Why do STD implementations of mt19937 have double sizeof as boost version?

Question

I have this simple C++ program with unexpected output:

#include<random>
#include<iostream>
#include "boost/random/mersenne_twister.hpp"
#include "boost/random/uniform_int_distribution.hpp"

int main(){
    std::cout << sizeof(std::mt19937) << std::endl;
    std::cout << sizeof(std::mt19937_64) << std::endl;
    std::cout << sizeof(boost::random::mt19937) << std::endl;
    std::cout << sizeof(boost::random::mt19937_64) << std::endl;
}

both clang and gcc output

5000

2504

2504

2504

What I find interesting is that sizeof standard implementation of mt19937(32bit one) is around 2x the the boost version, while 64bit ones match perfectly.

Since MT uses a lot of space it is not really a small difference.

Also it is weird that implementation of a strictly specified algorithm would have such different sizeof, we are not talking about std::string where implementers might pick different SSO buffer size...

My best guess would be that boost either has a bug or that it implements some slightly different version of mt19937, but wikipedia says this, suggesting boost might be right:

Relatively large state buffer, of 2.5 KiB,

edit: both boost and std version seem to satisfy requirement that 1000th generated value is 4123659995, so there seems to be no bug in boost.

Answer 1

This is the standard definition:

mersenne_twister_engine<
    uint_fast32_t, // element of the buffer
    32,
    624,           // size of the buffer
    397, 31,
    0x9908b0df, 11,
    0xffffffff, 7,
    0x9d2c5680, 15,
    0xefc60000, 18, 1812433253>

The issue is that GNU chose that std::uint_fast32_t is a 64 bit type on 64 bit systems (whether that is a good or bad choice is a separate discussion). Thus, the buffer is twice as big as one would have expected if the buffer contained 32 bit integers.

This is the Boost definition:

mersenne_twister_engine<
    uint32_t,
    32,
    624,
    397, 31,
    0x9908b0df, 11,
    0xffffffff, 7,
    0x9d2c5680, 15,
    0xefc60000, 18, 1812433253>

Which is identical, except for using a fixed width element which is alway same on all systems.

---

You can use std::mersenne_twister_engine directly with std::uint_least32_t element to get around this issue. Using this alias is preferable to the fixed alias because it is required to be supported on all systems.