c++ - template using code error

Question

I've recently started learning C++ and have a problem with understanding template mechanism. The task is first of all to build a user-defined binary tree. I can't compile my code.

#include <iostream>
using namespace std;
template <typename DataType> struct TreeNode
{
    TreeNode(DataType val, TreeNode *leftPtr = null, TreeNode *rightPtr = null)
    {
        left = leftPtr;
        right = rightPtr;
        data = val;
    }
    TreeNode *left, *right;
    DataType data;
};
template <typename DataType> TreeNode *BuildTree()
{
    TreeNode *root = new TreeNode(10, new TreeNode(20), new TreeNode(30));

    TreeNode *curRoot = root;
    curRoot = curRoot->left;
    curRoot->left = new TreeNode(40);
    curRoot->left->left = new TreeNode(70);
    curRoot->right = new TreeNode(50);
    curRoot = curRoot->right;
    curRoot->left = new TreeNode(80, new TreeNode(100), new TreeNode(110));
    curRoot = root->right;
    curRoot->left = new TreeNode(60);
    curRoot = curRoot->left;
    curRoot->right = new TreeNode(90, new TreeNode(120), new TreeNode(130));

    return root;
}
int main()
{
    TreeNode *treeRoot = BuildTree<int>();
    cin.get();
    return 0;
}

Function BuildTree builds the concrete instance of tree, with concrete data type. Please help me to understand my mistake. Please don't pay attention to the fact I don't free memory after using. That's just a draft.

Answer 1

Since TreeNode is a template class, you have to specify the specialization when you use it:

template <typename DataType> TreeNode *BuildTree()

should be

template <typename DataType> TreeNode<DataType> *BuildTree()

and

TreeNode *root = new TreeNode(10, new TreeNode(20), new TreeNode(30));

should be

TreeNode<DataType> *root = new TreeNode(10, new TreeNode(20), new TreeNode(30));

and others in the method, plus in main:

TreeNode<int> *treeRoot = BuildTree<int>();

instead of

TreeNode *treeRoot = BuildTree<int>();

Answer 2

I think your code should look like this:

#include <iostream>
using namespace std;
template <typename DataType> struct TreeNode
{
    TreeNode(DataType val, TreeNode<DataType> *leftPtr = null, TreeNode<DataType> *rightPtr = null)
    {
        left = leftPtr;
        right = rightPtr;
        data = val;
    }
    TreeNode<DataType> *left, *right;
    DataType data;
};
template <typename DataType> TreeNode<DataType> *BuildTree()
{
    TreeNode<DataType> *root = new TreeNode(10, new TreeNode<DataType>(20), new TreeNode<DataType>(30));

    TreeNode<DataType> *curRoot = root;
    curRoot = curRoot->left;
    curRoot->left = new TreeNode<DataType>(40);
    curRoot->left->left = new TreeNode<DataType>(70);
    curRoot->right = new TreeNode<DataType>(50);
    curRoot = curRoot->right;
    curRoot->left = new TreeNode<DataType>(80, new TreeNode<DataType>(100), new TreeNode<DataType>(110));
    curRoot = root->right;
    curRoot->left = new TreeNode<DataType>(60);
    curRoot = curRoot->left;
    curRoot->right = new TreeNode<DataType>(90, new TreeNode<DataType>(120), new TreeNode<DataType>(130));

    return root;
}
int main()
{
    TreeNode<int> *treeRoot = BuildTree<int>();
    cin.get();
    return 0;
}