std::default_random_engine generator;
//连续均匀分布
std::uniform_real_distribution<double> xy_rand(-4, 4.0);
double a = xy_rand(generator);
//高斯分布
std::normal_distribution<double> noise_pdf(0., 1. / 1000.); //均值为0, 方差为1/1000
double b = noise_pdf(generator);