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3809. Is std::subtract_with_carry_engine<uint16_t> supposed to work?

Section: 29.5.4.4 [rand.eng.sub] Status: WP Submitter: Jonathan Wakely Opened: 2022-11-02 Last modified: 2023-11-22

Priority: 3

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Discussion:

The standard requires subtract_with_carry_engine<T> to use:

linear_congruential_engine<T, 40014u, 0u, 2147483563u>

where each of those values is converted to T.

This appears to mean subtract_with_carry_engine cannot be used with uint16_t, because 2147483563u cannot be converted to uint16_t without narrowing.

What is the intention here? Should it be ill-formed? Should the seed engine be linear_congruential_engine<uint_least32_t, …> instead? The values from the linear_congruential_engine are used modulo 2^32 so getting 64-bit values from it is pointless, and getting 16-bit values from it doesn't compile.

[Kona 2022-11-12; Set priority to 3]

[Kona 2022-11-25; Jonathan provides wording]

[2023-05; reflector poll]

Status to Tentatively Ready after six votes in favour.

[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify the class synopsis in 29.5.4.4 [rand.eng.sub] as indicated:

   namespace std {
     template<class UIntType, size_t w, size_t s, size_t r>
     class subtract_with_carry_engine {
     public:
       // types
       using result_type = UIntType;
       // engine characteristics
       static constexpr size_t word_size = w;
       static constexpr size_t short_lag = s;
       static constexpr size_t long_lag = r;
       static constexpr result_type min() { return 0; }
       static constexpr result_type max() { return m − 1; }
       static constexpr result_typeuint_least32_t default_seed = 19780503u;
       // constructors and seeding functions
       subtract_with_carry_engine() : subtract_with_carry_engine(default_seed0u) {}
       explicit subtract_with_carry_engine(result_type value);
       template<class Sseq> explicit subtract_with_carry_engine(Sseq& q);
       void seed(result_type value = default_seed0u);
       template<class Sseq> void seed(Sseq& q);
   

2. Modify 29.5.4.4 [rand.eng.sub] p7 as indicated:

   explicit subtract_with_carry_engine(result_type value);

   -7- Effects: Sets the values of $X_{-r},\dots ,X_{-1}$, in that order, as specified below. If $X_{-1}$ is then $0$, sets $c$ to $1$; otherwise sets $c$ to $0$.

        To set the values $X_k$, first construct e, a linear_congruential_engine object, as if by the following definition:

   linear_congruential_engine<result_typeuint_least32_t,
                              40014u,0u,2147483563u> e(value == 0u ? default_seed : value);
   

        Then, to set each $X_k$, obtain new values $z_0,\dots ,z_{n-1}$ from $n=\lceil w⁄32\rceil$ successive invocations of e. Set $X_k$ to $({\sum }_{j=0}^{n-1}{z}_j\bullet 2^{32j})modm$.