How to construct an class template with std::allocator construct - c++

Question

How can i construct my class template in my member function newNode (in std=++98) without new operator ?
I tried to put an constructor of Node or the variable key but it doesn't work .
template <class T>
class Node{
public:
Node(){
_left = NULL;
_right = NULL;
_height = 0;
};
Node(const Node &ref);
Node &operator=(const Node &ref);
~Node(){};
T _key;
Node *_left;
Node *_right;
int _height;
};
template <class T, class A = std::allocator<Node<T> > >
class Tree{
public:
[...]
//attribut
Node<T> *head;
A _alloc;
Node<T> *newNode(T key){
Node<T> *node = _alloc.allocate(sizeof(Node<T>));
_alloc.construct(node, ???);
return (node);
}
};
What can i put in my 2nd argument of construct ?

Answer 1

allocate() already calculates the memory required for objects, you shouldn't use sizeof there. Also, don't use construct(), it was removed in C++20. Use placement new instead:
Node<T> *newNode(T key){
Node<T> *node = _alloc.allocate(1); // allocate 1 * sizeof(Node<T>)
new(node) Node<T>(/*arguments to Node<T> constructor, but Node<T> only has default constructor, so nothing goes here*/);
return node;
}
With C++20, you can also use std::construct_at(), which has easier syntax than placement new:
Node<T> *newNode(T key){
Node<T> *node = _alloc.allocate(1); // allocate 1 * sizeof(Node<T>)
node = std::construct_at(node);
return node;
}
To use construct() before C++11, you need to create an object to be copied:
Node<T> *newNode(T key){
Node<T> *node = _alloc.allocate(1); // allocate 1 * sizeof(Node<T>)
_alloc.construct(node, Node<T>());
return node;
}