How do I switch between local and global settings for the initial state of a C++11 RNG?

Question

In the code given below, I would like to implement a flag (or something equally simple) that has the same effect as commenting out the local setting and using the global setting some times (yielding two different numbers in this example), and using the local setting at other times (yielding two identical numbers in this example).

I have tried the obvious "if" and "switch" structures without success.

#include <iostream>
#include <random> 

void print();

std::seed_seq seed{1, 2, 3, 4, 5}; 
std::mt19937 rng(seed); // *global* initial state
std::uniform_real_distribution<> rand01(0, 1); 

int main()
{
    print();
    print();

    return 0; 
}

void print()
{
    std::mt19937 rng(seed); // *local* initial state
    std::cout << rand01(rng) << std::endl;
}

Answer 1

Use a ternary and a reference:

std::mt19937& ref = flag ? rng : local;

Here, flag is the condition to test, rng is the "global" random object, and local is the more localised one.

Binding a reference to the result of a ternary is syntactically valid: you can't do it using an if statement, or similar as those are not expressions of the correct type.

From that point, use ref. This will be valid so long as rng and local remain in scope.