Using shared_ptr for unique ownership (kind of) - is this good practice?

Question

this is quite hard to explain but I\'ll try my best. So, I have a RenderComponent, EventManager and RenderSystem. In my RenderComponents constructor, I raise a renderCompone

Answer 1

There's certainly nothing wrong with using std::weak_ptr to grant access to an object that might be destroyed, that's what it was invented for. But it does necessitate that the object itself be held by a std::shared_ptr. Not only does this mask your intent to have the object lifetime controlled by its parent, it forces dynamic allocation of the object and precludes it from being a member variable of the parent.

An alternate approach is to keep track of the pointer through a handle, and have a handle manager that tracks whether the object is alive or dead. The safest way to implement this is to make the manager a base class of the object you're tracking, that way RAII ensures it is always up to date. Here's a sample implementation of that concept. Note: untested.

template<class Derived>
class HandleBased
{
public:
    typedef uint64_t handle_t;

    HandleBased() : m_Handle(NextHandle())
    {
        Map()[m_Handle] = this;
    }

    ~HandleBased()
    {
        auto it = Map().find(m_Handle);
        Map().erase(it);
    }

    handle_t ThisHandle()
    {
        return m_Handle;
    }

    static Derived* FindPtr(handle_t h)
    {
        auto it = Map().find(h);
        if (it == Map().end())
            return null_ptr;
        return static_cast<Derived*>(it->second);
    }

private:
    static handle_t NextHandle()
    {
        static handle_t next = 0;
        return next++;
    }

    static std::unordered_map<handle_t, HandleBased*>& Map()
    {
        static std::unordered_map<handle_t, HandleBased*> the_map;
        return the_map;
    }

    handle_t m_Handle;
};

And here's an example of how you'd use it:

class RenderNode : public HandleBased<RenderNode>
{
};

class RenderComponent
{
    std::unique_ptr<RenderNode> node1;
    RenderNode node2;

public:
    void Setup(RenderSystem& rs)
    {
        node1 = new RenderNode;
        rs.nodes.push_back(node1->ThisHandle());
        rs.nodes.push_back(node2.ThisHandle());
    }
};

class RenderSystem
{
public:
    std::list<RenderNode::Handle> nodes;

    void DoRender()
    {
        for (auto it = nodes.begin(); it != nodes.end(); )
        {
            RenderNode* p = RenderNode::FindPtr(*it);
            if (p == NULL)
                it = nodes.erase(it);
            else
            {
                p->DoSomething();
                ++it;
            }
        }
    }
};