Making my binary tree class iterable with unique_ptr

I wanna make my class iterable for this function (assume I can't change this)

// basically my class wrapped in a unique_ptr
using tree_ptr = typename std::unique_ptr<binary_search_tree::binary_tree<T>>;

// then passed as a reference to this tester function along with a the correct sequence
static void test_sort(const tree_ptr<T> &tree, const std::vector<T> &expected)
{
    std::vector<T> actual;
    for (auto& x : *tree) {
        actual.push_back(x);
    }
    REQUIRE(expected == actual); 
}

Here is my class created in the namespace binary_search_tree

namespace binary_search_tree {

template <typename T> 
class binary_tree {

private:
    T t_data, min;
    std::unique_ptr<binary_tree<T>> t_left, t_right;

public:
    binary_tree(T d) : t_data(d), t_left(nullptr), t_right(nullptr)
    {}
    
    void insert(T data) {
        if(data <= t_data) {
            if(t_left == nullptr) {
                t_left = std::unique_ptr<binary_tree<T>>(new binary_tree<T>(data));
            } else {
                t_left->insert(data);
            }          
        } else {
            if(t_right == nullptr)
                t_right = std::unique_ptr<binary_tree<T>>(new binary_tree<T>(data));
            else
                t_right->insert(data);
        }
    }
    
    const T data() const {
        return t_data;
    }
    
    const std::unique_ptr<binary_tree<T>>& left() const {
        return t_left;
    }
    
    const std::unique_ptr<binary_tree<T>>& right() const {
        return t_right;
    }
    
//    const std::unique_ptr<binary_tree<T>>& begin() const {
//        
//        std::unique_ptr<binary_tree<T>>& leaf = this;
//        while(leaf) {
//            leaf = leaf->left();
//        }
//        return leaf;
//    }
    
//    const std::unique_ptr<binary_tree<T>>& end() const {
//        return this;
//    }
//    
//    bool operator!=(const leaf &rhs) {
//        return this;
//    }
//    
//    bool operator*(const leaf &rhs) {
//        return this;
//    }

};

}  // namespace binary_search_tree

The begin() should keep calling left() until it reaches a nullptr, while end() should do the same with right(). I'm confused on what these functions should be returning. I know I need to define an incrementer at some point as well.