我正在使用以下 g++ 版本在 Rocky Linux 环境中编译 C++ 程序：
gcc version 8.5.0 20210514 (Red Hat 8.5.0-3) (GCC)
我按如下方式调用 g++：
g++ -std=c++17 -Wall -Wpedantic -Wextra -Ofast main.cpp
我的程序使用 C++ 标准库测试无符号 64 位伪随机整数的生成。我遇到了问题，我正在尝试确定是否发现了编译器/库错误或者我是否做错了什么。问题是伪随机生成器永远不会生成小于 27,917,522,695（大约 2^34.7）的数字。当然，考虑到 0..2^64-1 的大小，这可能需要一些时间，但我已经生成了超过 11 万亿个伪随机数，以确保在一切正常的情况下对成功有极高的期望。
下面的源代码是独立的，符合标准（C++17），并且编写是为了尽可能清楚地演示这个问题。这是我的源代码：
#include <array>
#include <cmath>
#include <cstdint>
#include <iomanip>
#include <iostream>
#include <limits>
#include <random>
#include <type_traits>
using namespace std;
// This singleton is a uniform pseudorandom number generator.
// This singleton is lazily initialized.
template<typename T>
class prng_s
{
public:
prng_s(const prng_s &) = delete;
void operator=(const prng_s &) = delete;
prng_s(prng_s &&) = delete;
void operator=(prng_s &&) = delete;
prng_s *operator&() = delete;
T rand()
{
return di(dre);
}
static prng_s<T> &get_instance()
{
// This is the instance of this singleton.
// Putting this inside get_instance() makes this a lazily-initialized
// singleton.
static prng_s<T> instance;
return instance;
}
private:
prng_s() {}
~prng_s() {}
random_device rd;
default_random_engine dre{rd()};
// Prepare a uniform random number generator. This should be used in
// conjunction with the engine as follows:
// T prn{di(dre)};
uniform_int_distribution<T> di{
0,
numeric_limits<T>::max()
};
};
using my_uint_t = uint64_t;
static_assert(
sizeof(my_uint_t) == 8 &&
is_integral_v<my_uint_t> &&
is_unsigned_v<my_uint_t>,
"my_uint_t must be a 64-bit unsigned integer type"
);
int main()
{
prng_s<my_uint_t> &prng{prng_s<my_uint_t>::get_instance()};
my_uint_t total_count{0};
my_uint_t update_count{0};
my_uint_t min_n{numeric_limits<my_uint_t>::max()};
long double min_n_lg{log2(static_cast<long double>(min_n))};
my_uint_t max_n{0};
long double max_n_lg{log2(static_cast<long double>(max_n))};
cout << "update_count = " << setw(3) << setfill(' ') << right << update_count
<< " total_count = " << setw(15) << setfill(' ') << right << total_count
<< " min_n = " << setw(20) << setfill(' ') << right << min_n
<< " min_n_lg = " << fixed << setprecision(5) << setw(8) << setfill(' ') << right << min_n_lg
<< " max_n = " << setw(20) << setfill(' ') << right << max_n
<< " max_n_lg = " << fixed << setprecision(10) << setw(13) << setfill(' ') << right << max_n_lg;
while (true)
{
bool update_needed{false};
bool periodic_update_needed{false};
my_uint_t one_num{prng.rand()};
++total_count;
if (one_num > max_n)
{
max_n = one_num;
max_n_lg = log2(static_cast<long double>(max_n));
update_needed = true;
}
if (one_num < min_n)
{
min_n = one_num;
min_n_lg = log2(static_cast<long double>(min_n));
update_needed = true;
}
if (update_needed)
++update_count;
if (! (total_count & 0x0000000000FFFFFFULL) && ! update_needed)
{
periodic_update_needed = true;
update_needed = true;
}
if (update_needed)
{
if (periodic_update_needed)
cout << "\r" << flush;
else
cout << endl;
cout << "update_count = " << setw(3) << setfill(' ') << right << update_count
<< " total_count = " << setw(15) << setfill(' ') << right << total_count
<< " min_n = " << setw(20) << setfill(' ') << right << min_n
<< " min_n_lg = " << fixed << setprecision(5) << setw(8) << setfill(' ') << right << min_n_lg
<< " max_n = " << setw(20) << setfill(' ') << right << max_n
