Class CartesianProductGenerator

tests/gtest/gtest.h:11142–11255

template <typename... T>
class CartesianProductGenerator
    : public ParamGeneratorInterface<::std::tuple<T...>> {
 public:
  typedef ::std::tuple<T...> ParamType;

  CartesianProductGenerator(const std::tuple<ParamGenerator<T>...>& g)
      : generators_(g) {}
  ~CartesianProductGenerator() override {}

  ParamIteratorInterface<ParamType>* Begin() const override {
    return new Iterator(this, generators_, false);
  }
  ParamIteratorInterface<ParamType>* End() const override {
    return new Iterator(this, generators_, true);
  }

 private:
  template <class I>
  class IteratorImpl;
  template <size_t... I>
  class IteratorImpl<IndexSequence<I...>>
      : public ParamIteratorInterface<ParamType> {
   public:
    IteratorImpl(const ParamGeneratorInterface<ParamType>* base,
             const std::tuple<ParamGenerator<T>...>& generators, bool is_end)
        : base_(base),
          begin_(std::get<I>(generators).begin()...),
          end_(std::get<I>(generators).end()...),
          current_(is_end ? end_ : begin_) {
      ComputeCurrentValue();
    }
    ~IteratorImpl() override {}

    const ParamGeneratorInterface<ParamType>* BaseGenerator() const override {
      return base_;
    }
    // Advance should not be called on beyond-of-range iterators
    // so no component iterators must be beyond end of range, either.
    void Advance() override {
      assert(!AtEnd());
      // Advance the last iterator.
      ++std::get<sizeof...(T) - 1>(current_);
      // if that reaches end, propagate that up.
      AdvanceIfEnd<sizeof...(T) - 1>();
      ComputeCurrentValue();
    }
    ParamIteratorInterface<ParamType>* Clone() const override {
      return new IteratorImpl(*this);
    }

    const ParamType* Current() const override { return current_value_.get(); }

    bool Equals(const ParamIteratorInterface<ParamType>& other) const override {
      // Having the same base generator guarantees that the other
      // iterator is of the same type and we can downcast.
      GTEST_CHECK_(BaseGenerator() == other.BaseGenerator())
          << "The program attempted to compare iterators "
          << "from different generators." << std::endl;