On Tue, Apr 11, 2017 at 09:27:53PM +0100, Jonathan Wakely wrote: > > All what there is seems to be > > http://www.cplusplus.com/reference/random/mersenne_twister_engine/ > > Use a better reference. http://en.cppreference.com is much better than > cplusplus.com > Actually I also noted that some time ago (the above was just what came out first from the search engine (unfortunately)). > I don't understand what you're trying to do or what the difficulty is. > > The output of the mersenne_twister_engine is specified by the > standard, there should be no need to reimplement it. If you don't get > repeatable values for any implementation of mersenne_twister_engine > then it's a bug in that implementation. > > The template arguments are just parameters for the internal algorithm, > so different parameters create different outputs. std::mt19937 is > simply a specialization of the mersenne_twister_engine with parameters > that are known to produce good pseudo-random numbers. > > The behaviour of uniform_int_distribution is also specified by the > standard, but only the mathematical properties, not a specific > implementation. That means different implementations can give > different outputs. If you use your own uniform_int_distribution then > you ensure repeatable outputs across implementations. I repeat, there > should be no need to do that with the engines themselves. Consider the following program MT.cpp: #include <iostream> #include <random> #include <vector> #include <cstdint> int main() { typedef std::mt19937_64::result_type ui; std::seed_seq s {ui(10),ui(10),ui(0)}; std::vector<std::uint32_t> seeds(3); s.generate(seeds.begin(), seeds.end()); for (const auto x : seeds) std::cout << x << " "; std::cout << "\n"; std::mt19937_64 g(s); std::cout << g() << " " << g() << "\n"; } Compiled with > g++ --std=c++11 -Wall MT.cpp it yields with gcc 4.7.4 and gcc 5.4 > ./a.out 927367489 2598207009 681269367 14538134740155241067 15440504459514664889 As far as I understand it, only the first line is defined by the standard, but not the second: how the seed-sequence is used by the generator g is up to the compiler. Thus the need to do the seeding ourselves. Oliver
