class action {
   public:
   virtual void act() = 0;
};

class base {
   public:
   void act_f() { act_->act(); }

   std::unique_ptr<action> act_;
};


template <typename F>
class poly_act : public action {
   public:
   void act() override {
      std::invoke(f);
   }

   F f;
};

int main() {

   using lam_t = decltype([]() { std:: cout << 5;});
   using poly_f_t = poly_act<lam_t>;

   base a;
   auto act_f = std::make_unique<poly_f_t>();
   a.act_ = std::move(act_f);

   a.act_f();

   return 0;
}