# 28 Numerics library [numerics]

## 28.5 Random number generation [rand]

### 28.5.5 Random number engine adaptor class templates [rand.adapt]

#### 28.5.5.4 Class template shuffle_order_engine[rand.adapt.shuf]

A shuffle_order_engine random number engine adaptor produces the same random numbers that are produced by some base engine e, but delivers them in a different sequence.
The state x of a shuffle_order_engine engine adaptor object x consists of the state e of its base engine e, an additional value Y of the type delivered by e, and an additional sequence V of k values also of the type delivered by e.
The size of the state is the size of e's state plus .
The transition algorithm permutes the values produced by e.
The state transition is performed as follows:
• Calculate an integer .
• Set Y to and then set to e().
The generation algorithm yields the last value of Y produced while advancing e's state as described above.
namespace std { template<class Engine, size_t k> class shuffle_order_engine { public: // types using result_type = typename Engine::result_type; // engine characteristics static constexpr size_t table_size = k; static constexpr result_type min() { return Engine::min(); } static constexpr result_type max() { return Engine::max(); } // constructors and seeding functions shuffle_order_engine(); explicit shuffle_order_engine(const Engine& e); explicit shuffle_order_engine(Engine&& e); explicit shuffle_order_engine(result_type s); template<class Sseq> explicit shuffle_order_engine(Sseq& q); void seed(); void seed(result_type s); template<class Sseq> void seed(Sseq& q); // equality operators friend bool operator==(const shuffle_order_engine& x, const shuffle_order_engine& y); // generating functions result_type operator()(); void discard(unsigned long long z); // property functions const Engine& base() const noexcept { return e; } // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const shuffle_order_engine& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, shuffle_order_engine& x); private: Engine e; // exposition only result_type V[k]; // exposition only result_type Y; // exposition only }; }
The following relation shall hold: 0 < k.
The textual representation consists of the textual representation of e, followed by the k values of V, followed by the value of Y.
In addition to its behavior pursuant to subclause [rand.req.adapt], each constructor that is not a copy constructor initializes V[0], …, V[k-1] and Y, in that order, with values returned by successive invocations of e().