Pointer to an array of pointers.

[davermont]

I am trying to figure out a run-time exception I get on line 53 below. I am trying to create a new Node object using a pointer to an array of Node*. The Node* are all initialized to 0 in the constructor. Can anyone spot the error?

The run-time exception is:

Unhandled exception at 0x00df249f in LAN.exe: 0xC0000005: Access violation writing location 0xcccccccc.

// Main.h
int main() {
	Network network;
	const Node node;
	network.add(node);
	return EXIT_SUCCESS;
}

// Network.h
class Network {
public:
	Network();
	Network(const unsigned);
	bool add(const Node&);
	const unsigned size() const;
	~Network();

private:
	Node** nodes;
	unsigned node_count;
	unsigned capacity;
};

// Network.cpp
#include "Network.h"

Network::Network()
: node_count(0), capacity(10) {
	Network(10);
}

Network::Network(const unsigned init_capacity)
: node_count(0), capacity(init_capacity) {
	nodes = new Node*[capacity];
	for (unsigned i = 0; i < capacity; i++) {
		nodes[i] = 0;
	}
}

Network::~Network() {
	for (unsigned i = 0; i < node_count; i++){
		delete nodes[i];
	}
	delete [] nodes;
}

bool Network::add(const Node& node) {
	for (unsigned i = 0; i < node_count; i++) {
		if (nodes[i] == &node) {
			return false;
		}
	}
	nodes[node_count++] = new Node;
	return true;
}

[helios]

node_count is never checked to be <= capacity, so the buffer is being overflowed.

[davermont]

I don't see how this could be causing the problem. In main() I only add one node and node_count is initialized to 0, while capacity is initialized to 10; I have verified those value in the debugger just before line 53 executes. I can understand this being a problem for the 11th node, but not the first 10.

[helios]

I need to see the implementation of Node, but I already see a problem with Network::Network(). Constructors can't be called. I don't know how that compiled at all.

[davermont]

I was trying to save space by not including some code. Here is all of it:

// Main.cpp
#include <iostream>
#include <cstdlib>
#include "Network.h"

using namespace std;

int main() {
	Network network;
	Node node;
	network.add(node);
	system("pause");
	return EXIT_SUCCESS;
}

// Network.h
#ifndef NETWORK_H
#define NETWORK_H
#include "Node.h"

class Network {
public:
	Network();
	Network(const unsigned);
	bool add(const Node&);
	//const Node& remove(const Node&);
	const unsigned size() const;
	~Network();

private:
	void realloc();

	Node** nodes;
	unsigned node_count;
	unsigned capacity;
};
#endif

// Network.cpp
#include "Network.h"

Network::Network()
: node_count(0), capacity(10) {
	Network(10);
}

Network::Network(const unsigned init_capacity)
: node_count(0), capacity(init_capacity) {
	nodes = new Node*[capacity];
	for (unsigned i = 0; i < capacity; i++) {
		nodes[i] = 0;
	}
}

Network::~Network() {
	for (unsigned i = 0; i < node_count; i++){
		delete nodes[i];
	}
	delete [] nodes;
}

bool Network::add(const Node& node) {
	for (unsigned i = 0; i < node_count; i++) {
		if (nodes[i] == &node) {
			return false;
		}
	}
	if (node_count == capacity) {
		realloc();
	}
	nodes[node_count++] = new Node;
	return true;
}

void Network::realloc() {
	Node** new_nodes = new Node*[capacity *= 2];
	unsigned i = 0;
	for (; i < node_count; i++) {
		new_nodes[i] = nodes[i];
	}
	for (; i < capacity; i++) {
		new_nodes[i] = 0;
	}
	delete [] nodes;
	nodes = new_nodes;
}

const unsigned Network::size() const {
	return node_count;
}

// Node.h
#ifndef NODE_H
#define NODE_H
class Node {
public:
	Node();
	~Node();

private:
	Node* nodes;
	const unsigned id;
	static unsigned id_count;
};
#endif

// Node.cpp
#include "Node.h"

unsigned Node::id_count = 0;

Node::Node()
: id(id_count++), nodes(0) {
}

Node::~Node() {
}

[davermont]

It definitely compiles. I suppose I should have just included it all. Anyway, the run-time error now occurs on line 71.

[jsmith]

Line 44 does essentially nothing. You'll have to duplicate the code from your other constructor, or better yet:

class Network {
  public:
      explicit Network( unsigned int cap = 10 );
};

And kill two birds with one stone.

(Essentially your default constructor is not initializing nodes, and it is setting capacity to 10 whereas it is in fact zero.)

[davermont]

Thanks, that constructor was the problem.