Exploiting C++/WinRT CRTP: Property and event declarations

In C++/WinRT, properties are represented by method calls. Given this Windows Runtime class definition:

namespace MyNamespace
{
    runtimeclass Widget
    {
        Double Height;
    }
}

you can access the property like so:

Widget w;

// 0 parameters = get value
auto height = w.Height();

// 1 parameter = set value
w.Height(newHeight);

If you implement this class in C++/WinRT, the auto-generated header files use the Curiously Recurring Template Pattern (commonly known as CRTP) and expect your implementation class to follow the same pattern:

namespace winrt::MyNamespace::implementation
{
    struct Widget : WidgetT<Widget>
    {
        double m_height;
        auto Height() const { return m_height; }
        void Height(double const& height) { m_height = height; }
    };
}

If you have a lot of properties, writing these accessor methods can be quite tedious and therefore error-prone.

But we can exploit CRTP to make it easier.

You see, the CRTP wrappers don’t actually require Height() to be a method. It just requires that Height() be an expression that represents the property value. Similarly, Height(newValue) just needs to be an expression whose side effect is changing the property value.

// This code is autogenerated by C++/WinRT

int32_t get_Height(double* value) noexcept final try
{
    typename D::abi_guard guard(this->shim());
    *value = detach_from<double>(this->shim().Height());
    return 0;
}
catch (...) { return to_hresult(); }

int32_t put_Height(double value) noexcept final try
{
    typename D::abi_guard guard(this->shim());
    this->shim().Height(value);
    return 0;
}
catch (...) { return to_hresult(); }

In words, the autogenerated code expects d.Height() to produce something that can be stored in a double, and it expects d.Height(value) to do something with the value.

These look like method calls, but they don’t have to be. They can be anything, provided that sequence of tokens produces a valid result.

Specifically, they can be callable objects.

template<typename T>
struct winrt_property
{
    winrt_property(T const& initial =
        winrt_empty_value<T>()) :
        value(initial) {}

    T operator()() const { return value; }
    void operator()(T const& newValue) { value = newValue; }

    T value;
};

namespace winrt::MyNamespace::implementation
{
    struct Widget : WidgetT<Widget>
    {
        winrt_property<double> Height;
    };
}

When the C++/WinRT library does this->shim().Height(), this obtains the Widget implementation class and then looks up Height, which is a member variable, and then applies the () function call operator, which we overloaded to return the current wrapped value.

Similarly, when the C++/WinRT library does this->shim().Height(value), this invokes the overloaded single-parameter function call operator which updates the value.

In the implementation, you can access the value either by using the function call operator:

void Widget::IncreaseHeight(double increment)
{
    auto currentHeight = Height();
    Height(currentHeight + increment);
}

or you can access the value member, which was intentionally left public for this purpose.

void Widget::IncreaseHeight(double increment)
{
    Height.value += increment;
}

You can use the same trick for C++/WinRT event implementations.

template<typename D>
struct winrt_event : winrt::event<D>
{
    winrt_event() = default;

    using winrt::event<D>::operator();
    auto operator()(D const& handler)
        { return this->add(handler); }
    void operator()(winrt::event_token const& token)
        { this->remove(token); }
};

// Class definition

namespace MyNamespace
{
    runtimeclass Widget
    {
        Windows.Foundation.TypedEventHandler<Widget, Object> Closed;
    }
}

// Implementation
namespace winrt::MyNamespace::implementation
{
    // Just an abbreviation to save some typing.
    using ClosedHandler = Windows::Foundation::TypedEventHandler<
        MyNamespace::Widget, Windows::Foundation::IInspectable>;

    struct Widget : WidgetT<Widget>
    {
        winrt_event<ClosedHandler> Closed;
    };
}

Note the need to namespace-qualify Widget when it appears as the first template type parameter of Typed­Event­Handler because we want it to be the projected type and not the implementation type.