Re: static vector of pointers

From:

mlimber <mlimber@gmail.com>

Newsgroups:

comp.lang.c++

Date:

20 Apr 2007 14:12:51 -0700

Message-ID:

<1177103571.826864.200340@y80g2000hsf.googlegroups.com>

On Apr 20, 11:21 am, Jia <JiaLi...@gmail.com> wrote:

Hi all,

I have a class foo which has a static vector of pointers of type base
class, and a static function to set this vector.

#include <iostream>
#include <vector>
using namespace std;
class super{
protected:
        int a;
public:
        virtual void printOut(){ cout << "super " << endl;}

};

class subA : public super{
protected:
        int a_a;
public:
        virtual void printOut(){ cout << "subA" << endl;}

};

class subB : public super{
protected:
    int a_b;
public:
        virtual void printOut(){cout << "subB" << endl;}

};

class foo {
protected:
  static std::vector<super*> myList;
  static std::vector<static subA> templist;
  static int count;
public:
  static void setMyList();
  static vector<super*> getMyList();

};

std::vector<super*>foo::myList;
std::vector<subA> foo::templist;
int foo:: count = 0;
void foo::setMyList(){
  subA mysubA;
  templist.push_back(mysubA);
  myList.push_back(&(templist[count]));
  count++;
  cout<< count << " time call setMyList" << endl;

}

vector<super*> foo::getMyList()
{
  return myList;

}

main()
{

  foo::setMyList();
  foo::setMyList();

  vector<super*> newList;
  newList = foo::getMyList();
  newList[0]->printOut();}

What I really try to achieve is to set the myList to a vector of
pointers of super type, but actually points to the derived class
objects. I use a static vector of subA to store the sub object, so
that I could use its address to initilize myList( I know this is very
ugly, but I haven't figure out other method). But my problem is that
myList is not properly set as I expected. For example, as above
program will crush as try to execute newList[0]->printOut(). I
figured out that everytime templist calls push_back, its address has
been changed ( the first time is 0x0033c10, and second time after
push_back is called, its address has been changed) so that myList
first pointer points to nowhere, but I have already defined the
templist as static. Can anyone explain this to me? I hope I explain
myself clearly.

Apparently my previous response didn't go through or has been delayed.
The fundamental problem is that templist is being resized by the
push_back, which invalidates all pointers and iterators. Try this
instead:
#include <iostream>
#include <vector>
using namespace std;

struct super
{
   virtual ~super() {}
   virtual void printOut()
   { cout << "super\n";}
};

struct subA : super
{
   virtual void printOut()
   { cout << "subA\n"; }
};

class foo
{
   vector<super*> myList;
public:
   ~foo()
   {
     for( size_t n=0; n < myList.size(); ++n )
     {
       delete myList[n];
     }
   }

   void setMyList()
   {
     myList.push_back( new subA );
     cout<< "called setMyList" << endl;
   }

   vector<super*> getMyList() const
   {
       return myList;
   }
};

int main()
{
   foo f;
   f.setMyList();
   f.setMyList();

   const vector<super*> newList = f.getMyList();
   for( size_t n=0; n < newList.size(); ++n )
   {
     newList[n]->printOut();
   }
}

Cheers! --M