Is there a way to compose (or merge, aggregate) PODs between them ? One intuitive solution:
struct Base1 { int i; };
struct Base2 { char c; };
struct Derived : Base1, Base2 {};
// in a more generalized way
// template <class... Ts> struct Aggregate : Ts... {};
Except, we lose something:
int main()
{
Derived d {42, 'c'}; // OK
auto [i, c] = d; // Error
static_assert(std::is_standard_layout_v<Derived>); // Error
}
I see we could end up with some ambiguousities, conflicts, between the merged base classes. But it would be pretty nice to merge PODs into one. The result I try to achieve:
struct Expected { int i; char c; }; // I want Derived to behave exactly like this
Am I in the realm of reflection? Should I end up using macros?
CodePudding user response:
If we
don't care if the derived struct is a POD or not
, the task is pretty simple with Boost.PFR
- just convert your PODs to tuples and concatenate them:
template<typename... Ts> using Merge = decltype(std::tuple_cat(
boost::pfr::structure_to_tuple(std::declval<Ts>())...
));
A simple test:
int main() {
struct Base1 { int i, j; };
struct Base2 { char c; };
using Expected = Merge<Base1, Base2>;
static_assert(std::is_same_v<Expected, std::tuple<int, int, char>>);
Expected expected{42, 1337, 'c'};
auto&& [i, j, c] = expected;
}