Queue implementation bug C++ [duplicate]

Question

I am facing a very specific problem when trying to implement a generic queue in C++. Here is the code:

template <class T> class Queue  {

    private:
        T * queue;
        int front;
        int back;
        int MAX_SIZE;

    public:
        Queue(){
            Queue(SOME_PREDEFINED_CONSTANT);
        }

        Queue(int size){
            queue = new T[size];
            MAX_SIZE = size;
            front = 0;
            back = 0;
            cout << "inside constructor:" << " back= " << back <<  " front=" << front << endl;

        }

        void push(const T entry){

            if(back < MAX_SIZE){
                queue[back] = entry;
                back++;
            }
        }

        T peek(){

            return queue[front]; 
        }

        void pop(){

            if(size() > 0){
                front++;
            }
            if(front==back){
                front = 0;
                back = 0;
            }
        }

        int size(){
            cout << "size:" << " back=" << back <<  " front=" << front << endl;
            return (back-front);
        }
};

When I am initializing the queue, like this: Queue <int> q;, I get the correct values of back and front inside the constructor; The output:

inside constructor: back=0 front=0

But when I am asking for the size of the queue, like this: q.size() (without any push/pop operation), I get very weird values of back and front. One sample output:

size: back=32766 front=-746749592

Can anyone please point out what I am doing wrong here?

Answer 1

When you do

Queue(){
    Queue(SOME_PREDEFINED_CONSTANT);
}

What happens is that you inside the default constructor create a temporary object which immediately go out of scope and is destructed.

There are two ways of solving this:

1. The "new" way using a constructor initializer list and delegating constructors:

   Queue() : Queue(SOME_PREDEFINED_CONSTANT)
   { /* empty */ }
   

2. The "old" way using default arguments:

   template <class T> class Queue  {
       ...
   
   public:
       Queue(int size = SOME_PREDEFINED_CONSTANT){
           ...
       }
       ...
   };