Binary Search Tree recursion insertion

Question

I'm currently having difficulty inserting a node into a binary tree using recursion. I've been dwelling on this problem for a few days now and thought it was time I came looking for answers!

Node class (.h):

#ifndef STUDENT_MACROGUARD
#define STUDENT_MACROGUARD

#include <cstdlib>
#include <string>

namespace student_class
{
    class student
    {
        public:
            // Constructors / Destructors;

            student(const float entry = 0, std::string name = "", 
                        student* left_ptr = NULL, student* right_ptr = NULL);
            ~student(void){};

            // Mutating member functions;

            void set_grade(const float entry);
            void set_name(std::string entry);

            void set_left(student* node_ptr);
            void set_right(student* node_ptr);

            // Non mutating member functions;

            float grade(void);
            std::string name(void);

            student* left(void);
            student* right(void);

            // Const member functions;

            const student* left(void) const;
            const student* right(void) const;

    private:
            std::string student_name;
            float grade_field;
            student* left_ptr;
            student* right_ptr;
    };
}

#endif

BSTree class to implement the Binary Tree data structure (.h):

#ifndef BTTree_MACROGUARD
#define BTTree_MACROGUARD

#include <cstdlib>
#include <string>
#include <iostream>
#include "student.h"

using namespace student_class;

namespace BTTree_class
{
class BTTree
{
    public:
            // Constructors / Destructors; 

            BTTree(student* node_ptr = NULL);
            ~BTTree(void);

            // Mutating member functions;

            void insert(student* node_ptr = NULL, const float grade = 0, std::string name = "");
            void remove(student* node_ptr);

            // Non mutating member functions;

            student* grade_search(const float entry);
            student* name_search(const std::string entry);
            student* root(void);
            void print_tree(student* node_ptr);

            // Const member functions;

            const student* grade_search(const float entry) const;
            const student* name_search(const float entry) const;
            const student* root(void) const;

    private:
            student* root_ptr;
    };
}

#endif

The insert member function implementation I'm using to insert nodes into the tree:

    void BTTree::insert(student* node_ptr, const float grade, std::string name)
{
    if(root_ptr == NULL)
    {
        root_ptr = new student(grade, name);
        return;
    }

    if(node_ptr == NULL)
    {
        node_ptr = new student(grade, name);
        return;
    }
    else if(node_ptr->grade() > grade)
    {
        insert(node_ptr->left(), grade, name);
    }
    else
    {
        insert(node_ptr->right(), grade, name);
    }
}

I don't understand why this insertion isn't working. The code looks flawless and it's left me scratching my head. I've written an alternate insertion function which uses iteration, but recursion is a must.

Any help would be fantastic, thank you.

Answer 1

The problem is here:

if(node_ptr == NULL)
{
    node_ptr = new student(grade, name);
    return;
}

node_ptr is a local variable, because you pass it by value. Thus, the assignment is lost when you exit the function.

To fix it - pass by reference:

void BTTree::insert(student* &node_ptr, const float grade, std::string name)

That will require these changes, of course:

        student* & left(void);
        student* & right(void);