c++ copy() function

greg108 · 08-10-2005, 08:28 PM

Here is my copy constructor:

Code:

  Unit(const Unit &u)
    {
      std::cout << "Unit copy constructor invoked\n";
      std::cout << "list size before " << u.members->size() << "\n";
      ownerVal = u.ownerVal;
      rank = u.rank;
      members = new list<UnitMember>();
      copy(u.members->begin(), u.members->end(), members->begin()); 
      std::cout << "list size after " << members->size() << "\n";
    }

Am I missing something? My list doesn't get copied and the "list size after" is 0.

jayemef · 08-10-2005, 09:59 PM

Try:

Code:

std::copy(...)

Hivemind · 08-11-2005, 07:59 AM

Rewrite your call to copy as follows:

Code:

std::copy(u.m_list.begin(), u.m_list.end(), std::back_inserter(m_list));

I am not allocating the list object itself dynamically in this snippet, but it appears you are. Normally, that's not something you do not need to do. If you want to continue allocating the list object itself dynamically, replace . with ->
Here's a complete program that demonstrates how you should use std::copy.

Code:

#include <algorithm>
#include <iostream>
#include <list>

class Unit
{
public:
   Unit(int n)
   {
      m_list.push_back(n);
   }

   Unit(const Unit& u)
   {
      std::copy(u.m_list.begin(), u.m_list.end(), std::back_inserter(m_list));
   }

   void add_to_list(int n)
   {
      m_list.push_back(n);
   }

   void print_list() const
   {
      std::list<int>::const_iterator itr = m_list.begin();

      while(itr != m_list.end())
      {
         std::cout << *itr << std::endl;
         ++itr;
      }
   }

private:
   std::list<int> m_list;
};

int
main()
{
   Unit u1(4711);

   u1.add_to_list(123);
   u1.add_to_list(456);

   Unit u2(u1);

   std::cout << "Printing u1:" << std::endl;
   u1.print_list();

   std::cout << "Printing u2:" << std::endl;
   u2.print_list();

   return 0;
}

Sample output:

Code:

$ ./copyctor.exe 
Printing u1:
4711
123
456
Printing u2:
4711
123
456

greg108 · 08-11-2005, 11:13 AM

Thanks guys,
In my case this works:

Code:

std::copy(u.members->begin(), u.members->end(), std::back_inserter(*members));