Simply

A playground for external polymorphism in C++

• No macros
• Concise syntax for defining and using mixins
• Supports allocators
• constexpr in C++26

#include <simply/destructible.hpp>
#include <simply/dyn.hpp>

#include <iostream>
#include <numbers>
#include <vector>

struct insertable : simply::member_base {
  static auto fn(const auto &value, std::ostream &out) -> std::ostream & {
    return out << value;
  }
};

template <typename Self>
struct simply::iface<insertable, Self> {
  friend auto operator<<(std::ostream &out, const Self &self) -> std::ostream & {
    return simply::fn<insertable>(self, out);
  }
};

int main() {
  using namespace std::string_literals;

  struct mixins : simply::composes<insertable, simply::destructible> {};

  std::vector<simply::dyn<mixins>> values;
  values.emplace_back("Hello, world!"s);
  values.emplace_back(4);
  values.emplace_back(std::numbers::pi);

  for (const auto &value : values) {
    std::cout << value << '\n';
  }
}

Supported Compilers

• Clang 19, 20
• GCC 14, 15

Features

• dyn<Mixin, Storage, Dispatch> for type erasure of mixins
• Concepts:
  • mixin<Mixin>
  • fundamental<Mixin>
  • compound<Mixin>
  • composition<Mixin>
  • choice<Mixin>
  • mixin_for<Mixin, T>
  • has_mixin<T, Mixin>
• Predefined mixins for common use-cases:
  • composes<Mixins...>
  • chooses<Mixins...>
  • destructible
  • movable
  • copyable
  • invocable<Fn>
  • extractable<In>
  • insertable<Out>
  • identifiable
  • allocator_storage<Allocator>
  • indirect_dispatch
  • inplace_dispatch
• iface<Mixin, Self> to specialize interfaces of dyn
• impl<Mixin, T> to specialize implementations of a mixin

Planned Features

• inplace_storage<Size, Align>
• shared_storage
• copy_on_write_storage
• nullable
• retrievable
• trivially_copyable
• trivially_destructible
• equality_comparable<Policy>
• three_way_comparable<Ordering, Policy>
  • Policy determines behavior when erased types mismatch
• assignable_from<T> support for type erasing sinks
• slot_dispatch for vtable lookup using a static slot map key
• Mixins for iterators
• Specializations of impl to delegate member mixins through:
  • std::reference_wrapper
  • std::unique_ptr
  • std::shared_ptr
  • std::weak_ptr
  • std::indirect
  • std::polymorphic
  • std::variant (if every alternative has the mixin)
  • std::optional (if both T and std::nullopt_t have the mixin)
  • std::expected (if both T and E have the mixin)
  • std::pair (if either first or second exclusively has the mixin)
  • std::tuple (if one element exclusively has the mixin)
• Specializations of dyn that behave equivalently to:
  • std::function
  • std::copyable_function
  • std::move_only_function
  • std::function_ref
  • std::any
  • std::ranges::any_view