<< " max_n_lg = " << fixed << setprecision(10) << setw(13) << setfill(' ') << right << max_n_lg;
}
}
}
以下是该程序的一些当前输出：
update_count = 0 total_count = 0 min_n = 18446744073709551615 min_n_lg = 64.00000 max_n = 0 max_n_lg = -inf
update_count = 1 total_count = 1 min_n = 7002304003565306067 min_n_lg = 62.60254 max_n = 7002304003565306067 max_n_lg = 62.6025354054
update_count = 2 total_count = 2 min_n = 3783328042247973794 min_n_lg = 61.71436 max_n = 7002304003565306067 max_n_lg = 62.6025354054
update_count = 3 total_count = 3 min_n = 3783328042247973794 min_n_lg = 61.71436 max_n = 12978918679643380525 max_n_lg = 63.4928039951
update_count = 4 total_count = 4 min_n = 3783328042247973794 min_n_lg = 61.71436 max_n = 14336044164615022123 max_n_lg = 63.6362807898
update_count = 5 total_count = 5 min_n = 3783328042247973794 min_n_lg = 61.71436 max_n = 16226104930620721283 max_n_lg = 63.8149505260
update_count = 6 total_count = 12 min_n = 1656254512494108391 min_n_lg = 60.52263 max_n = 16226104930620721283 max_n_lg = 63.8149505260
update_count = 7 total_count = 15 min_n = 113413242023177263 min_n_lg = 56.65437 max_n = 16226104930620721283 max_n_lg = 63.8149505260
update_count = 8 total_count = 19 min_n = 113413242023177263 min_n_lg = 56.65437 max_n = 17611098941292711456 max_n_lg = 63.9331187397
update_count = 9 total_count = 20 min_n = 113413242023177263 min_n_lg = 56.65437 max_n = 17655211259205598036 max_n_lg = 63.9367278871
update_count = 10 total_count = 46 min_n = 113413242023177263 min_n_lg = 56.65437 max_n = 17873875422597147885 max_n_lg = 63.9544862770
update_count = 11 total_count = 83 min_n = 113413242023177263 min_n_lg = 56.65437 max_n = 17963711407470407736 max_n_lg = 63.9617192529
update_count = 12 total_count = 150 min_n = 113413242023177263 min_n_lg = 56.65437 max_n = 18015384043678543638 max_n_lg = 63.9658632097
update_count = 13 total_count = 201 min_n = 113413242023177263 min_n_lg = 56.65437 max_n = 18323125361920749911 max_n_lg = 63.9902994067
update_count = 14 total_count = 300 min_n = 113413242023177263 min_n_lg = 56.65437 max_n = 18328438497086837431 max_n_lg = 63.9907176827
update_count = 15 total_count = 390 min_n = 61992352998145645 min_n_lg = 55.78294 max_n = 18328438497086837431 max_n_lg = 63.9907176827
update_count = 16 total_count = 437 min_n = 28110783079001471 min_n_lg = 54.64197 max_n = 18328438497086837431 max_n_lg = 63.9907176827
update_count = 17 total_count = 470 min_n = 15530575099522685 min_n_lg = 53.78596 max_n = 18328438497086837431 max_n_lg = 63.9907176827
update_count = 18 total_count = 548 min_n = 15530575099522685 min_n_lg = 53.78596 max_n = 18348594481069843039 max_n_lg = 63.9923033584
update_count = 19 total_count = 655 min_n = 15530575099522685 min_n_lg = 53.78596 max_n = 18421702955696027995 max_n_lg = 63.9980402374
update_count = 20 total_count = 2738 min_n = 15530575099522685 min_n_lg = 53.78596 max_n = 18432143699169067459 max_n_lg = 63.9988576722
update_count = 21 total_count = 3187 min_n = 15353715552198611 min_n_lg = 53.76944 max_n = 18432143699169067459 max_n_lg = 63.9988576722
update_count = 22 total_count = 3307 min_n = 15353715552198611 min_n_lg = 53.76944 max_n = 18442503997275137543 max_n_lg = 63.9996683512
update_count = 23 total_count = 4793 min_n = 7449623236915790 min_n_lg = 52.72609 max_n = 18442503997275137543 max_n_lg = 63.9996683512
