Random engine state and multiple deterministic independent random sequences - c++

Question

The C++ TR1 random number generation scheme has improved the old C runtime library in terms of keeping a separate state for random engines in different threads, or for independent random sequences. The old library has a global state machine, and this is usually bad.
However, when implementing an algorithm that requires deterministic random sequences, I find it annoying to have to pass the engine down to any method that should be drawing numbers from such a sequence. From a design perspective, the code that initializes the random seed doesn't need to know which methods down the stack are using random numbers. Yet those inner methods cannot initialize their own random engines, because:
they lack the knowledge to create a unique reproducible seed
memory requirements prevent keeping a separate state for the many downstream clients
To clarify, the downstream methods do not need to draw numbers from the same sequence as the main method, but they do need to be independent and reproducible in different runs.
Any idea on how to solve this conundrum elegantly?
EDIT
Some code to clarify the situation
typedef std::mt19937 RandEng;
class PossibleRandomConsumer;
class RandomProvider {
public:
void foo() {
std::uniform_int<> uni;
uni(eng, 17); // using the random engine myself
std::for_each(children.begin(), children.end(), [](PossibleRandomConsumer& child) {
// may or may not need a random number. if it does, it has to be different than from other children, and from other providers
child.DoSomething(eng);
});
}
private:
RandEng eng; // unique seed per RandomProvider
std::array<PossibleRandomConsumer,10000> children; // lots of these...
};

Answer 1

Not an easy question without knowing some detail about your architecture. So this is just a try:
How about passing down references to an interface that knows how to provide random numbers. This interface may just have one function that returns an amount of random numbers if asked. In this way you can hide the implementation details from the downstream functions (methods) and passing constant references around is nearly for free. You can even decide at top level where those random numbers come from (file, random number generator, your room temperature, ...).
class RandomNumberProvider {
public:
typedef std::vector<int> RandomArray;
virtual RandomArray Generate(unsigned numNumbers) = 0;
};
void Consumer(const RandomNumberProvider& rndProvider) {
RandomNumberProvider::RandomArray rndNumber;
rndNumber = rndProvider(10); // get a sequence of 10 random numbers
...
}
something like this.