Pointers confusion - c++

Question

I'm confused about how pointers work here. I have a class called PrefixTree and a struct called TreeNode within this class. I have the following code to build a tree with a string, the problem is every time preorder() is called it doesn't return the root character correctly and after that it throws a segmentation fault.
I want to know if I'm correctly setting the root pointer or if I'm misusing it in the others files.
//prefixtree.cpp
PrefixTree::TreeNode* PrefixTree::buildTree(string& input)
{
char c = input[0];
input.erase(0,1);
TreeNode* node = new TreeNode();
node->character = c;
if (!root)
root = node;
if (c == '*')
{
node->left = buildTree(input);
node->right = buildTree(input);
}
return node;
}
void PrefixTree::preorder()
{
traverse(root);
}
void PrefixTree::traverse(TreeNode* node)
{
if (node)
{
cout << node->character << endl;
traverse(node->left);
traverse(node->right);
}
}
and
//prefixtree.h
class PrefixTree
{
private:
struct TreeNode
{
char character;
TreeNode* left;
TreeNode* right;
};
TreeNode* root;
void traverse(TreeNode* node);
public:
TreeNode* buildTree(string& input);
void preorder();
};
and
//main.cpp
PrefixTree tree;
string a = string("*a**!*dc*rb");
cout << tree.buildTree(a)->character << endl;
tree.preorder();

Answer 1

You need to initialize the pointer members, they are not automatically initialized and may contain garbage values instead of nullptr, which leads to your tests if(!root) and if(node) being useless, reporting uninitialized pointers as valid, which you will then dereference.
Add constructors:
class PrefixTree
{
private:
struct TreeNode
{
char character;
TreeNode* left;
TreeNode* right;
TreeNode() : character('a'), left(nullptr), right(nullptr) {};
};
TreeNode* root;
void traverse(TreeNode* node);
public:
PrefixTree() : root(nullptr) {};
TreeNode* buildTree(string& input);
void preorder();
};