Copy On Write Shared Ptr

cow.cpp

#include <iostream>
#include <memory>

struct vec3 {
    int x, y, z;
};

std::ostream& operator<<(std::ostream& out, const vec3& v)
{
    return out << "vec3{" << v.x << ", " << v.y << ", " << v.z << "}";
}

template <typename t>
class cow_ptr : public std::shared_ptr<t> {
public:
    t& operator*() noexcept
    {
        this->reset(new t(*this->get()));
        return *this->get();
    }

    const t& operator*() const noexcept
    {
        return *this->get();
    }

    t* operator->() noexcept
    {
        this->reset(new t(*this->get()));
        return this->get();
    }

    const t* operator->() const noexcept
    {
        return this->get();
    }
};

template <typename t>
std::ostream& operator<<(std::ostream& out, const cow_ptr<t>& cow)
{
    return out << cow.get()
               << " (" << cow.use_count() << ") "
               << *cow;
}

int main()
{
    cow_ptr<vec3> cow1, cow2;

    cow1.reset(new vec3{ 1, 2, 3 });
    cow2 = cow1;

    std::cout << "cow1: " << cow1 << "\n";
    std::cout << "cow2: " << cow2 << "\n\n";

    cow1->y = 5;

    std::cout << "cow1: " << cow1 << "\n";
    std::cout << "cow2: " << cow2 << "\n";
}