srand

Because the Mt19937 (“Mersenne Twister”) engine accepts only a single 32 bit integer as the seed, the number of possible random sequences is limited to just 2³² (i.e. 4,294,967,296), despite Mt19937’s huge period of 2¹⁹⁹³⁷-1.

When relying on either implicit or explicit random seeding, duplications will appear much earlier. Duplicated seeds are expected with 50% probability after less than 80,000 randomly generated seeds according to the birthday problem. A 10% probability of a duplicated seed happens after randomly generating roughly 30,000 seeds.

This makes Mt19937 unsuitable for applications where duplicated sequences must not happen with more than a negligible probability. If reproducible seeding is required, both the Random\Engine\Xoshiro256StarStar and Random\Engine\PcgOneseq128XslRr64 engines support much larger seeds that are unlikely to collide randomly. If reproducibility is not required, the Random\Engine\Secure engine provides cryptographically secure randomness.