How can I use mt19937 in a class without getting the same random sequence in each instance? [duplicate]

I'm writing code for initializing particle positions in a phase space using a density function. I would like to randomly sample their locations according to an arbitrary distribution, and the easiest way to do this that I found was to make a small class with everything needed to sample according to an array of probabilities that I give it. However, I've found that I get the same sequence of numbers every time I run the code.

vose *sampler = new vose(density, init_grid_count); for (unsigned int i=0; i<20; i++) { unsigned int index = sampler->alias_method(); cout << index << '\t'; } 

This is the definition of the function I use

unsigned int vose::alias_method() { double x = uniform(mt); unsigned int i = floor(array_size*x)+1; double y = array_size*x+1-i; if (y<distribution[i]) return i; else return alias[i]; } 

I found that if I do the same loop in the constructor for the class so that it runs when I initialize the vose object, it behaves as expected. This means that it generates a different random sequence each time. So why is it that when I do this in the constructor it works right, but when I use it in a member function it outputs the same sequence every time?

The problem is not that my compiler doesn't support the seed generation. I'm using the <chrono> library and when I print out the seed in the constructor, it is different every time. In case you think there could be something off in the constructor, here it is.

vose::vose(double *array, unsigned int size) { //Initialize auto seed = chrono::high_resolution_clock::now().time_since_epoch().count(); mt19937 mt(seed); // Seeds Mersenne Twister with Device RNG uniform_real_distribution<double> uniform(0, 1); distribution = array; array_size = size; alias = new unsigned int[size]; for (unsigned int i = 0; i<size; i++) { alias[i] = i+1; } } 

Edit: To be perfectly clear, everything works right when I run the method in the constructor. However, that's obviously not where I mean ultimately to run it so I need to figure out how to have a method run in the main section of code.

Edit: In response to the possibility that I hid the generator or something, here is the class declaration in the header file.

class vose { public: vose(double*, unsigned int); void demo(); unsigned int alias_method(); private: std::mt19937 mt; // I struggled with this for a while, you don't give it args // until you use it since it's not a function, it's a strange class std::uniform_real_distribution<double> uniform; double *distribution; unsigned int array_size; unsigned int *alias; };