Re: Layered data structures

16.7.2012 2:05, Wil Evers kirjoitti:


Here's a complete example program demonstrating both the problem and
its solution by Option 4 (read-only objects).

#include <cassert>
#include <iostream>
#include <memory>
#include <set>

typedef std::set<int> NumberSet;
typedef NumberSet::const_iterator Number_ConstIterator;

template <typename Type>
class ReadOnly
{
public:
      ReadOnly(Type&& data)
          : data_(new Type(std::move(data)))
      {
      }

      const Type& operator()() const
      {
          return *data_;
      }

      Type release()
      {
          assert(data_.use_count() == 1);
          return std::move(*data_);
      }

private:
      std::shared_ptr<Type> data_;
};

class NumberAnalysis
{
public:
      NumberAnalysis(
          ReadOnly<NumberSet> numberSet)
          : numberSet_(numberSet)
          , minEven_()
      {
          analyze();
      }

      Number_ConstIterator minEven() const
      {
          return minEven_;
      }

private:
      void analyze()
      {
          for (auto iter = numberSet_().cbegin();
              iter != numberSet_().cend();
              ++iter)
          {
              if ((*iter & 1) == 0)
              {
                  minEven_ = iter;
                  break;
              }
          };
      }

      ReadOnly<NumberSet> numberSet_;
      Number_ConstIterator minEven_;
};

int main()
{
      int numbers[] = {1, 5, 4, 2};

      NumberSet numberSet;
      numberSet.insert(std::begin(numbers), std::end(numbers));

      // Close for modification.
      ReadOnly<NumberSet> cNumberSet =
          std::move(numberSet);

      {
          // Analyze the read-only number-set.
          NumberAnalysis analysis(cNumberSet);

          if (analysis.minEven() != cNumberSet().cend())
          {
              std::cout << "The minimum even number in the set is "
                  << *analysis.minEven() << "." << std::endl;
          }
          else
          {
              std::cout << "There are no even numbers in the set."
                  << std::endl;
          }
      }

      // Recover the number-set for modification.
      numberSet = std::move(cNumberSet.release());
      numberSet.insert(10);

      return 0;
}

This demonstrates Option 4. Somehow I dislike this solution. Also,
when writing this program I first wrote "!= numberSet.cend()", which
caused the program to crash. It seems easy to get confused between the
read-only object 'cNumberSet' and the original object 'numberSet'.

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