Should I use a random engine seeded from std::random_device or use std::random_device every time

Question

I have a class that contains two sources of randomness.

std::random_device rd; std::mt19937 random_engine; 

I seed the std::mt19937 with a call to std::random_device. If I want to generate a number and I don't care about repeatability, should I call rd() or random_engine()?

In my particular case, I'm sure both would work just fine, because this is going to be called in some networking code where performance is not at a premium, and the results are not especially sensitive. However, I am interested in some "rules of thumb" on when to use hardware entropy and when to use pseudo-random numbers.

Currently, I am only using std::random_device to seed my std::mt19937 engine, and any random number generation I need for my program, I use the std::mt19937 engine.

edit: Here's an explanation for exactly what I am using this particular example for:

This is for a game playing program. This particular game allows the user to customize their 'team' prior to beginning a round against an opponent. Part of setting up a battle involves sending a team to the server. My program has several teams and uses the random number to determine which team to load. Each new battle makes a call to std::random_device to seed the pseudo-random number generator. I log the initial state of the battle, which includes this team that I'm randomly selecting and the initial seed.

The particular random number I'm asking about in this question is for the random team selection (where it is beneficial to not have the opponent know ahead of time what team I'm using, but not mission-critical), but what I'm really looking for is a rule of thumb. Is it fine to always use std::random_device if I don't need repeatability of my numbers, or is there a real risk of using up entropy faster than it can be collected?

Answer 1

The standard practice, as far as I am aware, is to seed the random number generator with a number that is not calculated by the computer (but comes from some external, unpredictable source). That should be the case with your rd() function. From then on, you call the pseudo-random number generator(PRNG) for each and every pseudo-random number that you need.

If you are worried about the numbers not being random enough, then you should pick a different PRNG. Entropy is a scarce and precious resource and should be treated as such. Although, you may not be needing that many random numbers right now, you may in the future; or other applications could need them. You want that entropy to be available whenever an application asks for it.

It sounds like, for your application, that the mersenne twister will suit your needs just fine. No one who plays your game will ever feel like the teams that are loaded aren't random.