In C++\/WinRT, properties are represented by method calls. Given this Windows Runtime class definition:<\/p>\n
namespace MyNamespace\r\n{\r\n runtimeclass Widget\r\n {\r\n Double Height;\r\n }\r\n}\r\n<\/pre>\nyou can access the property like so:<\/p>\n
Widget w;\r\n\r\n\/\/ 0 parameters = get value\r\nauto height = w.Height();\r\n\r\n\/\/ 1 parameter = set value\r\nw.Height(newHeight);\r\n<\/pre>\nIf you implement this class in C++\/WinRT, the auto-generated header files use the Curiously Recurring Template Pattern<\/a> (commonly known as CRTP) and expect your implementation class to follow the same pattern:<\/p>\nnamespace winrt::MyNamespace::implementation\r\n{\r\n struct Widget : WidgetT<Widget>\r\n {\r\n double m_height;\r\n auto Height() const { return m_height; }\r\n void Height(double const& height) { m_height = height; }\r\n };\r\n}\r\n<\/pre>\nIf you have a lot of properties, writing these accessor methods can be quite tedious and therefore error-prone.<\/p>\n
But we can exploit CRTP to make it easier.<\/p>\n
You see, the CRTP wrappers don’t actually require Height()<\/code> to be a method. It just requires that Height()<\/code> be an expression that represents the property value. Similarly, Height(newValue)<\/code> just needs to be an expression whose side effect is changing the property value.<\/p>\n\/\/ This code is autogenerated by C++\/WinRT\r\n\r\nint32_t get_Height(double* value) noexcept final try\r\n{\r\n typename D::abi_guard<\/a> guard(this->shim());\r\n *value = detach_from<double>(this->shim().Height());\r\n return 0;\r\n}\r\ncatch (...) { return to_hresult(); }\r\n\r\nint32_t put_Height(double value) noexcept final try\r\n{\r\n typename D::abi_guard guard(this->shim());\r\n this->shim().Height(value);\r\n return 0;\r\n}\r\ncatch (...) { return to_hresult(); }\r\n<\/pre>\nIn words, the autogenerated code expects d.Height()<\/code> to produce something that can be stored in a double<\/code>, and it expects d.Height(value)<\/code> to do something with the value<\/code>.<\/p>\nThese look like method calls, but they don’t have to be. They can be anything, provided that sequence of tokens produces a valid result.<\/p>\n
Specifically, they can be callable objects.<\/p>\n
template<typename T>\r\nstruct winrt_property\r\n{\r\n winrt_property(T const& initial =\r\n winrt_empty_value<\/a><T>()) :\r\n value(initial) {}\r\n\r\n T operator()() const { return value; }\r\n void operator()(T const& newValue) { value = newValue; }\r\n\r\n T value;\r\n};\r\n\r\nnamespace winrt::MyNamespace::implementation\r\n{\r\n struct Widget : WidgetT<Widget>\r\n {\r\n winrt_property<double> Height;\r\n };\r\n}\r\n<\/pre>\nWhen the C++\/WinRT library does this->shim().Height()<\/code>, this obtains the Widget<\/code> implementation class and then looks up Height<\/code>, which is a member variable, and then applies the ()<\/code> function call operator, which we overloaded to return the current wrapped value.<\/p>\nSimilarly, when the C++\/WinRT library does this->shim().Height(value)<\/code>, this invokes the overloaded single-parameter function call operator which updates the value.<\/p>\nIn the implementation, you can access the value either by using the function call operator:<\/p>\n
void Widget::IncreaseHeight(double increment)\r\n{\r\n auto currentHeight = Height();\r\n Height(currentHeight + increment);\r\n}\r\n<\/pre>\nor you can access the value<\/code> member, which was intentionally left public for this purpose.<\/p>\nvoid Widget::IncreaseHeight(double increment)\r\n{\r\n Height.value += increment;\r\n}\r\n<\/pre>\nYou can use the same trick for C++\/WinRT event implementations.<\/p>\n
template<typename D>\r\nstruct winrt_event : winrt::event<D>\r\n{\r\n winrt_event() = default;\r\n\r\n using winrt::event<D>::operator();\r\n auto operator()(D const& handler)\r\n { return this->add(handler); }\r\n void operator()(winrt::event_token const& token)\r\n { this->remove(token); }\r\n};\r\n\r\n\/\/ Class definition\r\n\r\nnamespace MyNamespace\r\n{\r\n runtimeclass Widget\r\n {\r\n Windows.Foundation.TypedEventHandler<Widget, Object> Closed;\r\n }\r\n}\r\n\r\n\/\/ Implementation\r\nnamespace winrt::MyNamespace::implementation\r\n{\r\n \/\/ Just an abbreviation to save some typing.\r\n using ClosedHandler = Windows::Foundation::TypedEventHandler<\r\n MyNamespace::Widget, Windows::Foundation::IInspectable>;\r\n\r\n struct Widget : WidgetT<Widget>\r\n {\r\n winrt_event<ClosedHandler> Closed;\r\n };\r\n}\r\n<\/pre>\n