4 ms·

Is it even possible to do it without indirection? The type parameter of a struct can change its shape hugely, so what you’re asking for is “vectors with variabl

by orf 6y ago

Is it even possible to do it without indirection? The type parameter of a struct can change its shape hugely, so what you’re asking for is “vectors with variable sized entries” which is typically done with indirection.

jcelerier 6y ago
In C++ we have things like Boost.PolyCollection to improve that specific case : https://www.boost.org/doc/libs/master/doc/html/poly_collection/an_efficient_polymorphic_data_st.html https://www.boost.org/doc/libs/master/doc/html/poly_collecti... With some metaclass / reflection magic maybe one could come up with an additional scheme that would save the per-object vtable pointer through creation of a "duplicate" structure - e.g. given struct Event { virtual ~Event(); virtual bool whateverAPI(); pair<double,double> globalPosition(); pair<double,double> localPosition(); private: double x{}, y{}; Widget& widget; }; struct MyEvent : Event { // or whatever, just an example bool whateverAPI() override; private: MouseButtons button{}; }; one would get a sub-array of things with a layout similar as struct synthesized_MyEvent { double x{}, y{}; Widget& widget; MouseButtons button{}; pair<double,double> globalPosition() { /* potentially UB magic */ } pair<double,double> localPosition() { /* potentially UB magic */ } bool whateverAPI() { /* potentially UB magic */ } }; ("potentially UB" being something like copying all the non-vtable things into a stack-allocated concrete MyEvent and calling the virtual function on that which could be worth it for small objects in terms of cache usage if the vtable is 8 bytes and the custom data something like 16 bytes and there's 350000 objects)
gpderetta 6y ago
Sometimes you do not need the vector to be runtime sized, then you can use tuples. In C++ variadic templates make it easy to work with compile time sized collections of objects. I'm sure you could do something similar with rust macros.
clappski 6y ago
In C++ you could do this with std::tuple and variadic templates, if you know all the event handlers at compile time. There would be zero indirection, although depending on how you implement the indexing into the std::tuple (i.e. by treating it as a std::variant and using std::visit) you could still inadvertently generate a vtable.
- gpderetta 6y ago
  this requires the set of all possible types to be known statically at the point the tuple is defined. True "virtual concepts" are an open set.

by orf 6y ago

jcelerier 6y ago

In C++ we have things like Boost.PolyCollection to improve that specific case : https://www.boost.org/doc/libs/master/doc/html/poly_collection/an_efficient_polymorphic_data_st.html https://www.boost.org/doc/libs/master/doc/html/poly_collecti... With some metaclass / reflection magic maybe one could come up with an additional scheme that would save the per-object vtable pointer through creation of a "duplicate" structure - e.g. given struct Event { virtual ~Event(); virtual bool whateverAPI(); pair<double,double> globalPosition(); pair<double,double> localPosition(); private: double x{}, y{}; Widget& widget; }; struct MyEvent : Event { // or whatever, just an example bool whateverAPI() override; private: MouseButtons button{}; }; one would get a sub-array of things with a layout similar as struct synthesized_MyEvent { double x{}, y{}; Widget& widget; MouseButtons button{}; pair<double,double> globalPosition() { /* potentially UB magic */ } pair<double,double> localPosition() { /* potentially UB magic */ } bool whateverAPI() { /* potentially UB magic */ } }; ("potentially UB" being something like copying all the non-vtable things into a stack-allocated concrete MyEvent and calling the virtual function on that which could be worth it for small objects in terms of cache usage if the vtable is 8 bytes and the custom data something like 16 bytes and there's 350000 objects)

gpderetta 6y ago

Sometimes you do not need the vector to be runtime sized, then you can use tuples. In C++ variadic templates make it easy to work with compile time sized collections of objects. I'm sure you could do something similar with rust macros.

clappski 6y ago

In C++ you could do this with std::tuple and variadic templates, if you know all the event handlers at compile time. There would be zero indirection, although depending on how you implement the indexing into the std::tuple (i.e. by treating it as a std::variant and using std::visit) you could still inadvertently generate a vtable.

gpderetta 6y ago
this requires the set of all possible types to be known statically at the point the tuple is defined. True "virtual concepts" are an open set.