Question

I am trying to build a BehaviourTree data structure. The “main” class is “BTNode”, and leaves (actions, commands or conditions) are “BTLeaf”. Because I want a BTLeaf to perform some action on my entities, I made it a template class which takes in an object and a member function pointer.

btnode.h:

#ifndef BTNODE_H
#define BTNODE_H

#include <QLinkedList>

class BTNode
{
public:
    BTNode();
    BTNode(QLinkedList<BTNode *> &children);

    ~BTNode();

    virtual bool execute() = 0;

protected:
    QLinkedList<BTNode*> _children;
};

#endif // BTNODE_H

btleaf.h:

#ifndef BTLEAF_H
#define BTLEAF_H

#include "btnode.h"

template <class T> class BTLeaf : public BTNode
{
public:
    BTLeaf(T* object, bool(T::*fpt)(void))
    { _object = object; _fpt=fpt; }

    /* Does not work either:
    BTLeaf(T* object, bool(T::*fpt)(void))
        : BTNode()
    { _object = object; _fpt=fpt; }
    */

    virtual bool execute()
    { return (_object->*_fpt)(); }

private:
    bool (T::*_fpt)(); //member function pointer
    T* _object;
};
#endif // BTLEAF_H

When I try to build the solution (using Qt Creator), I get:

spider.obj:-1: error: LNK2019: unresolved external symbol "public: __thiscall BTNode::BTNode(void)" (??0BTNode@@QAE@XZ) referenced in function "public: __thiscall BTLeaf<class Spider>::BTLeaf<class Spider>(class Spider *,bool (__thiscall Spider::*)(void))"

you can see my attempted solution commented out in my code. If I remove the public BTNode part and use my btleaf “manually”, I get the desired result. Any ideas?

Edit:
It might be worth nothing that I create a BTLeaf in my Spider class this way (temporarely, for testing purposes):

BTLeaf<Spider> test(this, &Spider::sayHello);
test.execute();

Answer 1

Presumably that BTNode default (no-argument) constructor you have declared is not defined anywhere (at least, not anywhere that your linker sees).