C++ template instantiation, error: member of non-class type 'int'

Question

I am trying to implement a binary search tree in C++. I have run into a problem while recursively calling function Node<T>::append in its own definition.

Here is a minimum reproducible example:

#include <iostream>
#include <string>
#include <memory> // std::unique_ptr<>

using namespace::std;

template<class T> class Node {
public:
    // constructors
    Node() {};
    Node(const T&);

    // operations
    void append(const T&);
    void print();

private:
    unique_ptr<T> value, left_child, right_child;
};

template<class T> class BinaryTree {
public:
    // constructors
    BinaryTree() {};
    BinaryTree(const T&);

    // operations
    void insert(const T&);
    void output();

private:
    Node<T> root;
    int size;
};

template<class T> Node<T>::Node(const T& in): value(new T (in)), left_child(nullptr), right_child(nullptr) {}

template<class T> void Node<T>::append(const T& in) {
    if (in < *value) {
        if (left_child)
            left_child->append(in);
        else
            left_child(new Node(in));
    } else if (in > *value) {
        if (right_child)
            right_child->append(in);
        else
            right_child(new Node(in));
    }
}

template<class T> void Node<T>::print() {
    cout << string(6,' ') << "( " << *value << " ) " << endl;
    if (left_child)
        left_child->print();
    if (right_child) {
        cout << string(10,' ');
        right_child->print();
    }
}

template<class T> BinaryTree<T>::BinaryTree(const T& in): root(in), size(1) {}

template<class T> void BinaryTree<T>::insert(const T& in) {
    root.append(in);
}

template<class T> void BinaryTree<T>::output() {
    root.print();
}

int main()
{
    BinaryTree<int> test(5);
    test.insert(3);
    test.insert(9);
    test.output();

    return 0;
}

g++ logs in the following error:

error: request for member 'append' in 
'*((Node<int>*)this)->Node<int>::left_child.std::unique_ptr<_Tp, _Dp>::operator-><int, std::default_delete<int> >()', 
which is of non-class type 'int' left_child->append(in);

I think the compiler sees the line left_child->append(in); not as a recursive call but as some functor to a function that does not exist.

How can I solve this issue? See online compilation: https://godbolt.org/z/Pna9e5

Answer 1

left_child and right_child do not point Node. The compiler explains this pretty clearly: which is of non-class type 'int' left_child, left_child is of type int, not class. The declarations

unique_ptr<T> value, left_child, right_child;

should be

unique_ptr<T> value;
unique_ptr<Node<T>> left_child, right_child;

The further issue: left_child(new Node(in));, left_child is not a function, the statement must be left_child.reset(new Node(in));.