错误：“模板”之前的预期主表达式

Question

Alright, so I'm trying to make a (for now) unbalanced binary tree. I'm getting a weird error that I can't seem to get rid of. The second half had the same error so I just got rid of it for now.

    void insert( binTreeNode < T >*& node, const T& obj) { // private version of insert( )
    if (obj < node->data) {
        if(node->left != NULL)
            insert(node->left, obj);
        else {
            binTreeNode<T> n;
            n = new binTreeNode(obj, NULL, NULL);
            node->left = n;
        }
    } else {
        if(node->right != NULL)
            insert(node->right, obj);
        else {

        }
    }
}

And the error

In file included from ../src/binTree.cc:2:0:
../src/binTree.h: In member function 'void binTree<T>::insert(binTreeNode<T>*&, const T&)':
../src/binTree.h:54:16: error: expected type-specifier before 'binTreeNode'
../src/binTree.h:54:16: error: expected ';' before 'binTreeNode'
src/subdir.mk:24: recipe for target `src/binTree.o' failed
make: *** [src/binTree.o] Error 1

binTreeNode's constructor is defined as

binTreeNode (const T& obj, binTreeNode < T >* leftObj = NULL, binTreeNode < T >* rightObj = NULL) {
    left = leftObj;
    right = rightObj;
    data = obj;
}

Thanks for your help.

Answer 1

        n = new binTreeNode(obj, NULL, NULL);

Should be

        n = new binTreeNode<T>(obj, NULL, NULL);

(also, n should be a pointer).

Classes don't autodeduce template parameters from constructors even when it's unambiguous. I'm not sure why. Maybe it's so that the code doesn't become brittle if you add another constructor.

Anyway, if this really bugs you, you can always add a helper function:

template<typename T>
binTreeNode<T>* newBinTreeNode(const T& obj, binTreeNode<T>* left, binTreeNode<T>* right) {
  return new binTreeNode<T>(obj, left, right);
}

Answer 2

You must precede the definition of insert() with the template<..> prefix when defining a member function outside of the class declaration:

template <typename T>
void binTree<T>::insert(binTreeNode < T >*& node, const T& obj) {
    if (obj < node->data) {
            if(node->left != NULL)
                insert(node->left, obj);
            else {
                binTreeNode<T> n;
                n = new binTreeNode(obj, NULL, NULL);
                node->left = n;
            }
    } else {
        if(node->right != NULL)
            insert(node->right, obj);
        else {

        }
    }
}