Intrusive value semantic polymorphism example.

poly_any.cpp

#include <memory>

using namespace std;

template <typename C> // Concept base class
class any {
  public:
    template <typename T>
    any(T x) : self_(new model<T>(move(x))) { }
    
    template <typename T, typename F>
    any(unique_ptr<T>&& x, F cpy) : self_(new model_unique<T, F>(move(x), move(cpy))) { }
    
    any(const any& x) : self_(x.self_->copy()) { }
    any(any&&) noexcept = default;
    
    any& operator=(const any& x) { any tmp = x; *this = move(tmp); return *this; }
    any& operator=(any&&) noexcept = default;
    
    C* operator->() const { return &self_->dereference(); }
    C& operator*() const { return self_->dereference(); }

  private:
    struct regular {
        virtual regular* copy() const = 0;
        virtual C& dereference() = 0;
    };
    
    template <typename T>
    struct model : regular {
        model(T x) : data_(move(x)) { }
        regular* copy() const { return new model(*this); }
        C& dereference() { return data_; }
        
        T data_;
    };
    
    template <typename T, typename F>
    struct model_unique : regular {
        model_unique(unique_ptr<T>&& x, F cpy) : data_(move(x)), copy_(move(cpy)) { }
        regular* copy() const { return new model_unique(copy_(*data_), copy_); }
        C& dereference() { return *data_; }
        
        unique_ptr<T> data_;
        F copy_;
    };
    
    unique_ptr<regular> self_;
};

/**************************************************************************************************/

#include <iostream>
#include <string>
#include <vector>

using namespace std;

class shape {
  public:
    virtual void draw(ostream& out, size_t position) const = 0;
};

class square : public shape {
    void draw(ostream& out, size_t position) const
    { out << string(position, ' ') << "square" << endl; }
};

class circle : public shape {
    void draw(ostream& out, size_t position) const
    { out << string(position, ' ') << "circle" << endl; }
};

int main() {
    any<shape> x = { unique_ptr<square>(new square()),
        [](const square& x) { return unique_ptr<square>(new square()); } };
    vector<any<shape>> a = { square(), circle(), x };
    for (const auto& e : a) e->draw(cout, 0);
}