8.15 — Global random numbers (Random.h)

What happens if we want to use a random number generator in multiple functions or files? One way is to create (and seed) our PRNG in our main() function, and then pass it everywhere we need it. But that’s a lot of passing for something we may only use sporadically, and in many different places. It would add a lot of clutter to our code to pass such an object around.

Alternately, you could create a static local std::mt19937 variable in each function that needs it (static so that it only gets seeded once). However, it’s overkill to have every function that uses a random number generator define and seed its own local generator, and the low volume of calls to each generator may lead to lower quality results.

What we really want is a single PRNG object that we can share and access anywhere, across all of our functions and files. The best option here is to create a global random number generator object (inside a namespace!). Remember how we told you to avoid non-const global variables? This is an exception.

Here’s a simple, header-only solution that you can #include in any code file that needs access to a randomized, self-seeded std::mt19937:

Random.h:

#ifndef RANDOM_MT_H
#define RANDOM_MT_H

#include <chrono>
#include <random>

// This header-only Random namespace implements a self-seeding Mersenne Twister.
// Requires C++17 or newer.
// It can be #included into as many code files as needed (The inline keyword avoids ODR violations)
// Freely redistributable, courtesy of learncpp.com (https://www.learncpp.com/cpp-tutorial/global-random-numbers-random-h/)
namespace Random
{
	// Returns a seeded Mersenne Twister
	// Note: we'd prefer to return a std::seed_seq (to initialize a std::mt19937), but std::seed can't be copied, so it can't be returned by value.
	// Instead, we'll create a std::mt19937, seed it, and then return the std::mt19937 (which can be copied).
	inline std::mt19937 generate()
	{
		std::random_device rd{};

		// Create seed_seq with clock and 7 random numbers from std::random_device
		std::seed_seq ss{
			static_cast<std::seed_seq::result_type>(std::chrono::steady_clock::now().time_since_epoch().count()),
				rd(), rd(), rd(), rd(), rd(), rd(), rd() };

		return std::mt19937{ ss };
	}

	// Here's our global std::mt19937 object.
	// The inline keyword means we only have one global instance for our whole program.
	inline std::mt19937 mt{ generate() }; // generates a seeded std::mt19937 and copies it into our global object

	// Generate a random int between [min, max] (inclusive)
	inline int get(int min, int max)
	{
		return std::uniform_int_distribution{min, max}(mt);
	}

	// The following function templates can be used to generate random numbers
	// when min and/or max are not type int
	// See https://www.learncpp.com/cpp-tutorial/function-template-instantiation/
	// You can ignore these if you don't understand them

	// Generate a random value between [min, max] (inclusive)
	// * min and max have same type
	// * Return value has same type as min and max
	// * Supported types:
	// *    short, int, long, long long
	// *    unsigned short, unsigned int, unsigned long, or unsigned long long
	// Sample call: Random::get(1L, 6L);             // returns long
	// Sample call: Random::get(1u, 6u);             // returns unsigned int
	template <typename T>
	T get(T min, T max)
	{
		return std::uniform_int_distribution<T>{min, max}(mt);
	}

	// Generate a random value between [min, max] (inclusive)
	// * min and max can have different types
	// * Must explicitly specify return type as template type argument
	// * min and max will be converted to the return type
	// Sample call: Random::get<std::size_t>(0, 6);  // returns std::size_t
	// Sample call: Random::get<std::size_t>(0, 6u); // returns std::size_t
	// Sample call: Random::get<std::int>(0, 6u);    // returns int
	template <typename R, typename S, typename T>
	R get(S min, T max)
	{
		return get<R>(static_cast<R>(min), static_cast<R>(max));
	}
}

#endif

And a sample program showing how it is used:

main.cpp:

#include "Random.h" // defines Random::mt, Random::get(), and Random::generate()
#include <cstddef> // for std::size_t
#include <iostream>

int main()
{
	// We can use Random::get() to generate random numbers

	std::cout << Random::get(1, 6) << '\n';   // returns int between 1 and 6
	std::cout << Random::get(1u, 6u) << '\n'; // returns unsigned int between 1 and 6

	// The following uses a template type argument
	// See https://www.learncpp.com/cpp-tutorial/function-template-instantiation/
	std::cout << Random::get<std::size_t>(1, 6u) << '\n'; // returns std::size_t between 1 and 6

	// We can access Random::mt directly if we have our own distribution

	// Let's create a reusable random number generator that generates uniform numbers between 1 and 6
	std::uniform_int_distribution die6{ 1, 6 }; // for C++14, use std::uniform_int_distribution<> die6{ 1, 6 };
	for (int count{ 1 }; count <= 10; ++count)
	{
		std::cout << die6(Random::mt) << '\t'; // generate a roll of the die here
	}

	std::cout << '\n';

	return 0;
}

Normally, defining variables and functions in a header file would cause violations of the one-definition rule (ODR) when that header file was included into more than one source file. However, we’ve made our mt variable and supporting functions inline, which allows us to have duplicate definitions without violating the ODR so long as those definitions are all identical. Because we’re #including those definitions from a header file (rather than typing them manually, or copy/pasting them), we can ensure they are identical. Inline functions and variables were added to the language largely to make doing this kind of header-only functionality possible.

The other challenge that we have to overcome is in how we initialize our global Random::mt object, as we want it to be self-seeding so that we don’t have to remember to explicitly call an initialization function for it to work correctly. Our initializer must be an expression. But in order to initialize a std::mt19937, we need several helper objects (a std::random_device and a std::seed_seq) which must be defined as statements. This is where a helper function comes in handy. A function call is an expression, so we can use the return value of a function as an initializer. And inside the function itself, we can have any combination of statements that we need. Thus, our generate() function creates and returns a fully-seeded std::mt19937 object (seeded using both the system clock and std::random_device) that we use as the initializer to our global Random::mt object.

Once “Random.h” has been included, we can use it in one of two ways:

  • We can call Random::get() to generate a random number between two values (inclusive).
  • We can access the std::mt19937 object directly via Random::mt and do whatever we want with it.
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