draconisplusplus/include/rfl/internal/Memoization.hpp

#ifndef RFL_INTERNAL_MEMOIZATION_HPP_
#define RFL_INTERNAL_MEMOIZATION_HPP_

#include <atomic>
#include <mutex>

namespace rfl {
  namespace internal {

    /// For a thread-safe memoization pattern.
    template <class T>
    class Memoization {
     public:
      Memoization() { flag_.clear(); }

      ~Memoization() = default;

     public:
      /// Returns the underlying value.
      template <class F>
      const T& value(const F& _f) {
        if (flag_.test()) { return value_; }

        std::lock_guard<std::mutex> guard(mtx_);

        if (flag_.test()) { return value_; }

        _f(&value_);

        flag_.test_and_set();

        return value_;
      }

     private:
      /// Signifies whether t_ has been set.
      std::atomic_flag flag_;

      /// A mutex, only needed for writing.
      std::mutex mtx_;

      /// The type to be initialized.
      T value_;
    };

  } // namespace internal
} // namespace rfl

#endif