Question

Just recently learned the correct application of inheritance of classes in Java and thought to apply it to some code I’m writing in C++.

The following is a header for an empty Node.cpp:

#ifndef Node_h
#define Node_h

#include <string>

using namespace std;

///////////////////////////////////////////////////////
//Class Declarations
class Array;
class Hash;
class Node;
    class BodyNode;
        class RealNode;
            class CharNode;
            class ComboNode;
        class EndNode;
    class HashNode;
    class NodeFollower;

///////////////////////////////////////////////////////
//Class Definitions
class Node{
public:
    virtual string toString(int info) = 0;
    };

class BodyNode: public Node{
public:
    };

class RealNode: public BodyNode{
protected:
    Hash *wordHash, *addressHash;
    Array *followers;
    int count;

//Initialization
    RealNode();
    ~RealNode();

//Hashes
    BodyNode getNext(int address);
    NodeFollower getWord(BodyNode *word);
    NodeFollower getAddress(int address);

    string toString(int info);
    };

#endif

From this code, I get the following error:

Node.h:43:11: error: invalid abstract return type for member function ‘BodyNode RealNode::getNext(int)’
Node.h:28:7: note:   because the following virtual functions are pure within ‘BodyNode’:
Node.h:25:17: note:     virtual std::string Node::toString(int)

Looked this up all over google, stackoverflow, and cplusplus, and while I find a lot of answers that surround this concern, I couldn’t find the exact answer that I needed to fix it. The cplusplus.com site that I was sourcing from is:

http://www.cplusplus.com/doc/tutorial/polymorphism/

Additionally, a few of the answers I found on stackoverflow indicated that

using namespace std;

is to be avoided, and it sounds like for good reason, but I can’t figure out how to include string. I tried

string::string

for each declaration, and I tried

using namespace std::string;

but both were to no avail. Thank you in advance for the help!

Answer 1

Your first problem is that BodyNode inherits from regular Node which has a toString() pure-virtual function. If it’s pure-virtual (the =0) then you MUST provide a definition for it in all derived classes, and you have not.

So, since you have not provided the pure-virtual function definition within BodyNode, it is also an abstract class and uninstantiable (an abstract-class has 1 or more pure-virtual methods so it cannot be instantiated).

You also have an issue that getNext() returns a BodyNode type, and since BodyNode is abstract, this isn’t possible – you COULD return a pointer to it instead of a by-copy-value, but I think your real problem is that it shouldn’t be abstract in the first place.

Provide a default toString() function in BodyNode to fix your problem.