P0330R2
Literal Suffixes for ptrdiff_t and size_t

1. Revision History

1.1. Revision 0 - November 21st, 2014

Initial release; published as N4254.

Published as P0330R0; summarized LEWG’s view re N4254; dropped the proposed suffix for ptrdiff_t; changed the proposed suffix for size_t to zu; added survey of existing literal suffixes.

1.2. Revision 1 - October 12th, 2017

Published as P0330R1; expanded the survey of existing literals. Synced the proposed wording with the Working Draft WG21/N4687. Moved the reference implementation from BitBucket to GitHub.

1.3. Revision 2 - October 1st, 2018

Published as P0330R2; change reply-to and point of contact from Rein Halbersma to JeanHeyd Meneide, who revitalized paper according to feedback from Rein Halbersma, and all of LWG. Overwhelming consensus for it to be a Language Feature instead, proposal rewritten as Language proposal with wording against [n4762].

2. Motivation

Consider this very simple code to print an index and its value:

std::vector<int> v{0, 1, 2, 3};
for (auto i = 0; i < v.size(); ++i) {
	std::cout << i << ": " << v[i] << '\n';
}

This code can lead to the following warnings:

main.cpp: In function 'int main()':
main.cpp:warning: comparison of integer expressions 
of different signedness: 'int' and 'long unsigned int' [-Wsign-compare]
    for (auto i = 0; i < v.size(); ++i) {
                     ~~^~~~~~~~~~

It grows worse if a user wants to cache the size rather than query it per-iteration:

std::vector<int> v{0, 1, 2, 3};
for (auto i = 0, s = v.size(); i < s; ++i) {
	/* use both i and v[i] */
}

Resulting in a hard compiler error:

main.cpp: In function 'int main()':
main.cpp:8:10: error: inconsistent deduction 
for 'auto': 'int' and then 'long unsigned int'
     for (auto i = 0, s = v.size(); i < s; ++i) {
          ^~~~

This paper proposes adding a zu literal suffix that deduces literals to size_t, making the following warning-free:

for (auto i = 0zu; i < v.size(); ++i) {
	std::cout << i << ": " << v[i] << '\n';
}

More generally:

• int is the default type deduced from integer literals without suffix;

• comparisons, signs or conversion ranks with integers can lead to surprising results;

• size_t -- and less frequently, ptrdiff_t -- are nearly impossible to avoid in the standard library for element access or .size() members;

• programmer intent and stability is hard to communicate portably with the current set of literals;

• surprises range from (pedantic) compiler errors to undefined behavior;

• existing integer suffixes (such as ul) are not a general solution, e.g. when switching compilation between 32-bit and 64-bit on common architectures;

• and, C-style casts and static_casts are verbose.

3. Design

Following the feedback from §7 Feedback on Revisions, we have dropped the std::support_literals User-Defined Literals and chose a Core Language Literal Suffix. We opine that it would better serve the needs of addressing the motivation.

As a language feature, the design of the suffixes becomes much simpler. The core language only has one format for its integer literal suffixes: the letter(s), with an optional u on either side of the letter(s) to make it unsigned, with the signed variant being the default on most architectures. This ruled out using s and sz, because that would produce an inconsistent set of suffixes with the rest of the language.

The literal suffixes z and uz/zu were chosen to represent signed and unsigned, respectively. decltype(0z) will yield ptrdiff_t and decltype(0uz)/decltype(0zu) will yield size_t. Like other case-insensitive language literal suffixes, it will accept both Z and z (and U and u alongside of it). This follows the current convention of the core language to be able to place u and z in any order / any case for the suffix.

3.1. What about the fixed/least/max (unsigned) int types?

This paper does not propose suffixes for the fixed size, at-least size, and max size integral types in the standard library or the language. This paper is focusing exclusively on ptrdiff_t and size_t. We have also been made aware of another paper which may handle this separately and considers all the design space necessary for such.

4. Impact on the Standard

This feature is purely an extension of the language and has, to the best of our knowledge, no conflict with existing or currently proposed features. z is currently not a literal suffix in the language.

5. Proposed wording and Feature Test Macros

The following wording is relative to [n4762].

5.1. Proposed feature Test Macro

The recommended feature test macro is __cpp_ptrdiff_t_suffix.

5.2. Intent

The intent of this paper is to propose 2 language suffixes for integral literals of specific types. One is for ptrdiff_t, one is for size_t. We follow the conventions set out for other literals in the standard. We define the suffix to produce types size_t and ptrdiff_t similar to how §5.13.7 Pointer Literals [lex.nullptr] introduces std::nullptr_t.

5.3. Proposed Wording

Modify §5.13.2 Integer Literals [lex.icon] with additional suffixes:

integer-suffix:
 unsigned-suffix long-suffix_opt
 unsigned-suffix long-long-suffix_opt
 unsigned-suffix ptrdiff-suffix_opt
 long-suffix unsigned-suffix_opt
 long-long-suffix unsigned-suffix_opt
  ptrdiff-suffix unsigned-suffix_opt

unsigned-suffix: one of
 u U

long-suffix: one of
 l L

long-long-suffix: one of
 ll LL

ptrdiff-suffix: one of
 z Z

Append to §5.13.2 Integer Literals [lex.icon]'s Table 7 two additional entries:

Suffix	Decimal literal	Binary, octal, or hexadecimal literal
z or Z	ptrdiff_t	ptrdiff_t
Both u or U and z or Z	size_t	size_t

Append to §14.8.1 Predefined macro names [cpp.predefined]'s Table 16 with one additional entry:

Macro name	Value
__cpp_ptrdiff_t_suffix	201811L

6. Acknowledgements

Thank you to Rein Halbersma, who started this paper and put in the necessary work for r0 and r1. Thank you to Walter E. Brown, who acted as locum on this paper before the Committee twice and gave us valuable feedback on wording. Thank you to Lounge<C++>'s Cicada for encouraging us to write this paper. Thank you to Hubert Tong for giving us a few pointers on where in the Core Language to modify things for such a paper. Thank you to Tim Song for wording advice.

We appreciate your guidance as we learn to be a better Committee member and represent the C++ community’s needs more more efficiently and effectively in the coming months.

7. Feedback on Revisions

Polls are in the form Strongly in Favor | Favor | Neutral | Against | Strongly Against. The polls on Revision 1 were as follows, from LWG at the WG21 Albuquerque meeting.

Proposal as presented, i.e., are we OK with the library solution going forward?

0 | 6 | 5 | 7 | 4

We translated this as strong discouragement to pursue this feature as a set of user-defined literals. A second poll was taken.

Do we want to solve this problem with a language feature?

2 | 15 | 0 | 2 | 2

We considered this overwhelming consensus for it to be a language feature instead, culminating in this paper after much feedback.