I cannot compile using template in C++

c++, templates

Solution

You're mixing up declarations and instantiations. When you declare a template, you don't specify a type immediately after its name. Instead, declare it like this:

template<class T>
class Node {
private:
  const T x_;
  Node *next_;
public:
  Node (const T& k, Node *next) : x_(k), next_(next) { }
  const T& data(){return x_;}
  Node *get_next(){return next_;}
};

Your original declaration also confuses `string`, `const char *`, and generic types that should be in terms of `T`. For a template like this, you probably want to let the user define the type of the member (`x_`). If you explicitly declare it as `const char *` or `string`, you're losing genericity by limiting what the user can use for `T`.

Notice that I changed the types of the instance variables, the parameters of the constructor and the return type of `data()` to be in terms of `T`, too.

When you actually instantiate a variable of the template type, you can provide a concrete type parameter, e.g.:

int main(int argc, const char **argv) {
    Node<char*> *tail = new Node<char*>("tail", NULL);
    Node<char*> *node = new Node<char*>("node", tail);
    // do stuff to mynode and mytail
}

Whenever you write the template name `Node` outside the template declaration, it's not complete until you provide a value for the parameter `T`. If you just say `Node`, the compiler won't know what kind of node you wanted.

The above is a little verbose, so you might also simplify it with a typedef when you actually use it:

typedef Node<char*> StringNode;
int main(int argc, const char **argv) {
    StringNode *tail = new StringNode("tail", NULL);
    StringNode *node = new StringNode("node", tail);
    // do stuff to mynode and mytail
}

Now you've built a linked list of two nodes. You can print out all the values in the list with something like this:

for (StringNode *n = node; n; n = n->get_next()) {
    cout << n->data() << endl;
}

If all goes well, this will print out:

node
tail

Problem

Original source