# 28 Numerics library [numerics]

## 28.5 Random number generation [rand]

### 28.5.3 Requirements [rand.req]

#### 28.5.3.2 Seed sequence requirements [rand.req.seedseq]

A seed sequence is an object that consumes a sequence of integer-valued data and produces a requested number of unsigned integer values i, , based on the consumed data.
[Note 1:
Such an object provides a mechanism to avoid replication of streams of random variates.
This can be useful, for example, in applications requiring large numbers of random number engines.
— end note]
A class S meets the requirements of a seed sequence if the expressions shown in Table 98 are valid and have the indicated semantics, and if S also meets all other requirements of this subclause [rand.req.seedseq].
In that Table and throughout this subclause:
• T is the type named by S's associated result_type;
• q is a value of type S and r is a value of type S or const S;
• ib and ie are input iterators with an unsigned integer value_type of at least 32 bits;
• rb and re are mutable random access iterators with an unsigned integer value_type of at least 32 bits;
• ob is an output iterator; and
• il is a value of type initializer_list<T>.
Table 98: Seed sequence requirements [tab:rand.req.seedseq]
 🔗 Expression Return type Pre/post-condition Complexity 🔗 S​::​result_type T T is an unsigned integer type of at least 32 bits. compile-time 🔗 S() Creates a seed sequence with the same initial state as all other default-constructed seed sequences of type S. constant 🔗 S(ib,ie) Creates a seed sequence having internal state that depends on some or all of the bits of the supplied sequence [ib,ie). O(ie−ib) 🔗 S(il) Same as S(il.begin(), il.end()). same as S(il.begin(), il.end()) 🔗 q.generate(rb,re) void Does nothing if rb == re. Otherwise, fills the supplied sequence [rb,re) with 32-bit quantities that depend on the sequence supplied to the constructor and possibly also depend on the history of generate's previous invocations. O(re−rb) 🔗 r.size() size_t The number of 32-bit units that would be copied by a call to r.param. constant 🔗 r.param(ob) void Copies to the given destination a sequence of 32-bit units that can be provided to the constructor of a second object of type S, and that would reproduce in that second object a state indistinguishable from the state of the first object. O(r.size())