C++ named requirements: RandomNumberEngine (since C++11)

< cpp‎ | named req
C++ named requirements
Type properties
Container Elements

Stream I/O
Random Numbers


A random number engine is a function object returning unsigned integer values such that each value in the range of possible results has (ideally) equal probability.

Any random number engine is also a UniformRandomBitGenerator, and therefore may be plugged into any random number distribution in order to obtain a random number (formally, a random variate).

[edit] Requirements

A type satisfies RandomNumberEngine if it satisfies UniformRandomBitGenerator and, given the following types and values, the semantic and complexity requirements in the table below are satisfied:

Type Definition
E a RandomNumberEngine type
T E::result_type
Value Definition
e a value of type E
v an lvalue of type E
x, y values of type (possibly const-qualified) E
s a value of type T
q a SeedSequence lvalue
z a value of type unsigned long long
os an lvalue whose type is a specialization of std::basic_ostream
is an lvalue whose type is a specialization of std::basic_istream
n the size of E's state
TA the transition algorithm of E
GA the generation algorithm of E
Expression Return type Semantics Complexity
E() N/A Creates an engine with the same initial state as all other default-constructed engines of type E. O(n)
E(x) Creates an engine that compares equal to x. O(n)
E(s) Creates an engine whose initial state is determined by s. O(n)
E(q) Creates an engine whose initial state is determined by a single call to q.generate. same as the complexity of q.generate called on a sequence whose length is n
e.seed() void Postcondition: e == E(). same as E()
e.seed(s) void Postcondition: e == E(s). same as E(s)
e.seed(q) void Postcondition: e == E(q). same as E(q)
e() T Advances e’s state from e
to e
(i.e. TA(e
) and returns GA(e
amortized constant
e.discard(z) void Advances e’s state from e
to e
by any means equivalent to z consecutive calls of e().
no worse than the complexity of z consecutive calls of e()
x == y bool For all positive integer i, if the ith consecutive calls of x() and y() return the same value, returns true. Otherwise returns false. O(n)
x != y bool !(x == y) O(n)
os << x decltype(os)&  With fmtflags set to std::ios_base::dec | std::ios_base::left and the fill character set to the space character, writes to os the textual representation of x's current state.

Postcondition: os's fmtflags and the fill character are the same as before.

is >> v decltype(is)& With fmtflags set to std::ios_base::dec, reads from is the textual representation of v's current state. If bad input is encountered, ensures that v’s state is unchanged by the operation and calls is.setstate(std::ios_base::failbit) (which may throw std::ios_base::failure).

Precondition: is provides a textual representation that was previously written using an output stream pr satisfying all following conditions:

  • is.getloc() == pr.getloc() is true.
  • std::is_same<decltype(is)::char_type,
    is true.
  • std::is_same<decltype(is)::traits_type,
    is true.

Postcondition: is's fmtflags are the same as before.


[edit] Standard library

The following standard library facilities satisfy RandomNumberEngine:

implements linear congruential algorithm
(class template) [edit]
implements Mersenne twister algorithm
(class template) [edit]
implements a subtract-with-carry (lagged Fibonacci) algorithm
(class template) [edit]
discards some output of a random number engine
(class template) [edit]
packs the output of a random number engine into blocks of a specified number of bits
(class template) [edit]
delivers the output of a random number engine in a different order
(class template) [edit]

The following standard library facilities satisfy UniformRandomBitGenerator but not RandomNumberEngine:

non-deterministic random number generator using hardware entropy source
(class) [edit]