Using random_device in classes

Question

What part of this code would be logical to call in your main{} class, and what part should be used within (for instance) the constructor of a new object (what should I pass as argument(s)), and why? The question is more along the lines of, what is the best way of passing random_device variables to objects (arguments, etc)?

srand ( time ( NULL ) );
std::random_device rd;
std::mt19937 mt(rd());
std::uniform_int_distribution<int> dist(1, 10);

for (int i=0; i<40; ++i)
    std::cout << (int)dist(mt) << " ";
std::cout << endl;

If you use the new PRNG functionality in C++11 you don't need `srand`. — Some programmer dude, Jan 24 '14 at 08:44
As for the question, it's very much a matter of design and individual preference, and it might change depending on what fits best in a certain project or even situation within a single project. — Some programmer dude, Jan 24 '14 at 08:48
What you're doing in the third line is seeding `mt19937` with a random value from `rd`. That replaces the `srand` step. — Joseph Mansfield, Jan 24 '14 at 08:53

score 2 · Accepted Answer · answered Jan 24 '14 at 09:20

If you are asking how to make the pseudo random number generator object available to all classes, you can use an externally defined global variable (like std::cout), or depending on your needs, a global in anonymous namespace, or a singleton.

However the safest and easiest method I found so far is to create one instance per thread. Here's how I do it:

std::mt19937 &mt()
{
  // initialize once per thread
  thread_local static std::random_device srd;
  thread_local static std::mt19937 smt(srd());
  return smt;  
}

Now each thread has its own random number generator.

Note: for visual studio, you can try using boost's thread_specific_ptr or wrap the random_device and mersenne twister in a class and use visual c++ TLS.

Thank you, I am going to research this. – staxas Jan 24 '14 at 10:38 — staxas, Jan 24 '14 at 10:38

Using random_device in classes

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