Question

I’d like to multithread my rng part of my code using C++11.

I create a bunch of RNG’s like this:

typedef std::mt19937 mersenne_twister;
typedef std::uniform_real_distribution<double> unidist;

// 8 rng engines with 8 consecutive seeds
const size_t Nrng = 8;
const array<uint32_t,Nrng> seed_values = {0,1,2,3,4,5,6,7};
array<mersenne_twister,Nrng> engines;
const array<unidist,Nrng> distributions; // default constructor is for [0,1] interval

const array<????,Nrng> rngs; // What type do I put here?

for( size_t i=0; i<Nrng; ++i )
    engines[i].seed(seed_values[i]);
    rngs[i] = std::bind(distributions[i], engines[i]);

With the idea I can later pass each of the RNG objects to a seperate std::thread that fills another array with some form of random numbers using the supplied generator. I don’t know what type I need to use for the rng array. I know how to create one rng, like so:

auto rng = std::bind(unidist, generator);

but I need to explicitely have a (decltyped?) type for the array.

Answer 1

One possible type would be std::function<double()>, but note that this doesn’t come free. The actual type of the result of bind is unknowable, and so you cannot just make a container with element type decltype(std::bind(...)), since the types are not guaranteed to be mutually convertible — all you know is that they all convert to function.

If this is performance-critical, I suggest you profile and compare the cost of std::function to the direct use of unidist(rngs[i]) in your target code. (The distribution is a stateless adapter that should be reentrant anyway, so you don’t need to keep more than one copy of it.)