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### 2716. Specification of `shuffle` and `sample` disallows lvalue `URNG`s

Section: 27.7.12 [alg.random.sample], 27.7.13 [alg.random.shuffle] Status: C++17 Submitter: Tim Song Opened: 2016-05-24 Last modified: 2017-07-30

Priority: 0

View all issues with C++17 status.

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 (28.5.3.3 [rand.req.urng]) type".

28.5.3.3 [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 27.7.12 [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 27.7.13 [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`.