Operator overloading unhandled exception - c++

Question

I am having some trouble overloading the + operator. I get a runtime error. Unhandled exception followed by a memory address.
Below is what I have coded:
#include <iostream>
#include <iomanip>
using namespace std;
class myVector{
int vsize, maxsize;
int* array;
void alloc_new();
friend ostream& operator<< (ostream &out, myVector&);
friend istream& operator>> (istream &in, myVector&);
public:
myVector();
myVector(int);
myVector(const myVector&); //copy constructor
~myVector();
void push_back(int);
int size();
int operator[](int);
myVector operator+(myVector&);
int at(int i);
};
myVector::myVector()
{
maxsize = 20;
array = new int[maxsize];
vsize = 0;
}
myVector::myVector(int i)
{
maxsize = i;
array = new int[maxsize];
vsize = 0;
}
myVector::myVector(const myVector& v){}
myVector::~myVector()
{
delete[] array;
}
void myVector::push_back(int i)
{
if (vsize + 1 > maxsize)
alloc_new();
array[vsize] = i;
vsize++;
}
int myVector::operator[](int i)
{
return array[i];
}
int myVector::at(int i)
{
if (i < vsize)
return array[i];
throw 10;
}
void myVector::alloc_new()
{
maxsize = vsize * 2;
int* tmp = new int[maxsize];
for (int i = 0; i < vsize; i++)
tmp[i] = array[i];
delete[] array;
array = tmp;
}
int myVector::size()
{
return vsize;
}
myVector myVector::operator+(myVector& a)
{
myVector result;
for (int i = 0; i < a.size(); i++)
result.array[i] = this->array[i] + a.array[i];
return result;
}
ostream& operator<< (ostream &out, myVector& a)
{
for (int i = 0; i < a.size(); i++)
out << a[i] << " ";
return out;
}
istream& operator>> (istream &in, myVector& a)
{
int tmp, lol;
cin >> tmp;
for (int i = 0; i < tmp; i++)
{
cin >> lol;
a.push_back(lol);
}
return in;
}
int main()
{
myVector vec;
myVector vec2;
myVector c;
int width = 15;
cout << "Input vector a\n";
cin >> vec; // In: 3 1 2 3
cout << "Input vector b\n";
cin >> vec2; // In: 3 4 5 6
cout << setw(width) << "Vector a: " << vec << endl;
cout << setw(width) << "Vector b: " << vec2 << endl;
cout << setw(width) << "c = a + b: " << c << endl;
c = vec + vec2;
system("PAUSE");
return 0;
}
How would I go about writing a copy constructor for a dynamic array? This is what I have right now:
myVector::myVector(const myVector &initial)
{
int* tmp = new int[3];
for (int i = 0; i < 3; i++)
tmp[i] = initial.array[i];
delete[] array;
array = tmp;
}

Answer 1

When you're assigning the results in your operator+, you've done nothing to reserve space in the results vector.
(You should slather your class in runtime checks (assert or similar) to sanity-check all the inputs to every method. That would show you that your result indexer passed in an index which didn't exist in the vector.)