Slides for P3666R3
Bit-precise integers

Document number:
P4157R0
Date:
2026-03-26
Audience:
LEWG
Project:
ISO/IEC 14882 Programming Languages — C++, ISO/IEC JTC1/SC22/WG21
Reply-To:
Jan Schultke <janschultke@gmail.com>
Source:
github.com/Eisenwave/cpp-proposals/blob/master/src/bitint-lewg-slides.cow

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Bit-precise integers
P3666R3

Jan Schultke  |  LEWG Slides for P3666R3 — Bit-precise integers  |  Croydon 2026  |  Slide 1

Introduction

C23 now has _BitInt type for N-bit integers (WG14 N2763, N2775):

// 8-bit unsigned integer initialized with value 255. // The literal suffix wb is unnecessary in this case. unsigned _BitInt(8) x = 0xFFwb;
Jan Schultke  |  LEWG Slides for P3666R3 — Bit-precise integers  |  Croydon 2026  |  Slide 2

P3666 Library decisions to be made

Jan Schultke  |  LEWG Slides for P3666R3 — Bit-precise integers  |  Croydon 2026  |  Slide 3

Anecdote: std::cmp_less

template<typename _Tp, typename _Up> constexpr bool cmp_less(_Tp __t, _Up __u) noexcept { static_assert(__is_signed_or_unsigned_integer<_Tp>::value); static_assert(__is_signed_or_unsigned_integer<_Up>::value); if constexpr (is_signed_v<_Tp> == is_signed_v<_Up>) return __t < __u; else if constexpr (is_signed_v<_Tp>) return __t < 0 || make_unsigned_t<_Tp>(__t) < __u; else return __u >= 0 && __t < make_unsigned_t<_Up>(__u); }
Jan Schultke  |  LEWG Slides for P3666R3 — Bit-precise integers  |  Croydon 2026  |  Slide 4

Alias templates

template<size_t N> using bit_int = _BitInt(N); template<size_t N> using bit_uint = unsigned _BitInt(N);
Jan Schultke  |  LEWG Slides for P3666R3 — Bit-precise integers  |  Croydon 2026  |  Slide 5

std::is_integral_v<_BitInt(N)>

integral types
   ├── signed or unsigned integer types
   │   ├── standard integers: int, unsigned, etc.
   │   ├── extended integers: __int128 etc.
   │   └── bit-precise integers: (unsigned) _BitInt(N)
   ├── character types: char, wchar_t, etc.
   └── bool
Jan Schultke  |  LEWG Slides for P3666R3 — Bit-precise integers  |  Croydon 2026  |  Slide 6