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Date: Wed, 12 Apr 2017 05:01:00 -0000
From: Oliver Kullmann <o.kullmann@swansea.ac.uk>
To: Jonathan Wakely <jwakely.gcc@gmail.com>
CC: Martin Sebor <msebor@gmail.com>, gcc-help <gcc-help@gcc.gnu.org>
Subject: Re: where are the implementations for <random>?
Message-ID: <20170412050132.GA22350@csltok.swansea.ac.uk>
References: <20170409173101.GA10474@csltok.swansea.ac.uk> <alpine.DEB.2.20.1704092015110.1870@stedding.saclay.inria.fr> <5277517b-1722-17b2-a33c-9abe6d625c0a@gmail.com> <20170411130242.GA7454@csltok.swansea.ac.uk> <20170411182615.GA16536@csltok.swansea.ac.uk> <20170411201729.GA18654@csltok.swansea.ac.uk> <CAH6eHdRHsFXfJgYCYvVV98YJRpZOCTCPQCumOMO6uFLg5NkNSQ@mail.gmail.com> <20170412034030.GA20421@csltok.swansea.ac.uk>
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I found

  http://eel.is/c++draft/rand.eng.mers

which, if it has been adopted by the Standard, seems in Points 6, 8 actually
to fully specify the seeding, from initial values as well as from
sequents (std::seed_seq), and thus, hopefully, actually seeding in both
forms seems well-defined by the standard. (Otherwise it would be a
strange oversight.)

Oliver

P.S. And it seems that the GCC implementation does what is described in
that document.


> 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
>