Convert (void **) to object in C++ - c++

Question

I am trying to convert a (void**) object into a queue in C++.
In one file HashTableVoid.cc I use the method find to:
bool HashTableVoid::find( const char * key, void ** data)
{
// Add implementation here
int i = hash(key);
HashTableVoidEntry * node = _buckets[i];
while(node){
if(strcmp(node->_key,key) == 0)
{
*data = node->_data;
return true;
}
node = node->_next;
}
return false;
}
And in IRCServer.cc I used
void IRCServer::sendMessage(int fd, const char * user, const char * password, const char * args)
{
//Get user's room
//Write to room messages
char * temp;
temp = strdup(args);
void ** q;
queue<char*> data;
const char * room = //Found Room;
communication.find(room, q);
data = (queue<char*>) ((char *)q);
data.push(temp);
//Make sure only 100 elements or less in the list
while(data.size() > 100)
data.pop();
}
I am creating void ** q to pass as a parameter to communication. The variable communication is a HashTable where the key is the room name and the value is a unique queue of messages. I am having trouble converting the void object to a queue. I can't change the find method.

Answer 1

Two problems that I can see directly:
First you call find without initializing q, so it doesn't actually point anywhere. So in the find function when you dereference do *data you dereference an unknown pointer leading to undefined behavior. I think what you're supposed to do is to declare q to be a single pointer, and then pass the address of that to the find function (emulating pass by reference). Like
void *q;
...
communication.find(room, &q);
However, since you're programming in C++ I don't see a reasong to use double-pointer to emulate pass by reference, since C++ have it built-in:
bool HashTableVoid::find( const char * key, void *& data);
The above declares the argument data as a reference to a pointer.
The second problem is your assignment from the pointer q to the variable data:
data = (queue<char*>) ((char *)q);
What you have now (without the changes above), q is a pointer to a pointer, and you try to use it as a single pointer, which you then try to cast to a value. A pointer is not a value, and can never be.
Maybe you mean
data = *reinterpret_cast<std::queue<char*>*>(q); // After change detailed above