Random Number In-Class Initialisation

Question

I am currently making a class for which I'd like one of the private members to be initialised with a random number each time the object is created. The following code causes no problem:

private:
    unsigned random = rand() % 10;

I would, however, like to use the C++11 random engines and distributions to do this. I would like to be able to do something along the lines of the following code, which will not compile but will give a general idea of what I'm trying to do:

private:
unsigned random = distribution(mersenne_generator(seed));

static std::random_device seed_generator;
static unsigned seed = seed_generator(); //So that it's not a new seed each time.
static std::mt19937 mersenne_generator;
static std::uniform_int_distribution<unsigned> distribution(0, 10);

This code won't compile because I'm trying to define some of the static members in the class. I'm not sure where to define them, however. I could create a member function that initialises everything, but then I would have to run it in main, which I don't want to. I would like to just sort out all the random definitions in class so that when I create an object in main, it will create the random number implicitly. Any suggestions?

Answer 1

You need to define the static data members outside the class definition. For instance, this will work:

struct foo
{
private:
    unsigned random = distribution(mersenne_generator);    
    static std::random_device seed_generator;
    static unsigned seed;
    static std::mt19937 mersenne_generator;
    static std::uniform_int_distribution<unsigned> distribution;
};

std::random_device foo::seed_generator;
unsigned foo::seed = seed_generator();
std::uniform_int_distribution<unsigned> foo::distribution(0, 10);
std::mt19937 foo::mersenne_generator(foo::seed);

The definitions for the static data members should be placed in a .cpp file, else you run the risk of violating the one definition rule.

Live example