Using CRTP with virtual inheritance

问题:

I have a hierarchy of nodes, where "diamond" can occurred.

Every node must be clonable but I don't want to write clone method to every node. So I use CRTP.

class Node
{
public:
    Node(){}
    Node(Fill*) { }

    virtual ~Node() {}
    virtual Node * clone() const = 0;

    virtual void id() { std::cout << "Node\n"; }
};

//====================================================================

template <typename Base, typename Derived>
class NodeWrap : public Base
{
public:

    NodeWrap() { } 
    NodeWrap(Fill * arg1) : Base(arg1) { }

    virtual Node *clone() const
    {
        return new Derived(static_cast<Derived const &>(*this));
    }
};

works as follows:

class NodeA : public NodeWrap<Node, NodeA>
{
public:
    typedef NodeWrap<Node, NodeA> BaseClass;

    NodeA() { }
    NodeA(Fill * f) : BaseClass(f) { }

    virtual void id() { std::cout << "NodeA\n"; }

}; 

First question:

There is know BUG in VS when "covariance is used with virtual inheritance". Is there a way to overcome the bug, and still have covariant types is clone method?

I changed return type to be Node instead of Base. I can live with that, but I would like to have Base as return type

Second question: Problem occurred when multiple inheritance comes to play. I created new wrapper, which inherits virtually

template <typename Base, typename Derived>
class NodeWrapVirtual : public virtual Base
{
public:

    NodeWrapVirtual() { }
    NodeWrapVirtual(Fill * arg1) : Base(arg1) { }

    virtual Node *clone() const
    {
        return new Derived(static_cast<Derived const &>(*this));
    }
};

and now building diamond structure:

class NodeB : public NodeWrapVirtual<Node, NodeB>
{
public:
typedef NodeWrapVirtual<Node, NodeB> BaseClass;

NodeB() { }
NodeB(Fill * f) : BaseClass(f) { }

virtual void id() { std::cout << "NodeB\n"; }
};

//====================================================================

class NodeC : public NodeWrapVirtual<Node, NodeC>
{
public:
    typedef NodeWrapVirtual<Node, NodeC> BaseClass;

    using BaseClass::clone;

    NodeC() { }
    NodeC(Fill * f) : BaseClass(f) { }

    virtual void id() { std::cout << "NodeC\n"; }
};

and problematic diamond node:

class NodeD : public NodeWrap<NodeB, NodeD>,
              public NodeWrap<NodeC, NodeD>
{
public:

    typedef NodeWrap<NodeB, NodeD>  BaseClassB;
    typedef NodeWrap<NodeC, NodeD>  BaseClassC;

    NodeD() { }
    NodeD(Fill * f) : BaseClassB(f), BaseClassC(f) { }

    using BaseClassB::clone;  // (1)
    virtual NodeD *clone() const { return new NodeD(*this); }       // (2)

    virtual void id() { std::cout << "NodeD\n"; }
};

where are 2 lines I am curious about. (line (1) and (2))

If both lines are removed, there is oblivious compile error, because there is ambiguous clone method (from every parent). Since I don't use covariant return types, there should work clone method form each parent, so i use line (1) but it doesn't work. Still ambiguous.

So I use line (2) and it works.

Is there a nice way, to avoid writing line (2)?

HERE is full working example on ideone.

回答1:

First you should be very carefull to use virtual inheritance with members inside the virtual base (look at https://stackoverflow.com/a/1193516/1918154, "Effective C++", item 20: "Avoid data members in public interfaces" and http://www.parashift.com/c++-faq-lite/multiple-inheritance.html#faq-25.8). Your node gets an pointer to a fill which is not used, but it looks like you need it somewhere.

Your problem can be solved when you move the inhertance relationship (public virtual and public) in the base class for your NodeWrap.

template <typename Base>
class  InheritVirtual
    : public virtual Base
{};

template <typename... Bases>
class InheritBases
    : public Bases...
{
    virtual Node* clone() const = 0;
    virtual void id() const = 0;
};

class NodeB : public NodeWrap<InheritVirtual<Node>, NodeB>
{ 
   //...
};


class NodeC : public NodeWrap<InheritVirtual<Node>, NodeB>
{ 
   //...
};

class NodeD : public NodeWrap<InheritBases<NodeB,NodeC>, NodeD>
{ 
   //...
};

Running Example.

The pure virtual methods in InheritBases are needed because the so called domination rule (Dominance in virtual inheritance).

The problem to be solved is a way to transfer paramters to the right constructor in case of multiple bases. Unlike Node (wich is a virtual base) it is ok to let NodeB and NodeC have member variables and non trivial constructors.

回答2:

Each virtual function must have a unique final overrider in each derived class. This has nothing to do with name lookup (the requirement is for the functions, not for their names), thus using is irrelevant.

Use a multi-base-classed node class template:

 template <class Derived, class Base1, class Base2>
 class node2 : //  etc
 // or use a variadic template if you have more than two bases

As for covariant returns, they are strictly unnecessary, if convenient. You can always split each virtual function into a private virtual and a public non-virtual. This comes handy if you want to return covariant smart pointers, which is not supported by the regular covariant return machinery at all.