Runtime error (eos-vm system failure) on using C++ `virtual` keyword?

Question

In my contract I have a data structure that involves a node struct and a LeafNode and InnerNode struct that inherit from node.

struct node {
        ...
        virtual string to_string() const {

            string str("node");

            ...

            return str;
        }
    };

struct LeafNode : public node {
        ...
        virtual string to_string() const {

            string str("LeafNode");

            ...

            return str;
        }
    };

struct InnerNode : public node {
        ...
        virtual string to_string() const {

            string str("InnerNode");

            ...

            return str;
        }
    };

Assuming I have an initialized node* nodeptr;:

When calling eosio::print(nodeptr->to_string());, I want it to call the child's to_string() method if the nodeptr is holding a LeafNode or InnerNode struct instead of just calling the base node::to_string() method.

In vanilla C++, this can be done using the virtual keyword. However, after adding it to my struct methods as shown above, I run into the following error:

Error 3070002: Runtime Error Processing WASM
Error Details:
eos-vm system failure
pending console output: ...

Is this because the virtual keyword is not allowed in EOSIO C++? Or is there something else going on here?

Answer 1

You are using virtual keyword incorrectly. Please, refer to this and this for better understanding.

The virtual specifier specifies that a non-static member function is virtual and supports dynamic dispatch.

If you would like to override node::to_string() method. You have to:

1. create a method with same signature in child class
2. initialize a pointer of parent type as the child type
3. call the method

In other words it should look like this:

struct node {
         virtual string to_string() const {
             string str("node");
             return str;
         }
};

struct LeafNode : public node {
        string to_string() const {
            string str("LeafNode");
            return str;
        }
};

Then initialize pointer as:

node* ptr;
LeafNode leaf;
ptr = &leaf;

And call method:

ptr->to_string();