Error in that.vect.push_back(0) and that.vect.begin() in overloading "+" orerator in C++

Question

class Polinom {
     public:
        std::vector<double> vect;
        Polinom operator +(const Polinom &that) {
            if (this->vect.size() > that.vect.size()) {
                for (int i = that.vect.size(); i < this->vect.size(); i++)
                    that.vect.push_back(0);//here
            }
            else if (that.vect.size() > this->vect.size()) {
                for (int i = this->vect.size(); i < that.vect.size(); i++)
                    this->vect.push_back(0);
            }
            std::vector<double> sum;
            std::vector<double>::iterator i2 = that.vect.begin();//here
            for (std::vector<double>::iterator i1 = this->vect.begin(); i1 != this->vect.end(); ++i1)
                sum.push_back(*i2++ + *i1);
            return sum;
        }
        Polinom();
        Polinom(std::vector<double> vect) {
            this->vect = vect;
        }
        ~Polinom();
};

Severity Code Description Project File Line Suppression State Error (active) E1087 no instance of overloaded function "std::vector<_Ty, _Alloc>::push_back [with _Ty=double, _Alloc=std::allocator]" matches the argument list and object (the object has type qualifiers that prevent a match)

Answer 1

Polinom operator +(const Polinom &that) {
                   ^^^^^

that is a const reference.

that.vect.push_back(0);//here

Since that is const, so are member access expressions through that reference. Thus that.vect is a const expression.

push_back is a non-const member function. It modifies the vector. The function cannot be called on const vectors.

std::vector<double>::iterator i2 = that.vect.begin();//here

std::vector<T>::begin() const returns a std::vector<T>::const_iterator. It is not implicitly convertible to std::vector<T>::iterator

Solution 1: Don't attempt to push elements into const vectors.

Solution 2: Use a non-const reference when you intend to modify the referred object.

In this case, solution 1. seems to be more sensible. Modifying operands of operator+ would be counter intuitive. Furthermore, you should probably make the function const qualified, and avoid modifying this, for the same reason.