# 26 Numerics library [numerics]

## 26.5 Random number generation [rand]

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

#### 26.5.4.2 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 xi of a discard_block_engine engine adaptor object x consists of the state ei 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 r > 0 values from each block of pr values delivered by e. The state transition is performed as follows: If nr, advance the state of e from ei to ei+p-r and set n to 0. In any case, then increment n and advance e's then-current state ej to ej+1.

The generation algorithm yields the value returned by the last invocation of e() while advancing e's state as described above.

```template<class Engine, size_t p, size_t r>
class discard_block_engine{
public:
// types
typedef typename Engine::result_type 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);

// generating functions
result_type operator()();
void discard(unsigned long long z);

// property functions
const Engine& base() const noexcept { return e; };

private:
Engine e;   // exposition only
int 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 section [rand.req.adapt], each constructor that is not a copy constructor sets n to 0.