# 28 Numerics library [numerics]

## 28.5 Random number generation [rand]

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

#### 28.5.5.2 Class template discard_block_engine[rand.adapt.disc]

A discard_block_engine random number engine adaptor produces random numbers selected from those produced by some base engine e.
The state x of a discard_block_engine engine adaptor object x consists of the state e of its base engine e and an additional integer n.
The size of the state is the size of e's state plus 1.
The transition algorithm discards all but values from each block of p  ≥ r values delivered by e.
The state transition is performed as follows: If n  ≥ r, advance the state of e from e to e and set n to 0.
In any case, then increment n and advance e's then-current state e to e.
The generation algorithm yields the value returned by the last invocation of e() while advancing e's state as described above.
namespace std { template<class Engine, size_t p, size_t r> class discard_block_engine { public: // types using result_type = typename Engine::result_type; // engine characteristics static constexpr size_t block_size = p; static constexpr size_t used_block = r; static constexpr result_type min() { return Engine::min(); } static constexpr result_type max() { return Engine::max(); } // constructors and seeding functions discard_block_engine(); explicit discard_block_engine(const Engine& e); explicit discard_block_engine(Engine&& e); explicit discard_block_engine(result_type s); template<class Sseq> explicit discard_block_engine(Sseq& q); void seed(); void seed(result_type s); template<class Sseq> void seed(Sseq& q); // equality operators friend bool operator==(const discard_block_engine& x, const discard_block_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 discard_block_engine& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, discard_block_engine& x); private: Engine e; // exposition only size_t n; // exposition only }; }
The following relations shall hold: 0 < r and r <= p.
The textual representation consists of the textual representation of e followed by the value of n.
In addition to its behavior pursuant to subclause [rand.req.adapt], each constructor that is not a copy constructor sets n to 0.