What to do when 2 classes are interdependant?

I'm implementing an NN, and I've made a class for the connection between nodes and a class for nodes. In the class for nodes, there is a vector of connections and in the class for connections, there is a node* for the sender and receiver.

Node.hpp is the first to get processed by the compiler, b/c it is included in directly in main. Connection is included only by Node.hpp.

Originally Posted by GCC

error: invalid use of incomplete type ‘struct Node’
error: forward declaration of ‘struct Node’

Code:

#ifndef NODE_HPP_INCLUDED
#define NODE_HPP_INCLUDED

#include <vector>
#include "Connection.hpp"

class Node
{
    public:

    void Recieve()
    {
        for(unsigned int i = 0;i < input.size();i++)
            if((input[i])->Empty() == true)
                return;

        // TODO: Process input ...
    }

    private:
    std::vector<class Input*> input;
    std::vector<Connection*> output;
};

#endif // NODE_HPP_INCLUDED

Code:

#ifndef CONNECTION_INCLUDED
#define CONNECTION_INCLUDED

#include <cstdlib>
#include <ctime>
#include <map>
#include <queue>
#include <set>

#include "Node.hpp"

typedef bool Message;

class Connection
{
    public:
    Connection(class Node *sender, class Node *reciever) : reciever(reciever), sender(sender) {}

    protected:
    class Node *reciever, *sender;
};

class Input : public Connection
{
    public:
    Input(class Node *sender, class Node *reciever) : Connection(sender, reciever) {}

    bool Empty()
    {
        return this->queue.empty();
    }

    virtual Message Recieve()
    {
        Message t = this->queue.front();
        this->queue.pop();
        return t;
    }

    protected:
    std::queue<Message> queue;
};

class Output : public Input
{
    public:
    Output(class Node *sender, class Node *reciever) : Input(sender, reciever) {srand(time(NULL));}

    Message Recieve()
    {
        int uid = _alloc_uid();
        Message t = this->queue.front();
        this->queue.pop();
        this->sent.insert(this->sent.begin(), std::pair<int, Message>(uid, t));
        return t;
    }

    void Send(Message m)
    {
        this->queue.push(m);
        this->reciever->Recieve();
    }

    protected:
    std::map<int, Message> sent;

    private:
    static std::set<int> _uid_list;

    static int _alloc_uid()
    {
        int t;
        while(_uid_list.count((t = rand())) != 0);
        _uid_list.insert(_uid_list.end(), t);
        return t;
    }

    static void _free_uid(int uid)
    {
        _uid_list.erase(uid);
    }
};

#endif // CONNECTION_INCLUDED

> #include "Node.hpp"
Remove this from the connection.hpp file, and replace it with

class Node;

I still get the same error messages. Notice that in Output::Send(), I call a method in Node.

To fix your problem, you have to move the problematic code into a .cpp file. In order for your code to work correctly as is now, you need to have it all in the same header otherwise it will not work.

Headers are for declarations, cpp files are for definitions

I have a habit of doing my headers before implementing the definitions behind them. Sometimes I get to prototyping and then BOOM, everything blows up in my face. Thanks, I've got all sorted out into separate files and it compiles fine.