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?

Is this question just how to define/initialize static data members? Could you post a SSCCE? — dyp
– dyp, Commented Mar 18, 2014 at 19:52
I'm trying to implement the same functionality as the first block of code (which generates a random number for that member each time an object is created) using C++11 random generators and engines. I have no idea how to do so, the static data members are just my attempt, but it doesn't compile. I included it as a sort of pseudocode to show my intention. — JamesLens
– JamesLens, Commented Mar 18, 2014 at 19:56

Praetorian · Accepted Answer · 2014-03-18 19:53:39Z

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

TIL: name lookup in brace-or-equal-initializers for NSDM is the same as name lookup inside member function definitions (it's postponed until the end of the class definition).
I would encapsulate the static data members into a static member function so as to avoid the requirement for separate out-of-class definitions, like so. Make the linker do the work ;)
Awesome, that did work. Thanks a lot, static members always confuse me with definitions vs declarations. Edit: Thanks Casey, that is some sexy code.

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