How can I box a std::optional into a C++/WinRT IInspectable?

Say you have a std::optional<T> in your hand, and you want to convert it to an IInspectable, say because you want to put it inside a PropertySet.

void SaveHeight(winrt::PropertySet const& set)
{
    std::optional<int> height = GetHeight();

    set.Insert(L"height", /* what goes here? */);
}

Well, one thing you shouldn’t do is go right for the value:

// Code in italics is wrong.
void SaveHeight(winrt::PropertySet const& set)
{
    std::optional<int> height = GetHeight();

    set.Insert(L"height",
        winrt::box_value(height.value()));
}

Calling value() on an empty std::optional throws the std::bad_optional_access exception. In that case, your SaveHeight method throws an exception instead of saving a nullptr into the property set. If this exception crosses an ABI boundary, C++/WinRT and C++/CX will convert it to E_FAIL, and WIL will convert it to ERROR_UNHANDLED_EXCEPTION. But really, it doesn’t matter how the C++ exception is converted to an ABI HRESULT because you didn’t want an exception in the first place. You wanted the std::nullopt to convert to a null pointer.

You could manually check for an empty std::optional:

void SaveHeight(winrt::PropertySet const& set)
{
    std::optional<int> height = GetHeight();

    set.Insert(L"height",
        height ? winrt::box_value(*height) : nullptr);
}

But there’s an even easier way.

C++/WinRT provides a conversion operator from std::optional<T> to IReference<T> which does the obvious thing: An empty std::optional becomes nullptr and a std::optional with a value becomes an IReference that holds the value.

void SaveHeight(winrt::PropertySet const& set)
{
    std::optional<int> height = GetHeight();

    set.Insert(L"height",
        winrt::IReference(height));
}

We are taking advantage here of class template argument deduction (CTAD), which lets us write just winrt::IReference(...) and let the compiler infer that we are constructing a winrt::IReference<int>.