Default parameter in a function wouldnt compile/link without static inline

Question

i was writing a BST class in C++ and wanted to add a default parameter to a function but, when i tried to the visual studio compiler gave compile and linking errors, so after a bit of googling, i made the root variable static inline and it worked, i want to know what does static inline change in this specific situation.

here's the BST class code:

#pragma once

template<typename T>
class BST
{
   template<typename T>
   struct node
   {
    T data;
    node<T>* left;
    node<T>* right;

    node(T d)
    {
        data = d;
        left = right = nullptr;
    }
  };


public:

BST()
{
    root = nullptr;
}

~BST()
{
    delete root;
}

node<T>* insert(T d)
{
    if (root == nullptr)
    {
        root = new node<T>(d);

        return root;
    }
    
    node<T>* current = root;
    node<T>* k = nullptr;
    

    while (current != nullptr)
    {
        k = current;

        if (current->data > d)
        {
            current = current->left;
        }
        else if (current->data < d)
        {
            current = current->right;
        }
    }

    if (k->data > d)
    {
        k->left = new node<T>(d);
    }
    else if (k->data < d)
    {
        k->right = new node<T>(d);
    }

    
    return root;
}


bool find(T d)
{
    if (root == nullptr)
    {
        std::cerr << "Tree is empty! : ";

        return false;
    }

    node<T>* current = root;
    node<T>* k = nullptr;

    while (current->data != d)
    {
        k = current;

        if (current->left == nullptr || current->right == nullptr && current->data != d)
        {
            std::cout << "Not Found! : ";
            
            return false;
        }

        if (current->data > d)
        {
            current = current->left;
        }
        else if (current->data < d)
        {
            current = current->right;
        }
    }

    return true;
}

node<T>* findmax()
{
    node<T>* temp = root;
    while (temp->right != nullptr)
    {
        temp = temp->right;
    }

    return temp;
}

node<T>* findmin(node<T>* temp = root)
{
    
    while (temp->left != nullptr)
    {
        temp = temp->left;
    }

    return temp;
}

private:
static inline node<T>* root;

};

Answer 1

This is because

The keyword this shall not be used in a default argument of a member function

and non static member of a class is bound to this (i.e. root is equivalent to this->root). More about default parameters restrictions here.

In your case you could have the following workaround:

node<T>* findmin(node<T>* temp = nullptr)
{
    if (temp == nullptr) temp = root;

however I am not sure if you should use it, for the sake of clarity.