ngcpp/proxy/basic-facade-builder

template <template <class> class Skill>
    requires(/* see below */)
using add_skill = Skill<basic_facade_builder>;

#include <iostream>
 
#include <proxy/proxy.h>
 
template <class FB>
using SharedSlim = typename FB                                //
    ::template restrict_layout<sizeof(void*), alignof(void*)> //
    ::template support_copy<pro::constraint_level::nothrow>   //
    ::template add_skill<pro::skills::as_weak>;
 
struct SharedFormattable : pro::facade_builder              //
                           ::add_skill<SharedSlim>          //
                           ::add_skill<pro::skills::format> //
                           ::build {};
 
int main() {
  // Raw pointer does not have shared semantics
  static_assert(!pro::proxiable<int*, SharedFormattable>);
 
  // The implementation of std::shared_ptr is usually larger than sizeof(void*)
  static_assert(!pro::proxiable<std::shared_ptr<int>, SharedFormattable>);
 
  // The built-in shared pointer is guaranteed to be "slim"
  pro::proxy<SharedFormattable> p1 =
      pro::make_proxy_shared<SharedFormattable>(123);
  pro::weak_proxy<SharedFormattable> p2 = p1;
  pro::proxy<SharedFormattable> p3 = p2.lock();
  std::cout << std::format("{}\n", *p3); // Prints "123"
}