C++ double free error from function copy

Question

I'm working through the Stroustrup C++ 11 book, and I ran into a double free exception. I understand that it's freeing the memory twice, but what I don't understand is why it's happening for a function that's passing by copy:

#include <iostream>

using namespace std;

namespace ALL_Vector { 

  class Vector {
    public:
      // Intitialize elem and sz before the actual function
      Vector(int size) :elem {new double[size]}, sz {size} {};
      ~Vector() {delete[] elem;};

      double& operator[](int i) {
        return elem[i];
      };
      int size() {return sz;};
    private:
      double* elem;
      int sz;
  };


  void print_product(Vector& y) {
    double result {1};

    for (auto x = 0; x < y.size() ; x++){
      if (y[x] > 0) {result *= y[x]; };
    }

    cout << "The product of Vector y is: " << result << ", or so it would appear ;)\n";
  } 

}


/*
  Self test of the Vector class.  
*/

int main(){  
    ALL_Vector::Vector myVector(15);
    cout << "The size of Vector y is: " << myVector.size() << "\n"; 
    myVector[0] = 12;
    myVector[2] = 7;
    myVector[3] = 19;
    myVector[4] = 2;

    ALL_Vector::print_product(myVector);

  return 0;
}

print_product() is taking the Vector class and creating a new Vector with duplicated contents? Why would this cause a double free? I'm assuming that RIIA in this instance is somehow interacting with the Vector::~Vector(), something like a race condition?

I know if I change this to pass it's argument by reference it will avoid the double free. I'm trying to better understand the issue with passing by copy.

Thanks!

Answer 1

Actually you are calling print_product with a reference to myVector, so everything is fine.
Troubles begin with passing myVector by value because default copy constructor will copy elem pointer instead of duplicating the whole array.
Both ALL_Vector::Vector elem pointer will refer to same memory storage and thus be deleted twice.
To tackle this issue, you have to implement copy constructor to create a new array and copy all elements.