Trying to overload << in a templated Binary Tree

This is a discussion on Trying to overload << in a templated Binary Tree within the C++ Programming forums, part of the General Programming Boards category; So that if I have a binary tree: Code: BinaryTree<int> *t = new BinaryTree<int>(5); And there have been more numbers ...

  1. #1
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    Trying to overload << in a templated Binary Tree

    So that if I have a binary tree:

    Code:
    BinaryTree<int> *t = new BinaryTree<int>(5);
    And there have been more numbers inserted I can just say:

    Code:
    cout << bn;
    I have overloaded this operator before, but never with templates, so I am unsure where I should start.

    Here is my BinaryTree class:

    Code:
    template <class T>
    class BinaryTree{
    	public:
    		BinaryTree(T); //constructor
    		BinaryTree<T>* insert(T, BinaryTree<T>*);
    		void inorder(BinaryTree<T>*); //prints using inorder traversal
    		int countNodes(BinaryTree<T>*);
    	private:
    		T data;
    		BinaryTree* left;
    		BinaryTree* right;
    };
    I believe the function should be something like this:
    Code:
    friend ostream& operator<< <>(ostream&, const BinaryTree<T>&)
    I know you can't put << or >> overloaded functions as a member function, so does this not require a prototype, or just put it outside of the class and above the implementation?

    Hope this makes sense, thanks.

    *EDIT* Actually, I am supposed to make it a friend function, so I would put it in the class
    Last edited by chinesepirate; 11-23-2008 at 10:28 AM.

  2. #2
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    Code:
    template <typename T>
    ostream& operator<< (ostream&, const BinaryTree<T>&);
    should work.

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  3. #3
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    If i do this:

    Code:
    template <class T>
    ostream& operator<< (ostream& out, const BinaryTree<T>& bt){
    	out << bt.data;
    	return out;
    }
    cout << t; just prints a memory address

  4. #4
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    is t a tree, or a pointer to tree. If the latter, you either need a function that takes a tree pointer, or use *t to print the actual tree.

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  5. #5
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    Yes its a pointer, I tried this as well:

    Code:
    friend ostream& operator<<(ostream&, const BinaryTree<T>*);
    And implementation:

    Code:
    template <class T>
    ostream& operator<<(ostream& out, const BinaryTree<T>* bt){
    	out << bt->data;
    	return out;
    }
    But that gives me some fatal error

  6. #6
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    So what is the ACTUAL (complete) code?

    I suspect you haven't actually got a valid pointer to data.

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  7. #7
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    binarytree.cpp
    Code:
    #include <iostream>
    using namespace std;
    
    template <class T>
    class BinaryTree{
    	public:
    		BinaryTree(T);
    		BinaryTree<T>* insert(T, BinaryTree<T>*);
    		void inorder();
    		int countNodes(BinaryTree<T>*);
    		BinaryTree<T>* reverse(BinaryTree<T>*);
    		friend ostream& operator<<(ostream&, const BinaryTree<T>*);
    	private:
    		T data;		
    		BinaryTree* left;
    		BinaryTree* right;
    };
    
    template <class T>
    BinaryTree<T>::BinaryTree(T x){
    	data=x;
    	left=NULL;
    	right=NULL;
    }
    
    template <class T>
    BinaryTree<T>* BinaryTree<T>::insert(T x, BinaryTree<T>* bt ){
    	if( bt == NULL ){
    		bt = new BinaryTree<T>(x);
        }
        else if( x < bt->data ){
            bt->left = insert( x, bt->left );
        }
        else if( x > bt->data ){
            bt->right = insert( x, bt->right );
        }
        return bt;
    }
    
    template <class T>
    void BinaryTree<T>::inorder(){
        if(this!=NULL){
            this->left->inorder();
            cout << this->data << endl;
            this->right->inorder();
       }
    }
    
    template <class T>
    int BinaryTree<T>::countNodes(BinaryTree<T>* bt) {
    	if ( bt == NULL ) return 0;
        else {
    		int count = 1;
            count += countNodes(bt->left);
            count += countNodes(bt->right);
            return count;
        }
    }
    
