Using same random number generator across multiple functions

I am given to believe that random number generators (RNGs) should only be seeded once to ensure that the distribution of results is as intended.

I am writing a Monte Carlo simulation in C++ which consists of a main function ("A") calling another function ("B") several times, where a large quantity of random numbers is generated in B.

Currently, I am doing the following in B:

void B(){
    std::array<int, std::mt19937::state_size> seed_data;
    std::random_device r;

    std::generate(seed_data.begin(), seed_data.end(), std::ref(r));
    std::seed_seq seq(std::begin(seed_data), std::end(seed_data)); //perform warmup
    std::mt19937 eng(seq);

    std::uniform_real_distribution<> randU(0,1);

    double myRandNum = randU(eng);

    //do stuff with my random number
}

As you can see, I am creating a new random number generator each time I call the function B. This, as far as I can see, is a waste of time - the RNG can still generate a lot more random numbers!

I have experimented with making "eng" extern but this generates an error using g++:

error: ‘eng’ has both ‘extern’ and initializer extern std::mt19937 eng(seq);

How can I make the random number generator "global" so that I can use it many times?