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2716. Specification of shuffle and sample disallows lvalue URNGs

Section: 26.7.12 [alg.random.sample], 26.7.13 [alg.random.shuffle] Status: C++17 Submitter: Tim Song Opened: 2016-05-24 Last modified: 2017-07-30

Priority: 0

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Discussion:

std::shuffle and std::sample each accepts a UniformRandomNumberGenerator&& argument and says that "UniformRandomNumberGenerator shall meet the requirements of a uniform random number generator (29.5.3.3 [rand.req.urng]) type".

29.5.3.3 [rand.req.urng]/2 in turn starts with "A class G satisfies the requirements of a uniform random number generator if […]".

If an lvalue is passed, UniformRandomNumberGenerator is a reference type, not a class, and in fact expressions like G::min() will not compile if G is a reference type.

[2016-06 Oulu]

Moved to P0/Ready during issues prioritization.

Friday: status to Immediate

Proposed resolution:

This wording is relative to N4582.

  1. Edit 26.7.12 [alg.random.sample]/1 as indicated:

    template<class PopulationIterator, class SampleIterator,
         class Distance, class UniformRandomNumberGenerator>
  SampleIterator sample(PopulationIterator first, PopulationIterator last,
                        SampleIterator out, Distance n,
                        UniformRandomNumberGenerator&& g);

    -1- Requires::

    1. […]
    2. (1.6) — remove_reference_t<UniformRandomNumberGenerator> shall meet the requirements of a uniform random number generator type (26.6.1.3) whose return type is convertible to Distance.
    3. […]

  2. Edit 26.7.13 [alg.random.shuffle]/2 as indicated:

    template<class RandomAccessIterator, class UniformRandomNumberGenerator>
  void shuffle(RandomAccessIterator first,
               RandomAccessIterator last,
               UniformRandomNumberGenerator&& g);

    -1- […]

    -2- Requires: RandomAccessIterator shall satisfy the requirements of ValueSwappable (17.6.3.2). The type remove_reference_t<UniformRandomNumberGenerator> shall meet the requirements of a uniform random number generator (26.6.1.3) type whose return type is convertible to iterator_traits<RandomAccessIterator>::difference_type.