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  1. #include <iostream>
  2. #include <cstdint>
  3. #include <random> // http://e...content-available-to-author-only...e.com/w/cpp/numeric/random
  4. #include <ctime>
  5. #include <stdexcept>
  6.  
  7. int main()
  8. {
  9.     constexpr std::size_t NSEEDS = 5 ;
  10.     std::uint32_t seeds[NSEEDS] = { 0 } ;
  11.  
  12.     // see: http://w...content-available-to-author-only...s.com/forum/beginner/103051/#msg554829
  13.     try
  14.     {
  15.         std::random_device rdev ;
  16.         // if( rdev.entropy() < 0.05 ) throw std::runtime_error( "not a random device" ) ;
  17.         // commented out because GCC reports zero entropy even for a true std::random_device
  18.         for( auto& s : seeds ) s = rdev() ;
  19.         std::cout << "seeds generated with a random device\n" ;
  20.     }
  21.     catch( const std::exception& )
  22.     {
  23.         std::cout << "no random device / bogus random device / random device has no entropy\n" ;
  24.         auto t = std::time(nullptr) ;
  25.         for( std::size_t i = 0 ; i < NSEEDS ; ++i )
  26.             if( seeds[i] == 0 ) seeds[i] = t * (i+3) ;
  27.     }
  28.  
  29.     std::seed_seq seed_seq( seeds, seeds+NSEEDS ) ;
  30.     std::mt19937 twister(seed_seq) ;
  31.     std::uniform_int_distribution<int> dist( 1, 1000000 ) ;
  32.  
  33.     for( int i = 0 ; i < 10 ; ++i ) std::cout << dist(twister) << ' ' ;
  34.     std::cout << '\n' ;
  35. }
  36.  
Success #stdin #stdout 0s 3032KB
comments (?)
stdin 
Standard input is empty
stdout 
seeds generated with a random device
811479 64016 360491 298008 779442 293644 673381 467266 522115 134440
https://ideone.com/ks0vgD
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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