update_count = 24 total_count = 7456 min_n = 7449623236915790 min_n_lg = 52.72609 max_n = 18446639273540196852 max_n_lg = 63.9999918037
update_count = 25 total_count = 12849 min_n = 2162335386263772 min_n_lg = 50.94151 max_n = 18446639273540196852 max_n_lg = 63.9999918037
update_count = 26 total_count = 13094 min_n = 942673241417463 min_n_lg = 49.74375 max_n = 18446639273540196852 max_n_lg = 63.9999918037
update_count = 27 total_count = 43816 min_n = 57864399722891 min_n_lg = 45.71774 max_n = 18446639273540196852 max_n_lg = 63.9999918037
update_count = 28 total_count = 60784 min_n = 7726706646700 min_n_lg = 42.81299 max_n = 18446639273540196852 max_n_lg = 63.9999918037
update_count = 29 total_count = 78943 min_n = 5239901122126 min_n_lg = 42.25268 max_n = 18446639273540196852 max_n_lg = 63.9999918037
update_count = 30 total_count = 130162 min_n = 5239901122126 min_n_lg = 42.25268 max_n = 18446686842822731255 max_n_lg = 63.9999955240
update_count = 31 total_count = 643439 min_n = 2465330536850 min_n_lg = 41.16492 max_n = 18446686842822731255 max_n_lg = 63.9999955240
update_count = 32 total_count = 1808778 min_n = 2465330536850 min_n_lg = 41.16492 max_n = 18446698312622650728 max_n_lg = 63.9999964211
update_count = 33 total_count = 2139847 min_n = 2465330536850 min_n_lg = 41.16492 max_n = 18446699753595454691 max_n_lg = 63.9999965338
update_count = 34 total_count = 2510203 min_n = 2465330536850 min_n_lg = 41.16492 max_n = 18446734162995213110 max_n_lg = 63.9999992249
update_count = 35 total_count = 16777216 min_n = 2465330536850 min_n_lg = 41.16492 max_n = 18446743463819675053 max_n_lg = 63.9999999523
update_count = 36 total_count = 20323037 min_n = 1629952860633 min_n_lg = 40.56797 max_n = 18446743463819675053 max_n_lg = 63.9999999523
update_count = 37 total_count = 21727248 min_n = 1629952860633 min_n_lg = 40.56797 max_n = 18446743506769684113 max_n_lg = 63.9999999557
update_count = 38 total_count = 33554432 min_n = 957785218844 min_n_lg = 39.80091 max_n = 18446743506769684113 max_n_lg = 63.9999999557
update_count = 39 total_count = 33605538 min_n = 938457714767 min_n_lg = 39.77150 max_n = 18446743506769684113 max_n_lg = 63.9999999557
update_count = 40 total_count = 50331648 min_n = 152472548969 min_n_lg = 37.14976 max_n = 18446743506769684113 max_n_lg = 63.9999999557
update_count = 41 total_count = 83886080 min_n = 103080038550 min_n_lg = 36.58497 max_n = 18446743506769684113 max_n_lg = 63.9999999557
update_count = 42 total_count = 150994944 min_n = 103080038550 min_n_lg = 36.58497 max_n = 18446743519654686831 max_n_lg = 63.9999999567
update_count = 43 total_count = 201326592 min_n = 103080038550 min_n_lg = 36.58497 max_n = 18446743781649742097 max_n_lg = 63.9999999772
update_count = 44 total_count = 352321536 min_n = 103080038550 min_n_lg = 36.58497 max_n = 18446743987809785585 max_n_lg = 63.9999999933
update_count = 45 total_count = 369098752 min_n = 53687528131 min_n_lg = 35.64387 max_n = 18446743987809785585 max_n_lg = 63.9999999933
update_count = 46 total_count = 553648128 min_n = 53687528131 min_n_lg = 35.64387 max_n = 18446744017874791927 max_n_lg = 63.9999999956
update_count = 47 total_count = 587202560 min_n = 27917522695 min_n_lg = 34.70045 max_n = 18446744017874791927 max_n_lg = 63.9999999956
update_count = 48 total_count = 60800630784 min_n = 27917522695 min_n_lg = 34.70045 max_n = 18446744050087332335 max_n_lg = 63.9999999982
有人知道为什么这个伪随机生成器似乎永远不会生成小于27,917,522,695的数字吗？