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c++double-free
MrMowgli
MrMowgli

Reputation: 873

C++ double free error from function copy

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!

Upvotes: 1

Views: 1508

Answers (2)

Richard Dally
Richard Dally

Reputation: 1450

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.

Upvotes: 5

Luca Pizzamiglio
Luca Pizzamiglio

Reputation: 313

If you pass Vector per value, the copy constructor is called, not the constructor you have implemented. In this case, elem is not duplicated but the pointer is copied in the new object and then deleted twice by the destructor. You have to implement a copy constructor that allocate a new elem and copy all elements.

Upvotes: 1

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