Question: is there a better solution for such dynamic subclassing that solves 'a' and 'b' problems ?
If you have fixed set of sub-classes then you may consider using algebraic data type like boost::variant
. Store shared data separately and place all varying parts into variant.
Properties of this approach:
- naturally works with fixed set of "sub-classes". (though, some kind of type-erased class can be placed into variant and set would become open)
- dispatch is done via switch on small integral tag. Sizeof tag can be minimized to one
char
. If your "sub-classes" are empty - then there will be small additional overhead (depends on alignment), because boost::variant
does not perform empty-base-optimization.
- "Sub-classes" can have arbitrary internal data. Such data from different "sub-classes" will be placed in one
aligned_storage
.
- You can make bunch of operations with "sub-class" using only one dispatch per batch, while in general case with virtual or indirect calls dispatch will be per-call. Also, calling method from inside "sub-class" will not have indirection, while with virtual calls you should play with
final
keyword to try to achieve this.
self
to base shared data should be passed explicitly.
Ok, here is proof-of-concept:
struct ThingData
{
int f1;
int f2;
};
struct Summator
{
void doAction1(ThingData &self) { self.f1 += self.f2; }
const char* type_name() { return "Summator"; }
};
struct Substractor
{
void doAction1(ThingData &self) { self.f1 -= self.f2; }
const char* type_name() { return "Substractor"; }
};
using Thing = SubVariant<ThingData, Summator, Substractor>;
int main()
{
auto test = [](auto &self, auto &sub)
{
sub.doAction1(self);
cout << sub.type_name() << " " << self.f1 << " " << self.f2 << endl;
};
Thing x = {{5, 7}, Summator{}};
apply(test, x);
x.sub = Substractor{};
apply(test, x);
cout << "size: " << sizeof(x.sub) << endl;
}
Output is:
Summator 12 7
Substractor 5 7
size: 2
LIVE DEMO on Coliru
Full Code (it uses some C++14 features, but can be mechanically converted into C++11):
#define BOOST_VARIANT_MINIMIZE_SIZE
#include <boost/variant.hpp>
#include <type_traits>
#include <functional>
#include <iostream>
#include <utility>
using namespace std;
/****************************************************************/
// Boost.Variant requires result_type:
template<typename T, typename F>
struct ResultType
{
mutable F f;
using result_type = T;
template<typename ...Args> T operator()(Args&& ...args) const
{
return f(forward<Args>(args)...);
}
};
template<typename T, typename F>
auto make_result_type(F &&f)
{
return ResultType<T, typename decay<F>::type>{forward<F>(f)};
}
/****************************************************************/
// Proof-of-Concept
template<typename Base, typename ...Ts>
struct SubVariant
{
Base shared_data;
boost::variant<Ts...> sub;
template<typename Visitor>
friend auto apply(Visitor visitor, SubVariant &operand)
{
using result_type = typename common_type
<
decltype( visitor(shared_data, declval<Ts&>()) )...
>::type;
return boost::apply_visitor(make_result_type<result_type>([&](auto &x)
{
return visitor(operand.shared_data, x);
}), operand.sub);
}
};
/****************************************************************/
// Demo:
struct ThingData
{
int f1;
int f2;
};
struct Summator
{
void doAction1(ThingData &self) { self.f1 += self.f2; }
const char* type_name() { return "Summator"; }
};
struct Substractor
{
void doAction1(ThingData &self) { self.f1 -= self.f2; }
const char* type_name() { return "Substractor"; }
};
using Thing = SubVariant<ThingData, Summator, Substractor>;
int main()
{
auto test = [](auto &self, auto &sub)
{
sub.doAction1(self);
cout << sub.type_name() << " " << self.f1 << " " << self.f2 << endl;
};
Thing x = {{5, 7}, Summator{}};
apply(test, x);
x.sub = Substractor{};
apply(test, x);
cout << "size: " << sizeof(x.sub) << endl;
}