Function as argument with default value - c++

Question

I want to create class for binary trees:
struct TreeNode {
explicit TreeNode(int _value) : value(_value) {}
int value = 0;
TreeNode* left = nullptr;
TreeNode* right = nullptr;
};
class BTree {
public:
void Add(int value);
void PrintPostOrder(void (*func)() = print_current);
~BTree();
private:
TreeNode* root = nullptr;
void print_current();
void delete_node();
};
BTree::~BTree() {
PrintPostOrder(delete_node);
}
My idea - for destructor and for Printing I need to do binary tree traversal. So I want to create function Traversal and use function as parameter in it:
if I need to print func = print and for destructor func = delete_node.
Error is here:
void PrintPostOrder(void (*func)() = print_current);
the default argument of type "void (BTree :: ) ()" is incompatible
with the parameter of type "void () ()"
I don't know how to set default value for parameter when parameter is a function.

Answer 1

print_current and delete_node are member functions, so you need a member function pointer:
class BTree {
public:
void PostOrder(void (BTree::*fn)() = &BTree::print_current) {
std::invoke(fn, this);
}
~BTree() {
PostOrder(&BTree::delete_node);
}
private:
void print_current();
void delete_node();
};
For more flexibility you can make PostOrder a template:
struct TreeNode {};
class BTree {
public:
template<class Fn>
void PostOrder(Fn fn) {
std::invoke(fn);
}
void PostOrder() {
PostOrder([this] { print_current(); });
}
~BTree() {
TreeNode* node;
PostOrder([this, node] { delete_node(node); });
}
private:
void print_current();
void delete_node(TreeNode*);
};

Answer 2

In principle you set a default parameter for a function the way you did. The problem is that a member function is of different type than a free function.
This is a free function pointer void (*func)(), while print_current is a member function of type void (BTree :: ) ().
Either fix the parameter type or use a free function as default parameter.
Also do not forget that member functions are fundamentally different from free functions, because you need an instance to call them.