[SOLVED] How computationally expensive is generating a random number in C++?

How computationally expensive is generating a random number in C++?

The method that I am considering is from an answer to Generating random integer from a range.

#include <random>

std::random_device rd;     // only used once to initialise (seed) engine
std::mt19937 rng(rd());    // random-number engine used (Mersenne-Twister in this case)
std::uniform_int_distribution<int> uni(min,max); // guaranteed unbiased

auto random_integer = uni(rng);

I'm also willing to use the rand() approach with srand(time(NULL)).

How expensive are these approaches? Is one much faster than the other?

Solution

Performance depends greatly on the generator you use (which in turn depends greatly on the quality of the numbers you need).

For example, std::mt19937 is much faster than std::random_device, but it produces pseudo random numbers. This is fine for most purposes, if you don't need cryptographically safe random numbers. But even if you do, random_device can produce raw entropy at a rate of about 50 MB/sec on my machine—how much randomness do you really need? (mt19937 generates about orders of magnitudes more than that if needed).

Avoid rand(). It just has very poor properties and a very low period.