Proper copy of RNG in C++

Question

I have a class containing a RNG:

class Sampler{    
private:
    std::random_device rd_;
    std::mt19937 gen_;

public:
    Sampler();
    double sample();
};

The class is initialized as follows:

Sampler::Sampler() :
                   gen_(rd_())
{}

My problem is that I want to use this class (the member function sample()) inside another class but I cannot copy the class because std::random_device cannot be copied (the copy constructor is deleted since otherwise the same random numbers would be generated).

class Foo{
private:
    Sampler s_;

public:
    Foo(Sampler);

};

And then:

Foo::Foo(Sampler s) :
        s_(s) // does not work
{}

While this makes total sense I wonder how one would implement this in order to make it work. I've read that one opportunity would be to construct a proper copy constructor such that every time a copy is made, it gets seeded with a new seed but I don't know how to do this.

You need another random numbers generated in `Foo` that in `Sampler` instance? If not - why not store reference/pointer(smart pointer)? — ForEveR, Feb 02 '17 at 09:24
Why is `random_device` even a member? You only need it while the ctor is executing, so why are you keeping it alive? — MSalters, Feb 02 '17 at 09:28
You're right, so you mean I could also write: `std::mt19937 gen_(std::random_device())`? — beginneR, Feb 02 '17 at 09:48

Jonas · Accepted Answer · 2017-04-21T10:23:59.530

You could just remove the std::random_device member variable and use temporary std::random_device when constructing std::mt19937 gen_, like this:

#include <iostream>
#include <random>

class Sampler{    
private:
    std::mt19937 gen_;

public:
    Sampler();
    double sample();
};

Sampler::Sampler() : gen_(std::random_device{}()) {}

class Foo{
private:
    Sampler s_;

public:
    Foo(Sampler);
};

Foo::Foo(Sampler s) : s_(s) {} // Does work

Try it here.

Proper copy of RNG in C++

1 Answers1