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.

// 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.

// 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++, 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 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 often use /dev/random (which can block) or /dev/urandom. Both have their advantages.