Why is this random number generator generating same numbers?

Question

The first one works, but the second one always returns the same value. Why would this happen and how am I supposed to fix this?

int main() {
    std::random_device rd;
    std::mt19937 gen(rd());
    std::uniform_real_distribution<> dis(0, 1);

    for(int i = 0; i < 10; i++) {

        std::cout << dis(gen) << std::endl;
    }return 0;
}

The one dosen't work:

double generateRandomNumber() {
    std::random_device rd;
    std::mt19937 gen(rd());
    std::uniform_real_distribution<> dis(0, 1);

    return dis(gen);
}



int main() {
    for(int i = 0; i < 10; i++) {
        std::cout << generateRandomNumber() << std::endl;
    }return 0;
}

Answer 1

What platform are you working on? std::random_device is allowed to be a pseudo-RNG if hardware or OS functionality to generate random numbers doesn't exist. It might initialize using the current time, in which case the intervals at which you're calling it might be too close apart for the 'current time' to take on another value.

Nevertheless, as mentioned in the comments, it is not meant to be used this way. A simple fix will be to declare rd and gen as static. A proper fix would be to move the initialization of the RNG out of the function that requires the random numbers, so it can also be used by other functions that require random numbers.