Code with changes

#include <iostream>
#include <algorithm>


class vector {
    int sz;
    double* elem;
public:
    vector(int s)
        :sz{ s }, elem{ new double[s] }
    {
        for (int i = 0; i < sz; ++i) elem[i] = 0.0;
    }

    vector(std::initializer_list<double> lst)
        :sz{ int(lst.size())}, elem{new double[sz]}
    {
        std::copy(lst.begin(), lst.end(), elem);
    }

    // Example from book
    //
    /*vector(std::initializer_list<double> lst)
        :sz{ lst.size() }, elem{ new double[sz] } << lst.size() gives a narrowing conversion error. lst.size() returns size_t and not an int.
    {
        std::copy(lst.begin(), lst.end(), elem);
    }*/


    ~vector()
    {
        delete[] elem;
    }

    double& operator[](int index)
    {
        if (index >= sz) {
            std::cout << "index out of bounds, exiting";
            exit(0);
        }
        return elem[index];
    };



    int size() { return sz; }

    vector& operator=(const vector&);
    vector(const vector&);
};


vector& vector::operator=(const vector& a)
{
    if (this == &a)
        return *this;

    double* p = new double[a.sz];
    std::copy(a.elem, a.elem + a.sz, p);
    delete[] elem;
    elem = p;
    sz = a.sz;
    return *this;
}


// Example from book
//
//vector& vector::operator=(const vector& a)
//{
//    double* p = new double[a.sz];
//    std::copy(a.elem, a.elem + a.sz, elem);  copying to elem then deleting elem next line?
//    delete[] elem;
//    elem = p;
//    sz = a.sz;
//    return *this;
//}


vector::vector(const vector& arg)
    :sz{ arg.sz }, elem{ new double[arg.sz] }
{
    std::copy(arg.elem, arg.elem + arg.sz, elem);
}

//Example from book
//
//vector::vector(const vector& arg)
//    :sz{ arg.sz }, elem{ new double[arg.sz] }
//{
//
//    std::copy(arg, arg + sz, elem); can't do arg + sz
//}


int main()
{
    vector v(20);
    for (int i = 0; i < v.size(); ++i)
    {
        v[i] = i;
        std::cout <<"V: " << v[i] << '\n';
    }

    v = { 5,5,5,5,5 };

    for (int i = 0; i < v.size(); ++i)
    {
        std::cout << "V new: " << v[i] << '\n';
    }

    vector a{ v };
    for (int i = 0; i < a.size(); ++i)
    {
        std::cout << "A: " << v[i] << '\n';
    }
}