Bonus operations for C++/WinRT iterators: The IIterator<T>

Raymond Chen

Raymond

C++/WinRT provides iterators for a number of basic Windows Runtime collections. Let’s start with the lowest-level Windows Runtime iterator-ish thing: The IIterator<T>, which represents a cursor in a collection.

C++/WinRT adds two additional operators to the Windows Runtime-defined methods on the C++/WinRT IIterator<T> interface:

  • The dereferencing * operator fetches the current item.
  • The preincrement ++ operator moves the iterator to the next item. If the iterator has moved past the last item of the collection, then the iterator is set to nullptr.

This allows you to write code that consumes the contents of an IIterator in a somewhat more idiomatic way.

for (IIterator<int> it = start.HasCurrent() ? start : nullptr; it; ++it) {
  int value = *it;
  /* do something with the value */
}

This is still rather awkward because you have to check whether the starting point is already beyond the end of the collection and convert it to a nullptr. A little helper function can help with that.

template<typename T>
IIterator<T> as_cpp_iterator(IIterator<T> const& it)
{
  return it.HasCurrent() ? it : nullptr;
}

This simplifies the for loop slightly:

for (IIterator<int> it = as_cpp_iterator(start); it; ++it) {
  int value = *it;
  /* do something with the value */
}

These operators are added primarily for use in C++ standard algorithms that accept input iterators.

std::vector<int> to_vector(IIterator<int> const& it)
{
  std::vector<int> v;
  std::copy(as_cpp_iterator(it), {}, std::back_inserter(v));
  return v;
}

int sum(IIterator<int> const& it)
{
    return std::reduce(as_cpp_iterator(it), {}, 0);
}

Iterators are a rather clumsy way of walking through a collection. Next time, we’ll look at something better.

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