How do I implement "template" in classes? Is there something wrong with my code?

Question

Question:

I am trying to implement "template" in my Queue classes. I am getting errors. I am not sure if that is syntax or my implementation of "templates" has issue. Without "template", my program runs perfectly.

Question: I am not sure if the "template" implementation is the way it should be. Appreciate the comments and feedbacks.

This is SNode class

#include <string>
using namespace std;

template <class T>
class SNode {
private:
    int elem;
    T* next;
    explicit SNode();
    //friend class SQueue<T>;
};

template <class T>
SNode<T>::SNode() : elem(" "), next(nullptr) {}

This is my SQueue class which

#include <string>
#include "SNode.h"
using namespace std;

template <class T>
class SQueue {
public:
    SQueue();
    void enqueue(T);
    void dequeue();
    void print();

private:
    T* front;
    T* end;
};

template <class T>
SQueue<T>::SQueue() : front(NULL), end(NULL) {}

template <class T>
void SQueue<T>::enqueue(T e) {
    T* np = new T();
    np->elem = e;
    np->next = NULL;
    if (front == NULL && end == NULL) {
        front = end = np;
        return;
    }
    end->next = np;
    end = np;
}

template <class T>
void SQueue<T>::dequeue() {
    T* np = front;
    if (front == NULL) {
        cout << "The queue is empty-1!" << endl;
        return;
    }
    front = front->next;
    delete np;
}

template <class T>
void SQueue<T>::print() {
    T* np = front;
    if (front == NULL) {
        cout << "The queue is empty-2!" << endl;
        return;
    }
    for (T* temp = front; temp != NULL; temp = temp->next) {
        if (temp != front) {
            cout << " <- ";
        }
        cout << temp->elem;
    }
    cout << "  [Queue: FIFO- First In First Out]" << endl;
}

This is my main/ Test file.

#include <iostream>
#include "SQueue.h"

using namespace std;
int main() {
    typedef SQueue<int> SQueue;
    SQueue SQ;
    cout << "Queue 1: ";
    SQ.enqueue(1);
    SQ.enqueue(2);
    SQ.enqueue(3);
    SQ.enqueue(3);
    SQ.print();
    SQ.dequeue();
    SQ.dequeue();
    SQ.print();
return 0;
}

ERROR MESSAGE
Severity    Code    Description Project File    Line    Suppression State
Error   C2227   left of '->elem' must point to class/struct/union/generic type  DeleteLinkedList    SQueue.h    24  

Answer 1

First, your node...

//...
template <class T>   // I assume T is the as-yet-unknown type stored in the node.
class SNode {
private:
    int elem;  // Why int ??
    T* next;   // Why T*

    // Should be:
    T elem;                // The user data 
    SNode<T>* next;        // the next node.

    explicit SNode();      // Why explicit?  Do you have a _very_ good reason for that? 

    // Should read:
    SNode() : elem(T()), next(nullptr) {}

    friend class SQueue<T>;
};

Now, you are delcaring a class with a private default construtor... That's a very strong indication that SNode should be declared within SQueue, as in:

template<class T> 
class SQueue
{
public:
    // Ideally, this inner class should be declared 'private', and SQueue
    // should declare some form of iterator to hide these details from
    // the caller... It's best to get everything else running first, and
    // add this complexity on a solid base.
    // 
    struct SNode  // all members are public.
    {
        SNode() : elem(T()), next(nullptr) {}
        T elem;
        SNode* next;            
    };

public:
    // define your constructors and access functions..

private:
    SNode* front;  // the queue stores a single-linked list of nodes. 
                   // Only one pointer is necessary to do that.
                   // UNLESS you want to create a list that's optimized for 
                   // appending elements at the end.  But optimizations should be 
                   // done after the unoptimized code has been tested and proven                
                   // to work flawlessly.
};