Update: I discovered several bugs with the initial implementation, so I've updated this post with a more complete solution.
Properties as seen in Python and C# are a pretty nice language feature, they allow you to have the syntax of accessing a public member variable, yet have that access still go through a getter or setter function.
I have to say I'm not totally sold on RW properties (I feel that if you are intending to change the state of a class, you should be forced to make it clear by calling a setter function) but I like RO properties, they remove the need to call a function to just access a value, and still allow you to check pre/post conditions or log access.
C++ doesn't have properties, if you Google around there's a general consensus that they don't fit the language well. However, I got a bit tired of my game engine's API having an excessive number of function calls. Here's an example:
TextureID rtt = window().new_texture(false);
window().mesh(rect_mesh)->set_texture_on_material(0, rtt);
It would be far nicer to be able to write:
TextureID rtt = window->new_texture(false);
window->mesh(rect_mesh)->set_texture_on_material(0, rtt);
... and still have the control that a getter function provides. So I wrote a Property template class, it looks like this..
template<typename Container, typename T>
class Property {
public:
Property(Container* _this, T Container::* member):
this_(_this),
getter_([member](Container* self) -> T& {
return self->*member;
}) {
}
Property(Container* _this, T* Container::* member):
this_(_this),
getter_([member](Container* self) -> T& {
return *(self->*member);
}) {
}
Property(Container* _this, std::shared_ptr<T> Container::* member):
this_(_this),
getter_([member](Container* self) -> T& {
return *(self->*member);
}) {
}
Property(Container* _this, std::function<T& (Container*)> getter):
this_(_this),
getter_(getter) {
}
/*
* We can't allow copy construction, because 'this_' will never be initialized
*/
Property(const Property& rhs) = delete;
Property operator=(const Property& rhs) {
assert(this_); //Make sure that this_ was initialized
getter_ = rhs.getter_;
// Intentionally don't transfer 'this_'
}
inline operator T&() const { return getter_(this_); }
inline T* operator->() const { return &getter_(this_); }
private:
Container* this_ = nullptr;
std::function<T& (Container*)> getter_;
};
Container is the class you are adding the property to, T is the type of the property.
Thanks to C++11 allowing initialization of members in the body of the class, and the awesome syntax of lambdas, we can use the Property template like so:
Property<WindowBase, Watcher> watcher = {
this, [](const WindowBase* self) -> Watcher& {
if(!self->watcher_) {
throw LogicError("Watcher has not been initialized");
} else {
return *self->watcher_.get();
}
}
};
Or, if you just want to wrap a member variable:
Property<WindowBase, Console> console = { this, &WindowBase::console_ };
Unfortunately we can't override the '.' operator in C++ so we can't get the exact same syntax as C# or Python, but the -> operator is good enough (and as a lot of my engine's accessors return pointers, it actually made things more consistent).
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For example, suppose we have a class Person with a property name that is of type string. We can declare this property using the auto keyword as follows:
class Person {
public:
auto name = std::string{};
};
In this example, the type of the name property is inferred from the std::string{} value that is assigned to it. This makes it easy to declare properties without having to write out their type explicitly.
One thing to keep in mind when using class properties in C++11 is that they are not compatible with older versions of the C++ standard. If you need to write code that is compatible with older versions of C++, you will need to declare your properties using the traditional method.
Overall, class properties in C++11 can be a useful tool for simplifying the declaration of properties in your code. If you need help with implementing class properties or any other C++11 features, reach out to AccessPrep, a leading provider of C++ development services.