Template error in list

Question

I am trying to implement Koch's Snowflake. I have made a generic list, for practice sake, but I am having some issues.

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <gl/glut.h>


template <typename T> class Node {
public:
    T data;
    Node<T> *next;
    Node<T>(T d) {
        next = NULL;
        data = d;
    }
};
template <typename T> class List {
    Node<T> *head;
    int size;
public:
    List() {
        head = NULL;
        size = 0;
    }
    void append(T data){
        if(head == NULL) {
            head = new Node<T>(data);
        } else {
            Node<T> *n = head;
            while( n->next != NULL ) {
                n = n->next;
            }
            n->next = new Node<T>(data);
        }
        size++;
    }
    void appendAll(List<T> data) {
        if(data.getHead() == NULL)
            return;
        Node<T> *n = data.getHead();
        append(n->data);
        while(n->next != NULL){
            append(n->next->data);
            n = n->next;
        }   
    }
    Node<T>* getHead(){ return head; }
};
void myinit();
void display();
void draw_snowflake();
List<GLfloat[2]> divide_snowflake(GLfloat A[2], GLfloat B[2], int n);

GLfloat tri[3][2] = {{-1.0, -0.58}, {1.0, -0.58}, {0.0, 1.15}};
List<GLfloat[2]> snow;
int n;


int main(int argc, char **argv) {
    n = 0;
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowSize(500,500);
    glutCreateWindow("Koch Snowflake");
    glutDisplayFunc(display);
    myinit();
    glutMainLoop();

    return EXIT_SUCCESS;
}

void myinit(){
    // Initialize OpenGL
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(-2.0, 2.0, -2.0, 2.0);
    glMatrixMode(GL_MODELVIEW);
    glClearColor(1.0, 1.0, 1.0, 1.0);
    glColor3f(0.0,0.0,0.0);

    // Initialize list of line_loop
    snow.append(tri[0]);
    snow.append(tri[1]);
    snow.append(tri[2]);
}

void display(){
    glClear(GL_COLOR_BUFFER_BIT);
    glBegin(GL_LINE_LOOP);
    draw_snowflake();
    glEnd();
    glFlush();
}

void draw_snowflake(){
    List<GLfloat[2]> temp;
    temp.append(snow.getHead()->data);
    Node<GLfloat[2]> *curr = snow.getHead();
    while(curr->next != NULL) {
        temp.appendAll(divide_snowflake(curr->data, curr->next->data, n));
        temp.append(curr->next->data);
        curr = curr->next;
    }
    temp.appendAll(divide_snowflake(curr->data, snow.getHead()->data, n));

    Node<GLfloat[2]> *ptr = temp.getHead();
    printf("\n>Drawing %f, %f", ptr->data[0], ptr->data[1]);
    glVertex2fv(ptr->data);
    while(ptr->next != NULL) {
        printf("\n>Drawing %f, %f", ptr->next->data[0], ptr->next->data[1]);
        glVertex2fv(ptr->next->data);
        ptr = ptr->next;
    }
}

List<GLfloat[2]> divide_snowflake(GLfloat A[2], GLfloat B[2], int n) {
    GLfloat A_Mid[2] = {A[0] + (B[0] - A[0]) / 3,
                        A[1] + (B[1] - A[1]) / 3};
    GLfloat Mid[2] = {A[0] + (B[0] - A[0]) / 2,
                      A[1] + (B[1] - A[1]) / 2};
    GLfloat B_Mid[2] = {B[0] - (B[0] - A[0]) / 3,
                        B[1] - (B[1] - A[1]) / 3};
    GLfloat Peak[2] = {Mid[0] + (Mid[1] - B_Mid[1]) * sqrt(3.0),
                       Mid[1] + (Mid[0] - A_Mid[0]) * sqrt(3.0)};

    List<GLfloat[2]> temp;
    if(n > 0) temp.appendAll(divide_snowflake(A, A_Mid, n-1));
    temp.append(A_Mid);
    if(n > 0) temp.appendAll(divide_snowflake(A_Mid, Peak, n-1));
    temp.append(Peak);
    if(n > 0) temp.appendAll(divide_snowflake(Peak, B_Mid, n-1));
    temp.append(B_Mid);
    if(n > 0) temp.appendAll(divide_snowflake(B_Mid, B, n-1));
    return temp;
}

Here is the error I am getting:

Error 1 error C2440: '=' : cannot convert from 'GLfloat []' to 'float [2]'  13

When I was just initializing as List<GLfloat*> it would only set node's data as a single value; whereas, I want points. For practice purposes, I want to continue to use a generic list.

Answer 1

Let's consider what the code would be if you used a non-generic list: namely, a list that worked with GLFloat[2]s. Here's your node code:

class Node {
public:
    GLFloat[2] data;
    Node *next;
    Node(GLFloat[2] d) {
        next = NULL;
        data = d;
    }
};

An important think to note now is that Node's constructor doesn't actually take an array: it takes a GLFloat*. That's just the way C++ works in this regard. (Strangely to me, this is also the way it works when you let the argument type depend on a template parameter: apparently, an array is also treated as a pointer then.)

You are now trying, by doing data = d;, to assign a GLFloat* to a GLFloat[2]. This isn't anything meaningful: you can't just take a pointer and put its value into an array. You could explicitly put d[0] into data[0] and d[1] into data[1], but this wouldn't be very general, as your list would no longer work with non-array types.

What can be done? Well, one solution would be to specialise your list for arrays. The code would look something like...

template <typename T, std::size_t N>
class Node<T[N]> {
public:
    T data;
    Node<T> *next;
    Node<T>(T d) {
        next = NULL;
        for (std::size_t i = 0; i < N; ++i)
            data[i] = d[i];
    }
};

However, this may lead to a lot of code duplication, and leads to further issues like how you can return elements.

My advice would be to use std::pair<GLFloat, GLFloat> instead. This should work with your list as it is. Alternatively, you can use std::array if you want to store an arbitrary number of elements.

And, while I'm commenting on all this anyway: you may want to move the Node class template into the List class template, as it is an implementation detail. You also should definitely not be providing a getHead() function -- it does not make sense to break encapsulation like that. Provide an iterator class and a pair of begin() and end() functions for iteration. One possible implementation would be along the lines of:

struct iterator {
    friend List;
  private:
    Node* current;
    iterator(Node* c) : current(c) {}
  public:
    iterator& operator++() {
        current = current->next;
        return *this;
    }

    friend bool operator==(iterator const& lhs, iterator const& rhs) {
        return lhs.current == rhs.current;
    }

    T& operator*() {
        return current->data;
    }

    T* operator->() {
        return &current->data;
    }
};

This would provide a sufficient interface for everything you've done so far, and would be much cleaner. Implementing a more complete interface unfortunately requires a lot of boilerplate; at that point, you may wish to switch to a simple std::list.