# <random>

Defines many random number generators.

For a list of all members of this header, see <random> Members.

```
#include <random>
```

## Remarks

A random number generator is an object that produces a sequence of pseudo-random values. A generator that produces values uniformly distributed in a specified range is an engine. An engine can be combined with a distribution, either by passing the engine as an argument to the distribution's operator() or by using a variate_generator Class, to produce values that are distributed in a manner that is defined by the distribution.

Most of the random number generators are templates whose parameters customize the generator. The descriptions of generators that take a type as an argument use common template parameter names to describe the properties of the type that are permitted as an actual argument type, as follows:

IntType indicates a signed or unsigned integral type.

UIntType indicates an unsigned integral type.

RealType indicates a floating-point type.

An engine is a TR1 class or template class whose instances act as a source of random numbers uniformly distributed between a minimum and maximum value. An engine can be a simple engine or a compound engine. Every engine has the following members:

typedef numeric-type result_type is the type that is returned by the generator's operator().

result_type min() returns the minimum value that is returned by the generator's operator().

result_type max() returns the maximum value that is returned by the generator's operator(). When result_type is an integral type, this is the maximum value that can actually be returned; when result_type is a floating-point type, this is the smallest value greater than all values that can be returned.

void seed() The seed function initializes the engine with default seed values.

template <class InIt> void seed(InIt& first, InIt last) The seed function seeds the engine by using values of type unsigned long from the half-open sequence that is pointed to by [first, last). If the sequence is not long enough to fully initialize the engine, the function stores the value last in first and throws an object of type std::invalid_argument.

Note

Only engines retained for TR1 compatibility include this member.

result_type operator()() returns values that are uniformly distributed between min() and max().

min, max, and result_type are not described in detail for the engines that follow.

Beginning with Visual Studio 2010, every engine except those retained for TR1 compatibility also includes the following members:

An explicit constructor with argument result_type x0 that creates an engine seeded as if by calling seed(x0).

An explicit constructor with argument seed_seq& seq that creates an engine seeded as if by calling seed(seq).

void seed(result_type x0) The seed function seeds the engine with seed value x0.

void seed(seed_seq& seq) The seed function seeds the engine with seed values from seq.

void discard(unsigned long long count) effectively calls operator() count times and discards each value.

Every engine has a state that determines the sequence of values that will be generated by subsequent calls to operator(). The states of two objects of the same type can be compared by using operator== and operator!=. If the two states compare as equal, the objects will generate the same sequence of values. The state of an object can be saved to a stream as a sequence of 32-bit unsigned values by using the operator<< of the object. The state is not changed by saving it. A saved state can be read into an object of the same type by using operator>>.

A simple engine is an engine that produces random numbers directly. This library provides one class whose objects are simple engines. It also provides four class templates that can be instantiated by using values that provide parameters for the algorithm they implement, and nine predefined instances of those class templates. Objects of these types are also simple engines.

A compound engine is an engine that obtains random numbers from one or more simple engines and generates a stream of uniformly distributed random numbers by using those values. This library provides class templates for two compound engines.

A distribution is a TR1 class or template class whose instances transform a stream of uniformly distributed random numbers obtained from an engine into a stream of random numbers that have a particular distribution. Every distribution has the following members:

typedef numeric-type input_type is the type that should be returned by the engine passed to operator().

typedef numeric-type result_type is the type that is returned by the distribution's operator().

void reset() discards any cached values, so that the result of the next call to operator() does not depend on any values obtained from the engine before the call.

template <class Engine> result_type operator()(Engine& eng) returns values that are distributed according to the distribution's definition, by using eng as a source of uniformly distributed random values and the stored parameter package.

input_type, result_type, and reset are not described in detail for the distributions that follow.

Beginning with Visual Studio 2010, every distribution also has:

typedef unspecified-type param_type is the package of parameters passed to operator() to generate its return value.

A const param& constructor initializes the stored parameter package from its argument.

param_type param() const gets the stored parameter package.

void param(const param_type&) sets the stored parameter package from its argument.

template <class Engine> result_type operator()(Engine& eng, param_type par0) returns values distributed in accordance with the distribution's definition, using eng as a source of uniformly distributed random values and the parameter package par0.

A parameter package is an object that stores all the parameters needed for a distribution. It contains:

typedef distribution-type distribution_type is the type of its distribution.

One or more constructors that take the same parameter lists as the distribution constructors take.

The same parameter-access functions as the distribution.

Equality and inequality comparison operators.

The library can be built as a checked version and as an unchecked version. The checked version uses a macro similar to C's assert macro to test the conditions marked as Precondition in the functional descriptions. To use the checked version, define either the macro _RNG_CHECK or the macro _DEBUG to a non-zero numeric value in all code that uses the library.