[rand.req.seedseq]

# 29 Numerics library [[numerics]](./#numerics)

## 29.5 Random number generation [[rand]](rand#req.seedseq)

### 29.5.3 Requirements [[rand.req]](rand.req#seedseq)

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

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L1638)

A [*seed sequence*](#def:seed_sequence "29.5.3.2 Seed sequence requirements [rand.req.seedseq]") is an object
 that consumes a sequence
 of integer-valued data
 and produces a requested number
 of unsigned integer values i, 0≤i<232,
 based on the consumed data[.](#1.sentence-1)

[*Note [1](#note-1)*:

Such an object provides a mechanism
 to avoid replication of streams of random variates[.](#1.sentence-2)

This can be useful, for example, in applications
 requiring large numbers of random number engines[.](#1.sentence-3)

— *end note*]

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L1653)

A class S meets the requirements
of a seed sequence
if the expressions shown
in Table [126](#tab:rand.req.seedseq "Table 126: Seed sequence requirements") are valid and have the indicated semantics,
and if S also meets all other requirements
of [rand.req.seedseq][.](#2.sentence-1)

In Table [126](#tab:rand.req.seedseq "Table 126: Seed sequence requirements") and throughout this subclause:

- [(2.1)](#2.1)

T is the type named by S's associated result_type;

- [(2.2)](#2.2)

q is a value of type S and r is a value of type S or const S;

- [(2.3)](#2.3)

ib and ie are input iterators
 with an unsigned integer value_type of at least 32 bits;

- [(2.4)](#2.4)

rb and re are mutable random access iterators
 with an unsigned integer value_type of at least 32 bits;

- [(2.5)](#2.5)

ob is an output iterator;
 and

- [(2.6)](#2.6)

il is a value of type initializer_list<T>[.](#2.sentence-2)

Table [126](#tab:rand.req.seedseq) — Seed sequence requirements [[tab:rand.req.seedseq]](./tab:rand.req.seedseq)  

| [🔗](#tab:rand.req.seedseq-row-1)<br>**Expression** | **Return type** | **Pre/post-condition** | **Complexity** |
| --- | --- | --- | --- |
| [🔗](#tab:rand.req.seedseq-row-2)<br>S​::​result_type | T | T is an [unsigned integer type](basic.fundamental#def:type,unsigned_integer "6.9.2 Fundamental types [basic.fundamental]")  of at least 32 bits[.](#tab:rand.req.seedseq-row-2-column-3-sentence-1) |  |
| [🔗](#tab:rand.req.seedseq-row-3)<br>S() |  | Creates a seed sequence  with the same initial state as all other default-constructed seed sequences  of type S[.](#tab:rand.req.seedseq-row-3-column-3-sentence-1) | constant |
| [🔗](#tab:rand.req.seedseq-row-4)<br>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)[.](#tab:rand.req.seedseq-row-4-column-3-sentence-1) | O(ie−ib) |
| [🔗](#tab:rand.req.seedseq-row-5)<br>S(il) |  | Same as S(il.begin(), il.end())[.](#tab:rand.req.seedseq-row-5-column-3-sentence-1) | same as S(il.begin(), il.end()) |
| [🔗](#tab:rand.req.seedseq-row-6)<br>q.generate(rb,re) | void | Does nothing if rb == re[.](#tab:rand.req.seedseq-row-6-column-3-sentence-1)<br>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[.](#tab:rand.req.seedseq-row-6-column-3-sentence-2) | O(re−rb) |
| [🔗](#tab:rand.req.seedseq-row-7)<br>r.size() | size_t | The number of 32-bit units  that would be copied  by a call to r.param[.](#tab:rand.req.seedseq-row-7-column-3-sentence-1) | constant |
| [🔗](#tab:rand.req.seedseq-row-8)<br>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[.](#tab:rand.req.seedseq-row-8-column-3-sentence-1) | O(r.size()) |