---
title: Slides for P3666R3Bit-precise integers
document: P4157R0
date: 2026-03-26
audience: LEWG
reply-to:
  - "Jan Schultke < <janschultke@gmail.com>"
paper-type: proposal
---

# 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

=

0x

FF

wb

;

- implemented by GCC and Clang; max: `_BitInt(8'388'608)`
- forwarded by EWG to LEWG/CWG for C++29

Jan Schultke
 | 
LEWG Slides for P3666R3
—
Bit-precise integers
 | 
Croydon 2026
 | 
Slide 2

## P3666 Library decisions to be made

- How much library support to provide?
  - Aggressively minimal: just the core feature
  - Minimal but useful: P3666R3
  - More extensive: `<simd>`, `<atomic>`, etc.
- Add `std::bit_int` and `std::bit_uint` alias templates?
- Is `std::is_integral_v<_BitInt(N)>` true?

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

)

;

- deduction is supported
- library spec should prefer alias templates over keyword spelling

Jan Schultke
 | 
LEWG Slides for P3666R3
—
Bit-precise integers
 | 
Croydon 2026
 | 
Slide 5

## std::is_integral_v<_BitInt(N)>

```
int
```

- mostly matches C2y taxonomy
- *very surprising* if `std::is_integral_v<_BitInt(N)>` was `false`
- huge wording/teaching effort to treat `_BitInt(N)` specially

Jan Schultke
 | 
LEWG Slides for P3666R3
—
Bit-precise integers
 | 
Croydon 2026
 | 
Slide 6