P1272 Byteswapping for fun&&nuf

[jensmaurer]

P1272R0 Byteswapping for fun&&nuf (Isabella Muerte)

https://issues.isocpp.org/show_bug.cgi?id=560

[jensmaurer]

Titus Winters 2018-11-10 04:17:10 UTC
Discussed in San Diego: http://wiki.edg.com/bin/view/Wg21sandiego2018/P1272

This goes into <bit>.
SF F N A SA
2 6 3 0 0

Require that bit-width is a power-of-two, forward to LWG for C++20.
SF F N A SA
3 8 0 0 0

[mclow]

I'm going to schedule this in LWG.

[jensmaurer]

@mclow, this might need some CWG attention (at your discretion), similar to bit_cast. For example, what about padding bits in the object representation of an integer?

[mclow]

Interesting idea. Do we have any integral types that do not "completely fill" an even number of bytes? (bool and char/unsigned char, yes, I think we can just call disable them)

[jensmaurer]

@mclow, note that "byte" equals (the size of) char/unsigned char in the C++ standard by definition (and might not be related to an octet, i.e. an 8-bit quantity); see [intro.memory].
For example, an implementation with 8-bit chars and 31-bit integers would be conforming. With the byte-swap feature, you'd presumably read one padding bit (unspecified value) and turn it into a value bit. Should we prevent that?
My view: Yes, let's make sure the facility does what it should do on today's architectures, and let's just remove the funny stuff from the overload set so that you get a controlled error on alien platforms.

[Lawrence-Crowl]

SG6: Leave the issue to other committees.
No objection to unanimous consent.