[rand.util.seedseq]

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

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

### 29.5.8 Utilities [[rand.util]](rand.util#seedseq)

#### 29.5.8.1 Class seed_seq [rand.util.seedseq]

[ð](#lib:seed_seq)

namespace std {class seed_seq {public:// typesusing result_type = uint_least32_t; // constructors seed_seq() noexcept; template<class T> seed_seq(initializer_list<T> il); template<class InputIterator> seed_seq(InputIterator begin, InputIterator end); // generating functionstemplate<class RandomAccessIterator>void generate(RandomAccessIterator begin, RandomAccessIterator end); // property functions size_t size() const noexcept; template<class OutputIterator>void param(OutputIterator dest) const; // no copy functions seed_seq(const seed_seq&) = delete; void operator=(const seed_seq&) = delete; private: vector<result_type> v; // *exposition only*};}

[ð](#lib:seed_seq,constructor)

`seed_seq() noexcept;
`

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4198)

*Postconditions*: v.empty() is true[.](#1.sentence-1)

[ð](#lib:seed_seq,constructor_)

`template<class T>
  seed_seq(initializer_list<T> il);
`

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4211)

*Constraints*: T is an integer type[.](#2.sentence-1)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4215)

*Effects*: Same as seed_seq(il.begin(), il.end())[.](#3.sentence-1)

[ð](#lib:seed_seq,constructor__)

`template<class InputIterator>
  seed_seq(InputIterator begin, InputIterator end);
`

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4228)

*Mandates*: iterator_traits<InputIterator>​::​value_type is an integer type[.](#4.sentence-1)

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4233)

*Preconditions*: InputIterator meets the [*Cpp17InputIterator*](input.iterators#:Cpp17InputIterator "24.3.5.3 Input iterators [input.iterators]") requirements ([[input.iterators]](input.iterators "24.3.5.3 Input iterators"))[.](#5.sentence-1)

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4238)

*Effects*: Initializes v by the following algorithm:for (InputIterator s = begin; s != end; ++s) v.push_back((*s)mod232);

[ð](#lib:generate,seed_seq)

`template<class RandomAccessIterator>
  void generate(RandomAccessIterator begin, RandomAccessIterator end);
`

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4255)

*Mandates*: iterator_traits<RandomAccessIterator>​::​​value_type is an unsigned integer type capable of accommodating 32-bit quantities[.](#7.sentence-1)

[8](#8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4260)

*Preconditions*: RandomAccessIterator meets the [*Cpp17RandomAccessIterator*](random.access.iterators#:Cpp17RandomAccessIterator "24.3.5.7 Random access iterators [random.access.iterators]") requirements ([[random.access.iterators]](random.access.iterators "24.3.5.7 Random access iterators"))
 and the requirements of a mutable iterator[.](#8.sentence-1)

[9](#9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4266)

*Effects*: Does nothing if begin == end[.](#9.sentence-1)

Otherwise,
 with s=v.size() and n=endâbegin,
 fills the supplied range [begin,end) according to the following algorithm
 in which
 each operation is to be carried out modulo 232,
 each indexing operator applied to begin is to be taken modulo n,
 and T(x) is defined as x xor(x rshift27):

- [(9.1)](#9.1)

  By way of initialization,
 set each element of the range to the value 0x8b8b8b8b[.](#9.1.sentence-1)
  Additionally,
 for use in subsequent steps,
 let p=(nât)/2 and let q=p+t,
 where   
t=(nâ¥623)Â ?Â 11Â :Â (nâ¥68)Â ?Â 7Â :Â (nâ¥39)Â ?Â 5Â :Â (nâ¥7)Â ?Â 3Â :Â (nâ1)/2;

- [(9.2)](#9.2)

  With m as the larger of s+1 and n,
 transform the elements of the range:
 iteratively for k=0,â¦,mâ1,
 calculate values   
r1=1664525âT(begin[k]xorbegin[k+p]xorbegin[kâ1])r2=r1+â§âªâ¨âªâ©s,Â k=0kmodn+v[kâ1],Â 0<kâ¤skmodn,Â s<k and, in order,
 increment begin[k+p] by r1,
 increment begin[k+q] by r2,
 and
 set begin[k] to r2[.](#9.2.sentence-1)

- [(9.3)](#9.3)

  Transform the elements of the range again,
 beginning where the previous step ended:
 iteratively for k=m,â¦,m+nâ1,
 calculate values   
r3=1566083941âT(begin[k]+begin[k+p]+begin[kâ1])r4=r3â(kmodn) and, in order,
 update begin[k+p] by xoring it with r3,
 update begin[k+q] by xoring it with r4,
 and
 set begin[k] to r4[.](#9.3.sentence-1)

[10](#10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4339)

*Throws*: What and when RandomAccessIterator operations of begin and end throw[.](#10.sentence-1)

[ð](#lib:size,seed_seq)

`size_t size() const noexcept;
`

[11](#11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4351)

*Returns*: The number of 32-bit units
 that would be returned
 by a call to param()[.](#11.sentence-1)

[12](#12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4357)

*Complexity*: Constant time[.](#12.sentence-1)

[ð](#lib:param,seed_seq)

`template<class OutputIterator>
  void param(OutputIterator dest) const;
`

[13](#13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4369)

*Mandates*: Values of type result_type are writable ([[iterator.requirements.general]](iterator.requirements.general "24.3.1 General")) to dest[.](#13.sentence-1)

[14](#14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4373)

*Preconditions*: OutputIterator meets the [*Cpp17OutputIterator*](output.iterators#:Cpp17OutputIterator "24.3.5.4 Output iterators [output.iterators]") requirements ([[output.iterators]](output.iterators "24.3.5.4 Output iterators"))[.](#14.sentence-1)

[15](#15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4378)

*Effects*: Copies the sequence of prepared 32-bit units
 to the given destination,
 as if by executing the following statement:copy(v.begin(), v.end(), dest);

[16](#16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4387)

*Throws*: What and when OutputIterator operations of dest throw[.](#16.sentence-1)
