Document Number: P3248R0.
Date: 2024-04-17.
Reply to: Gonzalo Brito Gadeschi <gonzalob _at_ nvidia.com>.
Authors: Gonzalo Brito Gadeschi.
Audience: SG1, SG22, EWG, LEWG.

Require [u]intptr_t

Motivation

Proposals like P2835 and P3125 require an integer type capable of holding a pointer value in their APIs. However, [u]intptr_t being optional forces sub-optimal design choices such as making APIs optional or introducing workarounds.

The potential absence of [u]intptr_t compromises the portability of high-level software and attempts to address this introduce software engineering overheads and potential portability bugs, as seen in libvlc PR#1519.

This proposal advocates for requiring [u]intptr_t in C++ to ensure that all C++ code can rely on integer types capable of holding a pointer value.

Implementation Experience

A survey found almost unanimous support for [u]intptr_t in C++:

Platforms that do not provide [u]intptr_t types today would be impacted:

Therefore, we conclude that:

Design

Design alternatives:

  1. C++ requires [u]intptr_t.
  2. C++ adds new integer types - different from [u]intptr_t - capable of holding a pointer value.
  3. Do nothing.

This proposal advocates for Option 1, i.e., for C++ to require [u]intptr_t, because:

Usage Guideline

[u]intptr_t is well suited for C++ language or C++ Standard Library APIs that need an integer type capable of holding a pointer value, i.e., an integer type with a lossless conversion from/to pointer.

Some features or APIs may only need an integer type capable of holding a pointer address. C and C++ do not provide an integer type suited for this use case, but some standard libraries do, e.g., glibc's <stddef.h> header ptraddr_t on Arm CHERI.

Wording changes

Modify [cstdint.syn]:

  1. The header <cstdint> supplies integer types having specified widths, and macros that specify limits of integer types.

// all freestanding
namespace std {
  using int8_t         = signed integer type;   // optional
  using int16_t        = signed integer type;   // optional
  using int32_t        = signed integer type;   // optional
  using int64_t        = signed integer type;   // optional
  using intN_t         = see below;             // optional

  using int_fast8_t    = signed integer type;
  using int_fast16_t   = signed integer type;
  using int_fast32_t   = signed integer type;
  using int_fast64_t   = signed integer type;
  using int_fastN_t    = see below;             // optional

  using int_least8_t   = signed integer type;
  using int_least16_t  = signed integer type;
  using int_least32_t  = signed integer type;
  using int_least64_t  = signed integer type;
  using int_leastN_t   = see below;             // optional

  using intmax_t       = signed integer type;
  using intptr_t       = signed integer type;   // optional

  using uint8_t        = unsigned integer type; // optional
  using uint16_t       = unsigned integer type; // optional
  using uint32_t       = unsigned integer type; // optional
  using uint64_t       = unsigned integer type; // optional
  using uintN_t        = see below;             // optional

  using uint_fast8_t   = unsigned integer type;
  using uint_fast16_t  = unsigned integer type;
  using uint_fast32_t  = unsigned integer type;
  using uint_fast64_t  = unsigned integer type;
  using uint_fastN_t   = see below;             // optional

  using uint_least8_t  = unsigned integer type;
  using uint_least16_t = unsigned integer type;
  using uint_least32_t = unsigned integer type;
  using uint_least64_t = unsigned integer type;
  using uint_leastN_t  = see below;             // optional

  using uintmax_t      = unsigned integer type;
  using uintptr_t      = unsigned integer type; // optional
}

#define INTN_MIN         see below
#define INTN_MAX         see below
#define UINTN_MAX        see below

#define INT_FASTN_MIN    see below
#define INT_FASTN_MAX    see below
#define UINT_FASTN_MAX   see below

#define INT_LEASTN_MIN   see below
#define INT_LEASTN_MAX   see below
#define UINT_LEASTN_MAX  see below

#define INTMAX_MIN       see below
#define INTMAX_MAX       see below
#define UINTMAX_MAX      see below

#define INTPTR_MIN       see below              // optional
#define INTPTR_MAX       see below              // optional
#define UINTPTR_MAX      see below              // optional

#define PTRDIFF_MIN      see below
#define PTRDIFF_MAX      see below
#define SIZE_MAX         see below

#define SIG_ATOMIC_MIN   see below
#define SIG_ATOMIC_MAX   see below

#define WCHAR_MIN        see below
#define WCHAR_MAX        see below

#define WINT_MIN         see below
#define WINT_MAX         see below

#define INTN_C(value)    see below
#define UINTN_C(value)   see below
#define INTMAX_C(value)  see below
#define UINTMAX_C(value) see below

  1. The header defines all types and macros the same as the C standard library header <stdint.h> except that the types intptr_t and uintptr_t and the macros INTPTR_MIN, INTPTR_MAX, and UINTPTR_MAX are always defined and are not optional. See also: ISO/IEC 9899:2018, 7.20.

  2. All types that use the placeholder N are optional when N is not 8, 16, 32, or 64. The exact-width types intN_t and uintN_t for N = 8, 16, 32, and 64 are also optional; however, if an implementation defines integer types with the corresponding width and no padding bits, it defines the corresponding typedef-names. Each of the macros listed in this subclause is defined if and only if the implementation defines the corresponding typedef-name.
    [Note 1: The macros INTN_C and UINTN_C correspond to the typedef-names int_leastN_t and uint_leastN_t, respectively. — end note]