    template <class T>
    BinaryTree<T>* BinaryTree<T>::reverse(BinaryTree<T>* bt){
    	if(bt!=NULL){
    		BinaryTree<T>* temp = bt->left;
    		bt->left=bt->right;
    		bt->right=temp;
    	}
    	return bt;
    }
    
    
    template <class T>
    ostream& operator<<(ostream& out, const BinaryTree<T>* bt){
    	out << bt->data;
    	return out;
    }
    main.cpp
    Code:
    #include <iostream>
    #include "binarytree.cpp"
    using namespace std;
    
    int main(){
    	int choice;
    	int size;
    	bool play=true;
    	BinaryTree<int> *bn = NULL;
            while(play){
    		cout << "Menu Display" << endl << "-------------" << endl << "1) Enter Data" << endl << "2) Print Tree" << endl
    			<< "3) Reverse Tree" << endl << "4) Quit" << endl;
    		cout << "Enter your choice: ";
    		cin >> choice;
    		if(choice==1){
    			int num;
    			cout << "Enter data for tree: ";
    			cin >> num;
    			bn = bn->insert(num,bn);
    		}
    		if(choice==2){
    			size=bn->countNodes(bn);
    			cout << "Tree size = " << size << endl << "-------------" << endl;
    			cout << bn;
    		}
    		if(choice==3){
    			bn = bn->reverse(bn);
    		}
    		if(choice==4){
    			play=false;
    		}
    	}
    
    	system("PAUSE");
    	return 0;
    }

  8. #8
    C++ Witch laserlight's Avatar
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    bn is a pointer, so:
    Code:
    cout << bn;
    prints an address. As mentioned earlier, you should write:
    Code:
    cout << *bn;
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  9. #9
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    ^^

    error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'BinaryTree<T>' (or there is no acceptable conversion)

    Which doesn't make sense because I don't see anywhere with just BinaryTree<T>, it is all BinaryTree<T>* I think

  10. #10
    & the hat of GPL slaying Thantos's Avatar
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    You never create a valid root node:

    Code:
    BinaryTree<int> *bn = NULL;

  11. #11
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    Its created in the insert function

  12. #12
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    After I fix the warning about the friend function template like this:
    Code:
        template <typename X>
        friend ostream& operator<<(ostream&, const BinaryTree<X>*);
    it works for me:
    Code:
    E:\Temp>g++ -Wall main.cpp
    
    E:\Temp>a
    Menu Display
    -------------
    1) Enter Data
    2) Print Tree
    3) Reverse Tree
    4) Quit
    Enter your choice: 1
    Enter data for tree: 2
    Menu Display
    -------------
    1) Enter Data
    2) Print Tree
    3) Reverse Tree
    4) Quit
    Enter your choice: 2
    Tree size = 1
    -------------
    2Menu Display
    -------------
    --
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    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  13. #13
    C++ Witch laserlight's Avatar
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    Read this FAQ: Why do I get linker errors when I use template friends?

    As described, a fix goes like this:
    Code:
    template<typename T>
    class BinaryTree;
    
    template<typename T>
    std::ostream& operator<<(std::ostream& out, const BinaryTree<T>& tree);
    
    template<typename T>
    class BinaryTree {
    public:
        // ...
    
        friend std::ostream& operator<< <>(std::ostream& out, const BinaryTree<T>& tree);
    
        // ...
    Actually, it may be good to separate a BinaryTree from a BinaryTreeNode.

    Also, you should rename binarytree.cpp to binarytree.h and get rid of the using directive in it.
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  14. #14
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    Hey thanks, that worked

    I didn't know you had to have a separate template for the friend function

  15. #15
    & the hat of GPL slaying Thantos's Avatar
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    Quote Originally Posted by chinesepirate View Post
    Its created in the insert function
    The only call to insert that I see in your main is
    Code:
    bn = bn->insert(num,bn);
    But you are dereferencing a null pointer to make the call.

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