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  1. #include <atomic>
  2. #include <iostream>
  3.  
  4. template <class T> class Seqlock {
  5.     std::atomic<int> seq_;
  6.     T val_;
  7.  
  8. public:
  9.     Seqlock(T value = T())
  10.         : val_(value) {
  11.     }
  12.  
  13.     // concurrent calls are NOT allowed
  14.     void store(T value) {
  15.         const int seq0 = seq_.load(std::memory_order_relaxed);
  16.         seq_.store(seq0 + 1, std::memory_order_relaxed);
  17.  
  18.         std::atomic_thread_fence(std::memory_order_release);
  19.  
  20.         val_ = value;
  21.         std::atomic_thread_fence(std::memory_order_release);
  22.  
  23.         seq_.store(seq0 + 2, std::memory_order_relaxed);
  24.     }
  25.  
  26.     // concurrent calls are allowed
  27.     T load() const {
  28.         for (;;) {
  29.             const int seq0 = seq_.load(std::memory_order_relaxed);
  30.             if (seq0 & 1) {
  31.                 // cpu_relax()
  32.                 continue;
  33.             }
  34.  
  35.             std::atomic_thread_fence(std::memory_order_acquire);
  36.  
  37.             T ret = val_;
  38.             std::atomic_thread_fence(std::memory_order_acquire);
  39.  
  40.             const int seq1 = seq_.load(std::memory_order_relaxed);
  41.             if (seq0 == seq1) {
  42.                 return ret;
  43.             }
  44.         }
  45.     }
  46. };
  47.  
  48. int main() {
  49.     Seqlock<int> sl;
  50.     sl.store(123);
  51.     std::cout << sl.load() << "\n";
  52.     return 0;
  53. }
Success #stdin #stdout 0s 4508KB
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123
https://ideone.com/DZeIZv
language:
C++ (gcc 8.3)
created:
5 years ago
visibility:
public

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