Random number generators (RNGs) are indispensable in programming for tasks ranging from simulations and gaming to cryptography. Yet, the “randomness” they produce isn’t truly random. In this discussion, we’ll explore how RNGs function in C++, spotlight different types of RNGs, and explain why they can’t achieve complete randomness.
Understanding Random Number Generators
RNGs in programming are algorithms designed to produce numbers that appear random by lacking predictable patterns. In C++, RNGs are facilitated through the <random>
library, offering various engines and distributions that cater to different needs.
Popular RNG Engines in C++

Mersenne Twister (
std::mt19937
): 
Linear Congruential Generator (
std::minstd_rand0
): 
Cryptographically Secure RNGs (
std::random_device
):
How RNGs Work in C++
Here’s a quick example of using the Mersenne Twister in C++ to generate a random number within a specific range:
#include <random>
#include <iostream>
int main() {
std::mt19937 rng(std::random_device{}());
std::uniform_int_distribution<std::mt19937::result_type> dist6(1,6); // distribution in range [1, 6]
std::cout << dist6(rng) << std::endl; // generate a pseudorandom number
return 0;
}
This example uses std::random_device
to seed the Mersenne Twister, which then generates a pseudorandom number between 1 and 6.
The Myth of True Randomness
Despite their utility, the numbers produced by RNGs in C++ are pseudorandom because they stem from deterministic processes. Given the same seed, algorithms like the Mersenne Twister will reproduce the same sequence of numbers.
Why They Aren’t Truly Random:

Deterministic Algorithms: All standard RNGs in C++ are deterministic at their core.

Seed Dependence: The output is entirely contingent upon the seed value; identical seeds yield identical outputs.
Implications and Best Practices
For most applications, pseudorandom numbers suffice. However, in securitysensitive environments, where unpredictability is paramount, relying on basic RNGs can introduce vulnerabilities.
Enhancing Randomness:

Use Cryptographic RNGs: For critical security applications, use RNGs that are specifically designed for cryptography.

Mix Sources: Combining RNG outputs with highentropy environmental data can enhance randomness.
Conclusion
While RNGs like the Mersenne Twister serve well for general purposes in C++, their limitations in predictability necessitate careful consideration in securityfocused applications. Understanding the different types of RNGs available in C++ helps developers choose the right tool for the right job, ensuring both functionality and security are upheld.