Question

I was trying to practice on the following code:

#include <iostream>
#include <vector>

using namespace std;

class A
{
public:
   virtual void f(){cout<<"A"<<endl;}
   virtual ~A(){cout<<"destruct A"<<endl;}
};

int main()
{
   A o1,o2;
   vector <A > O;
   O.push_back(o1);
   cout<<"1"<<endl;
   O.push_back(o2);

cout<<"test"<<endl;
return 0;
}

The results turned out to be:

1
destruct A
test
destruct A
destruct A
destruct A
destruct A

Get confused about where the first destructor comes from.

Answer 1

After we had print statements to the copy constructor we get this:

constructor A(0x7fff6e21e800)
constructor A(0x7fff6e21e7f8)
copy A(0x10e700910: From 0x7fff6e21e800)
1
copy A(0x10e700920: From 0x10e700910)
copy A(0x10e700928: From 0x7fff6e21e7f8)
destruct A(0x10e700910)
test
destruct A(0x10e700920)
destruct A(0x10e700928)
destruct A(0x7fff6e21e7f8)
destruct A(0x7fff6e21e800)

So now lets look at the code:

int main()
{
   A o1,
     // constructor A(0x7fff6e21e800)
         o2;
     // constructor A(0x7fff6e21e7f8)
   vector <A > O;
   O.push_back(o1);
     // copy A(0x10e700910: From 0x7fff6e21e800)
   cout<<"1"<<endl;
     // 1

   O.push_back(o2);
    // copy A(0x10e700920: From 0x10e700910)    // O needs to expand.
                                                // So a new range is created and the old value
                                                // copied from the old range to the new range.

                                                // Now we push o2 into the vector
    // copy A(0x10e700928: From 0x7fff6e21e7f8)

                                                // Now the old range has to be destroyed.
    // destruct A(0x10e700910)


   cout<<"test"<<endl;
    // test


   return 0;
                 // Before we exit destroy the old vector (of two items)
    // destruct A(0x10e700920)
    // destruct A(0x10e700928)

                // Now destroy o2 then o1
    // destruct A(0x7fff6e21e7f8)
    // destruct A(0x7fff6e21e800)
}