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https://github.com/tbxfreeware/seed_randomly

Seed a C++ random number engine with random seeds from std:random_device. Works with any engine that can be seeded with a seed sequence, including those in the C++ Standard Library.
https://github.com/tbxfreeware/seed_randomly

cpp14 cpp14-library mt19937 random-number-generators seeder

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Seed a C++ random number engine with random seeds from std:random_device. Works with any engine that can be seeded with a seed sequence, including those in the C++ Standard Library.

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README 

        
# seed_randomly

Header `tbx.cpp14.seed_randomly.h` is a single-file "library" that provides tools for seeding a C++ random number engine. It works with any engine that can be seeded with a seed sequence, including all of the random number engines in the C++ Standard Library. It generates seeds using `std:random_device`.



This header uses only the features of C++14, nothing later.



 

## A good choice for `mt19937`

The tools here are particularly useful for seeding `std::mt19937` and `std::19937_64`. Those engines have 19,968 bits of state, so trying to seed them with a single seed of only 32 or 64 bits is borderline foolish. The functions in this header supply a full 19,968 bits.



Using one of the methods described in the following two sections is far superior to seeding with something like `std::time(nullptr)` or `std::chrono::high_resolution_clock::now().time_since_epoch().count()`, which, respectively, have only 32 and 64 bits of entropy.



 

## seed_randomly

This function takes a random number engine as argument, and seeds it with random seeds generated from `std::random_device`.

````cpp

// Example: Seed `mt19937`.

std::mt19937 mt;

tbx::seed_randomly( mt );



// Example: Seed `pcg32`, one of the PCG engines by Melissa O'Neill.

pcg32 e;

tbx::seed_randomly( e );

````



 

## class seed_seq_rd

This class mimics the interface of `std::seed_seq`, but uses `std::random_device` to generate seeds. Objects of this type are seed sequences that can be used as arguments to member function `seed` in a random number engine.



A `seed_seq_rd` object can also be used as an argument to the constructor of a random number engine, and anywhere else that a `std:seed_seq` object can be used.



Function `seed_randomly` is a trivial wrapper around a `seed_seq_rd` object.

````cpp

// Example: Seed `mt19937` with random seeds from `std::random_device`.

tbx::seed_seq_rd s;

std::mt19937 mt{ s };



// Example: Seed `mt19937_64` with random seeds from `std::random_device`.

tbx::seed_seq_rd s;

std::mt19937_64 mt64;

mt64.seed( s );



// Example: Seed `pcg32`, one of the PCG engines by Melissa O'Neill.

pcg32 e;

e.seed( s );  // `seed_seq_rd` object can be reused.



// Example: Implementation of `seed_randomly`:

template< typename RandomNumberEngine >

void seed_randomly( RandomNumberEngine& e );

{

    tbx::seed_seq_rd s;

    e.seed( s );

}

````



 

## Check `random_device`

Before using this library, you should satisfy yourself that `std::random_device` is a good source of entropy on your system. Sometimes, it is not.



[Microsoft Visual C++](https://learn.microsoft.com/en-us/cpp/standard-library/random-device-class?view=msvc-170), for instance, generates "non-deterministic and cryptographically secure" values, and never blocks, which is excellent. Prior to version 9.2, however, [MinGW distributions of GCC](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85494) used `std::mt19937` with a fixed seed! Those systems generated the same sequence every time. (Newer versions purport to have fixed the problem, but I have not checked.) [Unix-like systems](https://en.wikipedia.org/wiki//dev/random) often use `/dev/random` (which can block) or `/dev/urandom`. Both have their advantages.