Reference from a class to take a sub class

**a.mlw.walker** · 12-17-2014

Sorry, my title may not be very descriptive. Perhaps I can do better here

I have a class called Node. I am subclassing it to make a class called CustomNode.
Another class takes a reference to a vector of Node pointers:

Code:

void getNeighbouringNodes(Node *n, std::vector<Node*> &nodes);

I am calling it like this:

Code:

  std::vector<CustomNode*> *nodes;
  nodes = new (std::vector<CustomNode*>);
  graph->getNeighbouringNodes(n, *nodes);

I get the error message:

error: non-const lvalue reference to type 'vector<class graphDB::Node *>' cannot bind to a value of unrelated
type 'vector<class CustomNode *>'

I understand the error I think: the method getNeighbouringNodes is expecting a refernce to a vector of Node pointers, but I am passing it a reference to a vector of CustomNode pointers. CustomNode pointers is a subclass of Node so I expect somehow I can pass a reference vector of CustomNode pointers into something expecting a reference vector of Node pointers?????

Thanks
Alex

**grumpy** · 12-17-2014

Your CustomNode is derived from Node. However, a vector<CustomNode*> is NOT derived from vector<Node *>.

A vector<Node *> can have CustomNode *'s as elements though, which is probably what you need to do.

It is also not necessary for nodes to be a pointer, nor to dynamically allocate it. It can simply be an instance of a vector<Node *>.

**Elysia** · 12-17-2014

You can also use static polymorphism:

template<typename Node_t>
void GetNeighbouringNodes(Node_t* n, std::vector<Node_t*>& nodes);

**grumpy** · 12-17-2014

Originally Posted by Elysia

You can also use static polymorphism:

template<typename Node_t>
void GetNeighbouringNodes(Node_t* n, std::vector<Node_t*>& nodes);

Maybe, although - from info the OP has provided - I doubt it is warranted in this case.

For example, it's a fair bet that the GetNeighbouringNodes() member function attempts to copy additional pointers to Node into the array passed - which won't work if a vector<CustomNode *> is passed. Static polymorphism does not help with that (except in a few very specific circumstances).

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