C++: Random Number Generation Using <random>

Note: Please note that the random header has undergone substantial change for C++11 and later. This post is outdated and only meant for older versions of C++. For the latest, please refer to this page.

C++ can get very irritating if all you want is a simple solution. Take random number generation for example. The <random> header was added to C++ to provide all kinds of random number generation. While it does cover any kind of random generation you can dream of, it makes writing a simple random generator super-hard! Unlike C or C#, where you can quickly generate random numbers with little code, in C++ it takes a lot of reading, pondering and lots of ugly-looking code.


The <random> header file provides many kinds of classes:

  • Engine: It generates random numbers based on a certain algorithm. There are many engines defined in <random>. One example is linear_congruential_engine, which uses the algorithm that is typically used in the C rand() random number generator. A engine cannot be used alone, it should be attached to a distribution (see below).
  • Predefined Engine: It is an engine created with some good-quality initial values. This saves a lot of the pain of coming up with values to initialize the engine. There are many predefined engines in <random>. For example, minstd_rand is pre-defined on the linear_congruential_engine.
  • Distribution: It takes the output of an engine and shapes it to fit a certain distribution. An example is uniform_real_distribution, which creates an uniform distribution of real numbers. A distribution needs an engine to provide it with random number input.
  • Variate Generator: variate_generator is a wrapper to combine an engine (or predefined engine) with a distribution, to create a custom random number generator.

Simple Random Number Generation

Here is a simple random number generator that produces random float values. It uses the variate_generator to wrap a minstd_rand engine and uniform_real_distribution.

#include <random>
using namespace std;

typedef minstd_rand                             Engine;
typedef uniform_real_distribution<float>        Distribution;
typedef variate_generator<Engine, Distribution> Generator;

Generator gen( Engine(9999), Distribution( 10, 100 ) );

cout << "Random value: " << gen() << endl;

The minstd_rand engine requires a seed, here we seed it with 9999. We also provide the uniform_real_distribution an output range of [10.0,100.0).

Simple Random Generator Class

If all I want is to generate some random number simply and quickly, the C++ <random> looks truly foreboding! šŸ˜¦

Here is a simple random number generator class that wraps up the complexity to provide a simple interface:

#include <random>
using namespace std;

class MyRandom
	// Types
	typedef minstd_rand                             Engine;
	typedef uniform_real_distribution<float>        Distribution;
	typedef variate_generator<Engine, Distribution> Generator;

	// To hold the generator
	Generator* _gen;

	MyRandom( int seed, float minVal, float maxVal )
		_gen = new Generator( Engine( seed ), Distribution( minVal, maxVal ) );

		delete _gen;

	float next()
		return (*_gen)();

The above class interface is influenced by C# Random, which is how simple and accessible C++ should have been! Writing code using the above class is easy:

const int seed		= 9999;
const float minVal	= 10;
const float maxVal	= 100;
MyRandom myRand( seed, minVal, maxVal );
cout << "Random value: " << myRand.next() << endl;

I hope this post made digesting <random> a wee bit easier! šŸ™‚

7 thoughts on “C++: Random Number Generation Using <random>”

      1. Oh, yeah I notice the date only after my comment. šŸ˜‰
        Also the code doesn’t work with C++11 anymore, since there is no ‘variate_generator’, but it was still helpful after all. šŸ™‚


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s