Question

I’m using a vector of pointers to free up a series of node objects in the heap. The vector has all the node object addresses and there is a function, delete_nodes, which is used with the for_each loop to delete all nodes in the vector. For some reason I get the following error in eclipse cdt with the for_each loop underlined in red:

error: no matching function for call to 'for_each(__gnu_cxx::__normal_iterator<Node**, std::vector<Node*, std::allocator<Node*> > >, __gnu_cxx::__normal_iterator<Node**, std::vector<Node*, std::allocator<Node*> > >, <unresolved overloaded function type>)'

The code is for Huffman coding, and the for_each loop is at the very end. The nodes_delete vector is created right before the while loop.

void Huff::delete_nodes(Node*n){//this is used to delete all the nodes in the binary tree at the end of Huff::compress()
    delete n;
}
vector<Code>* Huff::compress(){
    //-------GETTING WEIGHTS/FREQUENCIES------
    vector<Node *>* nodes = new vector<Node*>; // Vector of nodes for later use
    map<char, int>* freq = new map<char, int>; //  Map to find weight of nodes
    for(unsigned int i = 0; i < content.length(); i++)
        (*freq)[content[i]]++; 
    CopyTo copyto(nodes); //sets vector<Node*> to copy to 
    for_each(freq->begin(), freq->end(), copyto); // Copies 
    delete freq;
    vector<Node *>::iterator beg = nodes->begin();

    //-------SETTING UP TO BUILD TREE------
    if(nodes->size() % 2 == 1){ //makes sure there are an even number of nodes
        Node* fill = new Node;
        fill->set_node(0, '*', NULL, NULL);
        nodes->push_back(fill);
    }
    huff_sort(nodes); // sort nodes by weight
    vector<Node*> nodes_delete(*nodes); //this is used to delete all the nodes in the binary tree at the end
    //-------BUILDING TREE------
    while(nodes->size() != 1){ //Sorts nodes by weight and then removes two of them and replaces them with one
        int w= (**beg).weight + (**(beg+1)).weight;
        Node* p = new Node;
        p->set_node(w, '*', *nodes->begin(), *(nodes->begin()+1)); //making it the parent node of the two lowest nodes
        nodes->erase(nodes->begin(), nodes->begin()+2);
        unsigned int i = 0;
        while(w > (*nodes)[i]->weight && i <= nodes->size()){ //finds where to insert the parent node based on weight
            i++;
        }
        if(i > nodes->size()) //if it needs to be inserted at the end
            nodes->push_back(p);
        else
            nodes->insert(nodes->begin()+i, p);
    }
    //-------TRAVERSING TREE------
    Node* root = (*nodes)[0];
    delete nodes;
    vector<Code>* codes = new vector<Code>;
    traverse(root, codes , "");
    delete root;
    for_each(nodes_delete.begin(), nodes_delete.end(), delete_nodes);
    return codes;
}

Answer 1

It look like your delete_nodes is a non-static member function. If so, you cannot just use delete_nodes as an argument for std::for_each. std::for_each requires a functor. Your delete_nodes is not a functor.

Firstly, to obtain a pointer to a non-static member function, & operator and a qualified name are always required. A mere name of non-static member function (just delete_nodes) is not a valid expression in C++. You have to do &Huff::delete_nodes.

Secondly, again, a pointer to a member function (as opposed to a pointer to an “ordinary” function) is not a functor. In order to turn it into a functor you can use std::mem_fun function. That will give you a binary functor, since std::mem_fun will turn the implicit this parameter into an explicit one. In order to turn it into a unary functor required by std::for_each you have to bind the first argument to a specific object pointer value (this probably?).

The end result of the above steps will look as

bind1st(mem_fun(&Huff::delete_nodes), this)

This is a unary functor that calls delete_nodes for this object.

So, the for_each call in your example should look as follows

for_each(nodes_delete.begin(), nodes_delete.end(),
  bind1st(mem_fun(&Huff::delete_nodes), this));

However, it looks like in your implementation delete_nodes can be turned into a static member function. A static member function is an “ordinary” function, meaning that it is a functor and it can be used directly. I.e. if you make delete_nodes static your code should work as is.

Decide what path you wish to follow and make the necessary changes.