std::shared_ptr Class Factory C++

Question

We have a base class.....

class Node
{
   public:
      std::string Name;
      Node(){};
      ~Node(){};
}

Filter is a derived class of Node. Filter is the same a node, except it has a 'filterId' property and it's constructor populates Node.Name='Filter'.

class Filter: public Node
{
   public:
      int filterId;
      Filter() : Node()
      {
         Name="Filter";
      }
}

I have a ClassFactory that returns std::shared_ptr<Node> when passed a string

Please find the code to that factory on CodeProject following this link.

http://www.codeproject.com/Articles/567242/AplusC-b-bplusObjectplusFactory.

The code seems to instantiate a Filter, but, returns a Node. i.e. Object slicing.

std::shared_ptr<Node> n = std::make_shared(Node);  // As expected, doesn't not have filter ID and node.Name is blank.
std::shared_ptr<Filter> f = std::make_shared(Filter);  // As expected, has filterID and node.Name is 'Filter'.

Now i want to handle an array of std::shared_ptr(Node) which may actually contain std::shared_ptr(Filter) depending on what the class is requested from the factory.

std::shared_ptr<Node> nf = std::make_shared(Filter);     // Error. Does not have filterId, but does have **Node**.Name='Filter'.

I even tried the simpler case..... and got the same problem.

Node n = Node();
Filter f = Filter();
Node nf =Filter();   // Error, nf does not have a filterID, but it's Name field is set to 'Filter'.

What am i doing wrong here? The Node is a base class for all of my derived classes. When i have a function that accepts a Node, i should be able to send it a Filter and have the filterId available.

Pointers are advised to help in situations like this, however the shared_ptr is a pointy as things will get.

Praetorian Praetorian · Accepted Answer · 2014-09-18T03:07:12

Node doesn't have any knowledge of additional data members and functions that derived classes may define. So you need to retrieve a shared_ptr<Filter> to be able to access filterId. Use std::dynamic_pointer_cast to do this.

std::shared_ptr<Node> n = ...
auto f = std::dynamic_pointer_cast<Filter>(n);

if(f) {
    // The managed object is a Filter
    std::cout << f->filter_id << '\n';
}

Live demo

And if you know that n always contains a Filter (or something derived from Filter), you can use std::static_pointer_cast instead.

std::shared_ptr Class Factory C++

2 Answers