C++ operator + overload

Question

I have a dynamic array and I need to make a function which overloads the operator + such that it should give you the possibility to add a new object element to the array in two ways:

array=array+element; and array=element+array;

Until now I have the function:

DynamicVector& DynamicVector::operator+(const TElement& e)
{
if (this->size == this->capacity)
     this->resize();
this->elems[this->size] = e;
this->size++;
return *this;
}

But this only works for the first case, when we do array=array+element;

How can I implement the problem to work for both the cases.

Answer 1

How can I implement the problem to work for both the cases.

You need to overload the function as a non-member function.

DynamicVector& operator+(const TElement& e, DynamicVector& v);

Ideally, make both of them non-member functions.

You can implement the second one using the first one.

DynamicVector& operator+(const TElement& e, DynamicVector& v)
{
   return v + e;
}

---

Suggestions for improvement.

1. Add a non-const operator+= member function to DynamicVector .
2. Allow operator+ functions work with const objets.

Member function.

DynamicVector& DynamicVector::operator+=(const TElement& e)
{
   if (this->size == this->capacity)
     this->resize();
   this->elems[this->size] = e;
   this->size++;
   return *this;
}

Non-member functions.

DynamicVector operator+(DynamicVector const& v,  TElement const& e)
{
   DynamicVector copy(v);
   return (copy += e);
}

DynamicVector operator+(TElement const& e, DynamicVector const& v)
{
   return v + e;
}

With those changes the overloaded operators work just like fundamental types.

int i = 0;
i += 3; // Modifies i
int j = i + 10;  // Does not modify i. It creates a temporary.

Answer 2

First off, operator+ is not supposed to modify the left-hand object being added to. It is supposed to return a new object that is a copy of the two inputs added together, eg:

DynamicVector DynamicVector::operator+(const TElement& e) const
{
    DynamicVector tmp(*this);
    if (tmp.size == tmp.capacity)
      tmp.resize();
    tmp.elems[tmp.size] = e;
    tmp.size++;

    /* NOTE: the above can be optimized to avoid unnecessary reallocations:

    DynamicVector tmp;
    tmp.capacity = size + 1; // or rounded up to some delta...
    tmp.size = size + 1;
    tmp.elems = new TElement[tmp.capacity];
    std::copy(elems, elems + size, tmp.elems);
    tmp.elems[size] = e;
    */

    return tmp;
}

You are thinking of operator+= instead, which is supposed to modify the object being added to, and return a reference to it:

DynamicVector& DynamicVector::operator+=(const TElement& e)
{
    if (size == capacity)
        resize();
    elems[size] = e;
    size++;
    return *this;
}

Now, that being said, to answer your question, you need to define a separate non-member overload of operator+ to handle the case where TElement is on the left-hand side, eg:

DynamicVector operator+(const TElement& e, const DynamicVector &v)
{
    return v + e;
}

Or:

DynamicVector operator+(const TElement& e, const DynamicVector &v)
{
    DynamicVector tmp(v);
    tmp += e;
    return tmp;

    /* alternatively:

    return DynamicVector(v) += e;
    */

    /* NOTE: the above can be optimized to avoid unnecessary reallocations:

    DynamicVector tmp;
    tmp.capacity = v.size + 1; // or rounded up to some delta...
    tmp.size = v.size + 1;
    tmp.elems = new TElement[tmp.capacity];
    std::copy(v.elems, v.elems + v.size, tmp.elems);
    tmp.elems[v.size] = e;
    return tmp;
    */
}