C++/WinRT's convention for getter is T Property(), and setter void Property(T). To implement one simple property, developers need to define at least 2 methods and 1 data member. The Property helper pattern has been developed to reduce such boilerplate.
Property helper pattern works by taking advangage of the fact that, in C++, function call syntax foo() works for all callable.
That is to say, the following is legal C++:
struct Cat
{
void Meow() {}
}
Cat().Meow();
And so is the following:
struct Meower
{
void operator()() {} // Overloads operator(), making *this callable with ()
}
struct Cat
{
Meower Meow;
}
Cat().Meow(); // Calls Meower's overloaded operator()
IOW, other than T Property() and void Property(T), a non-static data member whose type overloads T operator() and void operator(T) also satisfy C++/WinRT's property convention.
To take advantage of this syntax, property helper classes were developed, e.g. cppxaml::XamlProperty.
Idlgen recognizes data members whose type is a template and overloads operator() with function signatures matching getter and setter. By default, these operator() are treated as methods (that is why cppxaml::XamlEvent works automatically). By adding [[idlgen::property]] to such data members, or by configuring these templates as getters or properties in project properties, idlgen can treat these overloaded operator() as getters or setters.