How to access array elements of pointers - c++

Question

How can I access elements from my Node pointer array? The cout at the bottom returns an address "0xb7738ff4". Even if I make a constructor and set each element to NULL, I cannot modify any of them later on.
#include <iostream>
using namespace std;
class Node
{
public:
char character;
};
class Tree
{
public:
Node* nodes[26];
};
int main() {
Tree t;
//t.nodes[0]->character = 'a';
cout << "\"" << (t.nodes[0]) << "\"";
}
http://ideone.com/XvLme9

Answer 1

t.nodes[0] is returning a Node* pointer. You are passing that pointer as-is to cout, that is why it is printing a memory address (and a random one at that, because you are not initializing the array). If you want to print the character of a node, you have to dereference the Node* pointer, exactly like your commented out code is doing (which is the correct way to do it):
t.nodes[0]->character
You just have to make sure that nodes[0] returns a valid pointer to a real Node object to begin with:
Tree t;
t.nodes[0] = new Node; // <-- here
t.nodes[0]->character = 'a';
std::cout << "\"" << t.nodes[0]->character << "\"" << std::endl;
Don't forget to delete the node when you are done using it. Tree should have a destructor that frees the nodes it owns.
Try something more like this:
#include <iostream>
#include <stdexcept>
class Node
{
public:
char character;
Node(char c = 0);
};
class Tree
{
private:
Node* nodes[26];
int count;
public:
Tree();
~Tree();
Node* add(char c);
Node* get(int idx);
};
Node::Node(char c)
: character(c)
{
}
Tree::Tree()
: count(0)
{
for (int i = 0; i < 26; ++i)
nodes[i] = NULL;
}
Tree::~Tree()
{
for (int i = 0; i < count; ++i)
delete nodes[i];
}
Node* Tree::add(char c)
{
if (count == 26)
throw std::runtime_error("nodes array is at its max capacity");
Node *node = new Node(c);
nodes[count++] = node;
return node;
}
Node* Tree::get(int idx)
{
if ((idx < 0) || (idx >= count))
throw std::out_of_range("invalid index");
return nodes[idx];
}
int main()
{
Tree t;
t.add('a');
std::cout << "\"" << t.get(0)->character << "\"" << std::endl;
}
With that said, you should use std::list or std::forward_list instead of writing your own tree class:
#include <list>
int main()
{
std::list<char> l;
l.push_back('a');
std::cout << "\"" << l.front() << "\"" << std::endl;
}
Or:
#include <list>
class Node
{
public:
char character;
// other things here...
Node(char c = 0);
Node(const Node &src);
Node& operator=(const Node &rhs);
};
Node::Node(char c)
: character(c)
{
}
Node::Node(const Node &src)
: character(src.character)
{
}
Node& Node::operator=(const Node &rhs)
{
character = src.character;
return *this;
}
int main()
{
std::list<Node> l;
l.push_back('a');
std::cout << "\"" << l.front().character << "\"" << std::endl;
}