What is the best way to generate random numbers in C++?

Question

What is the best way to generate random numbers?

Answer 1

If and only if:

• you are not looking for "perfect uniformity" or

• you have no C++11 support and not even TR1 (thus you don't have another choice)

then you might consider using the following C-style solution, which (for the sake of the reputation of this community ~ see rand() Considered Harmful) is written in strike-through font:

Here's the simple C-style function that generates random number from the interval from min to max, inclusive. Those numbers seem to be very close to being uniformly distributed.

int irand(int min, int max) {
    return ((double)rand() / ((double)RAND_MAX + 1.0)) * (max - min + 1) + min;
}

and don't forget to call srand before you use it:

int occurences[8] = {0};

srand(time(0));
for (int i = 0; i < 100000; ++i)
    ++occurences[irand(1,7)];

for (int i = 1; i <= 7; ++i)
    printf("%d ", occurences[i]);

output: 14253 14481 14210 14029 14289 14503 14235

Also have a look at:
Generate a random number within range?
Generate random numbers uniformly over an entire range
and find some time and watch at least first 11 minutes of aforementioned video

Otherwise:

use <random> just like it was pointed out by Kerrek SB already.

Answer 2

You should use <random>:

#include <random>

typedef std::mt19937 rng_type;
std::uniform_int_distribution<rng_type::result_type> udist(0, 7);

rng_type rng;

int main()
{
  // seed rng first:
  rng_type::result_type const seedval = get_seed(); // get this from somewhere
  rng.seed(seedval);

  rng_type::result_type random_number = udist(rng);

  return random_number;
}

Pre C++11 you could find this either in TR1 (<tr1/random>, std::tr1::mt19937 etc.), or in Boost.random, with essentially the same interface (though there are minor differences).