C++ template class operator overloading

Question

So I'm trying to build a template class for a study project about group theory and logic. I have the class:

#pragma once
#include <iostream>
#include <assert.h>
using namespace std;
template <class T>
class CSet
{
private:
    T* arr;
    int size;
protected:

public:
    /*Constructors & Destructor*/
    CSet(int s = 0, T* a = NULL); //Default constructor
    CSet(const CSet<T> & obj_input); //Copy constructor
    ~CSet() { delete[] arr; } //Destructor

    /*Operators Overloading*/
    const CSet<T>& operator=(const CSet<T>& obj_input); // = overloading
    const CSet<T>& operator+=(const T& val_input); // += overloading
    const CSet<T>& operator-=(const T& val_input); // -= overloading

    /*Logic Operators Overloading*/
    const CSet<T>& operator|(const CSet<T>& obj_input);
    const CSet<T>& operator&(const CSet<T>& obj_input);
    const CSet<T>& operator-(const CSet<T>& obj_input);
    bool operator==(const CSet<T>& obj_input);
    bool operator!=(const T& val_input);
    bool operator>(const CSet<T>& obj_input);
    const CSet<T>& operator^(const CSet<T>& obj_input);

    //void DifWrite(const CSet<T>& obj_input); //does - and outputs to file

    friend ostream& operator<<(ostream& op, const CSet<T>& input) {
        for (int i = 0; i < input.size; i++)
        {
            op << input.arr[i] << " ";
        }
        return op;
    }
};

and I'm trying to make the | operator to simulate OR logic function. This means that if I make A={1,2,3} and B={3,4,5} then A|B={1,2,3,4,5} which is a new object. However I can't decipher how to allocate memory for the new object and return it. The function I currently have is changing 'this' instead of returning a new object:

template <class T>
const CSet<T>& CSet<T>::operator|(const CSet<T>& obj_input) {
    if (!arr)   
    {
        *this = obj_input;
        return *this;
    }
    else if (!obj_input.arr)
    {
        return *this;
    }
    else
    {
        for (int i = 0; i < size; i++)
        {
            temp += this->arr[i];
        }
        for (int i = 0; i < obj_input.size; i++)
        {
            temp += obj_input.arr[i];
        }
        *this = temp;
    }
    return *this;
}

You are doing it wrong: Your operators are binary operators which should return a CSet<T>. The mutating binary operators are @=, which should return CSet<T>&. — user2249683
– user2249683, Commented Jun 3, 2015 at 18:02

NathanOliver · Accepted Answer · 2015-06-04 14:30:26Z

You don't want to return a constant reference to an object you are creating in the function. What you should do is create a object in the function and then return it by value. To do that your code would be:

template <class T>
CSet<T> CSet<T>::operator|(const CSet<T>& obj_input) const
{
    CSet<T> temp;
    temp.size = *this.size + obj_input.size;
    temp.arr = new T[temp.size];
    int i = 0;
    for (; i < *this.size; i++)
        temp.arr[i] = *this.arr[i];
    for (; i - *this.size < obj_input.size; i++)
        temp.arr[i] = *this.arr[i];
    return temp;
}

If you instead use a std::vector instead of raw arrays your function would become:

template <class T>
CSet<T> CSet<T>::operator|(const CSet<T>& obj_input) const
{
    CSet<T> temp;
    temp.arr.insert(temp.arr.end(), *this.arr.begin(), *this.arr.end());
    temp.arr.insert(temp.arr.end(), obj_input.arr.begin(), obj_input.arr.end())
    return temp;
}

You miss the union-set logic, but idea is here. BTW, operator may be const
So can I use a const or not on the return value? and we are not allowed to use std::vector.
@adam if you are returning by value there is no reason to make the return constant. You can make the function constant thought.

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