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  1.     #include <cassert>
  2.     #include <cstdlib>
  3.  
  4.     #include <array>
  5.     #include <iostream>
  6.     #include <numeric>
  7.     #include <utility>
  8.  
  9.     // Wraps a std::array of TKey/TValue pairs and provides a method
  10.     // to randomly select a TKey with TValue bias.
  11.     template< typename TKey, typename TValue, std::size_t TSize >
  12.     class weights final
  13.     {
  14.         public:
  15.             using pair = const std::pair< const TKey, const TValue >;
  16.             using array = const std::array< pair, TSize >;
  17.  
  18.             weights( array values )
  19.                 : values_{ values }
  20.                 , sum_{ std::accumulate(values_.begin(), values_.end(), 0, [](TValue total, const pair& p){ return total + p.second; }) }
  21.             {}
  22.  
  23.             // Implements this algorithm
  24.             // http://stackoverflow.com/a/1761646/331024
  25.             const TKey get() const
  26.             {
  27.                 // The real code uses c++11 <random> features,
  28.                 // which I've removed for brevity.
  29.                 auto weight_rand = static_cast< TValue >( std::rand() % sum_ );
  30.  
  31.                 for ( std::size_t i = 0; i < TSize; ++i )
  32.                 {
  33.                     if (weight_rand < values_[i].second)
  34.                     {
  35.                         return values_[i].first;
  36.                     }
  37.                     weight_rand -= values_[i].second;
  38.                 }
  39.                 assert(false);
  40.             }
  41.  
  42.         private:
  43.             array values_;
  44.             const TValue sum_;
  45.     };
  46.  
  47.     enum class direction
  48.     {
  49.         NORTH,
  50.         SOUTH,
  51.         EAST,
  52.         WEST
  53.     };
  54.  
  55.     // For convenience create a type to map the above
  56.     // four-value enumeration to integer weights.
  57.     using w4i = weights< direction, int, 4 >;
  58.  
  59.     // Map the directions with a weight.
  60.     static const w4i direction_weights = w4i::array{
  61.         {
  62.             w4i::pair{ direction::NORTH, 2 },
  63.             w4i::pair{ direction::EAST, 1 },
  64.             w4i::pair{ direction::SOUTH, 3 },
  65.             w4i::pair{ direction::WEST, 1 }
  66.         }
  67.     };
  68.  
  69.     int main()
  70.     {
  71.         std::cout << (int)direction_weights.get() << std::endl;    
  72.  
  73.         return 0;
  74.     }
Compilation error #stdin compilation error #stdout 0s 0KB
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stdin
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Standard input is empty
compilation info 
prog.cpp: In instantiation of ‘weights<TKey, TValue, TSize>::weights(array) [with TKey = direction; TValue = int; unsigned int TSize = 4u; weights<TKey, TValue, TSize>::array = const std::array<const std::pair<const direction, const int>, 4u>]’:
prog.cpp:67:5:   required from here
prog.cpp:20:137: error: could not convert ‘values’ from ‘weights<direction, int, 4u>::array {aka const std::array<const std::pair<const direction, const int>, 4u>}’ to ‘const std::pair<const direction, const int>’
                 , sum_{ std::accumulate(values_.begin(), values_.end(), 0, [](TValue total, const pair& p){ return total + p.second; }) }
                                                                                                                                         ^
stdout
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Standard output is empty
https://ideone.com/s8mn40
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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