by restore_rep_count frame > > unsigned char _M_rep_count : 2; > > }; > > struct { // Used by restore_cur_results frame > > unsigned char _M_matched : 1; > > unsigned char _M_restore_first : 1; > > }; > > }; > > > > So that we use more precise names than just _M_bytes[0] (and could > > still extend it later with other alternatives if needed). > > > > This might be over-complicating things, but if we *did* want to do it, > > I think we should decide now because the difference between setting > > _M_bytes[0] = 0xff and setting _M_restore_first = true would not be > > compatible if we changed it later (old TUs might expect to find 0xff > > there and new TUs would only set one bit). > > I think slightly more flexible would be: > > union { > unsigned char _M_byte0; > struct { // Used by restore_rep_count frame > unsigned char _M_rep_count : 2; > }; > struct { // Used by restore_cur_results frame > unsigned char _M_matched : 1; > unsigned char _M_restore_first : 1; > }; > }; > unsigned char _M_bytes[6]; > > So that we could decide how to use each byte independently of the other > (e.g. maybe one byte could be for universal flags that apply to every > code).
Do we want to value-init the _M_byte0 member in the constructor, so that we know all the bit-fields are initially zero? I think we can use a NSDMI for union members even in C++11, so this should work: unsigned char _M_byte0 = 0;
