How can I transfer ownership of a C-style array to a Windows Runtime component?

Suppose you have a large C-style array, and you want to transfer ownership of that array to a Windows Runtime component. For concreteness, let’s say that we have this:

namespace Sample
{
    runtimeclass Widget
    {
        // The "indices" array will be very large.
        void SetIndices(Int32[] indices);
    }
}

You might be tempted to do something like this:

void SetWidgetIndices(Widget const& widget)
{
  winrt::com_array<int32_t> indices = CalculateIndices();
  widget.SetIndices(std::move(indices));
}

Using a std::move means that you are fine with the method stealing the resources out of the object, and you no-so-secretly hope that it will do so.

But it won’t.

If you look at the rules for passing C-style arrays across the Windows Runtime ABI boundary, you’ll see that a parameter declared as T[] v uses the PassArray pattern. In that pattern, ownership of the data remains with the caller, and the recipient must make a copy if it wants to access it beyond the end of the method.

So that’s not going to work.

The FillArray pattern doesn’t work either. That is for asking the method to fill a preallocated array, which is not what we’re doing here.

And the last pattern, ReceiveArray doesn’t work, because that is for transferring ownership from the method back to the caller.

So we’re stuck. How can we do this without incurring a copy of a large block of data?

One option is to express the data in the form of an IVector instead of a C-style array. Since IVector is an interface, the recipient can just AddRef the interface and continue using it later. The major downside of this is that it costs you a lot of performance, since access to each element of an IVector is a virtual method call.¹

Another option is to express the data in the form of a byte buffer, IBuffer. However, this works only for types that have no destructor (like integers). Furthermore, getting the data into and out of the buffer is a bit awkward, since you have to do some casting of the byte buffer to get it into the form you want.

auto data = reinterpret_cast<int32_t*>(m_buffer.data());
auto size = m_buffer.Length() / sizeof(int32_t);
auto view = winrt::array_view(data, data + size);
// access the data via the view

It’s also a problem for languages which do not have raw pointer types.

It occurred to me that there’s still a third option, but you have to change your point of view: Since the only ownership-transferring operation is from the method to its caller, reverse the roles so that the caller can “return” the array to the method.

namespace Sample
{
    // The "indices" array will be very large.
    delegate Int32[] WidgetIndicesProducer();

    runtimeclass Widget
    {
        void SetIndices(WidgetIndicesProducer producer);
    }
}

To provide the indices, you actually provide a callback that generates the indices and returns them via the ReceiveArray pattern.

void SetWidgetIndices(Widget const& widget)
{
  winrt::com_array<int32_t> indices = CalculateIndices();
  widget.SetIndices(
    [&] { return std::move(indices); });
}

The [&] capture assumes that the lambda will be called back before the indices variable is destructed. A safer version would be to store the indices inside the lambda itself.

void SetWidgetIndices(Widget const& widget)
{
  widget.SetIndices(
    [indices = CalculateIndices()]() mutable
    { return std::move(indices); });
}

You could simplify this to

void SetWidgetIndices(Widget const& widget)
{
  widget.SetIndices([] { return CalculateIndices(); });
}

but note that this changes the order of evaluation, since Calculate­Indices() is called from inside the call to Set­Indices().

Yes, it’s awkward, but at least it’s a workaround. You can make it slightly less awkward with a wrapper function:

void SetWidgetIndices(
    Widget const& widget,
    winrt::com_array<int32_t>&& indices)
{
  widget.SetIndices(
    [indices = std::move(indices)]() mutable
    { return std::move(indices); });
}

Next time, we’ll look at the implementation side of this method.

¹ You can use IVector::Get­Many() to slurp out the elements, but that’s still a copy operation, which we are trying to avoid.