Generating random numbers and shared_ptr handling

My problem is that I don't quite get generating random numbers implemented in the code, and want to ask you if everything here's okay - I mean should it generate new random numbers every time I compile the code? Moreover can you please explain me the difference between the three of following:

for(auto someObject:SomeClass)

for(auto &someObject:SomeClass)

and the third is shown in a method below.

Here's a bit of class definition and the method generating random numbers

class DiscretePDFGenerator {
.
.
.
    vector<std::shared_ptr<DiscreteProbabilityValue>>probabilities_;
    std::random_device seed_;
    std::mt19937 rng_;
    std::uniform_real_distribution<double> distribution_;
.
.
.
}


std::string DiscretePDFGenerator::generateGrade ()
{
    string value;
    double rng,sumProbability=0.0;
    rng=distribution_(rng_);
    for  (auto &&probability: probabilities_)
    {
        sumProbability+=probability->getProbability();
        if (rng<sumProbability)
        {
            value= probability->getValue();
            break;
        }
    }
    return value;
}

I want to ask if there's even need to make it double reference or single reference or not even a reference, if you get my point. Sorry if it's a dumb question and thank you for your help!