error while copying std::map defined in a class

Question

I have a class with a std::map used as a container. I want to add a function to copy the map between two objects. Since the map was declared as a private member of the class, I need a friend function to do so. Here is my code:

class Data;
void copyData(Data &, Data &);

class Data
{
  private:
    map<int, int> _data;

  public:
    friend void copyData(Data &, Data&);
};

void copyData(Data &a, Data &b)
{
  std::copy(a._data.begin(), a._data.end(), b._data.begin());
}

main()
{
   // initialization here
   Data A, B;
   copyData(A, B);
}

But there are many warnings while compiling with mingw32. How do I do this correctly?

Is there a reason you're not just using a copy constructor? And, passing b by value into copyData doesn't make much sense. — Corbin, Apr 07 '12 at 04:29
And we shouldn't have to guess what warnings your compiler gave you. Provide them. Also, your title says warning, your posts says error. Which is it? — Mahmoud Al-Qudsi, Apr 07 '12 at 04:43

score 2 · Accepted Answer · answered Apr 07 '12 at 05:51

The std::map<K,V>::value_type is defined as std::pair<const K,V>. This ensures that std::map<K,V>::iterator objects can't be used to assign to the key (or else you could use this to break the container invariants).

This means that map iterators don't satisfy the OutputIterator concept requirements and you can't use them as the third parameter of the std::copy() function. On the same note, there are lots of situations that would make this code break anyways, such as if one map has more associations than the other.

Besides, there is a much easier way to write your function:

void copyData(Data &a, Data &b)
{
  b._data = a._data;
}

error while copying std::map defined in a class

1 Answers1