c ++继承和运算符重载

Question

I have two classes, one is the parent of the other. They both need to have different operator<< functions. Each class has an output function that's unique to it. Here's how they are more or less set up:

template <class T>
class Tree {

    protected:
        T elem;
        vector< Tree<T>* > children;

    public:
        ostream &output(ostream& stream,int level) {
            stream << "Tree" << endl;
            for (int j=0;j<level;j++) stream << "    ";
            stream << '\'' << this->elem << '\'' << endl; 
            for (unsigned int i=0;i<this->children.size();i++) {
                this->children[i]->output(stream,level+1);
            }
            return stream;
        }

        template <class U>
        friend ostream &operator<<(ostream &cout,Tree<U> &obj);

};

template <class T>
ostream &operator<<(ostream &stream,Tree<T> &obj) {
    obj.output(stream,0);
    return stream;
};

---

template <class T>
class ParseTree : public Tree<T> {

    protected:
        ParseTree<T>* elemTree;
        vector< ParseTree<T>* > children;

    public:
        ostream &output(ostream& stream,int level) {
            stream << "ParseTree" << endl;
            if (elemTree == NULL) {
                for (int j=0;j<level;j++) stream << "    ";
                stream << '\'' << this->elem << '\'' << endl; 
            }
            else {
                elemTree->output(stream,level+1);
            }
            for (unsigned int i=0;i<this->children.size();i++) {
                this->children[i]->output(stream,level+1);
            }
            return stream;
        }

        template <class U>
        friend ostream &operator<<(ostream &cout,ParseTree<U> &obj);
};

template <class T>
ostream &operator<<(ostream &stream,ParseTree<T> &obj) {
    obj.output(stream,0);
    return stream;
};

Both output functions recursively print out the tree, but ParseTree's is slightly different. The problem I'm having is that when I try to cout << a ParseTree the first iteration is from ParseTree's output function (as confirmed by the stream << "ParseTree" << endl statement), but all subsequent requests seem to be Tree's ouput function (as confirmed by the stream << "Tree" << endl statement). Every object pushed onto the children vector is definitely a ParseTree. My guess is that ParseTree::children is different from Tree::children, and for some reason the context is getting switched. Any ideas?

Answer 1

Make output virtual and define operator<< only once with an argument of type Tree<T> . That should be sufficient.