Question

How can I make sure that, for each allocated new space on heap area, the recently created element of vector of pointers points to NULL ?

 Ex:
   vector < Sometype* >

   vector ----------------------
          |  |  |  | ...       |
          ----------------------

   new element is pushed back but no available area so double space

   index                         x  x+1            y
   vector -------------------------------------------
          |  |  |  | ...       |   |   | ...        |
          -------------------------------------------

                               ^^^^^^^^^^^^^^^^^^^^^
                                 recently created

         x, x+1, ... y all points to the NULL 

I want each space on recently created part point to NULL ?

Answer 1

This new space is part of the capacity of the vector, but not part of the size. You shouldn’t need to care what values it contains, since you’re not allowed to access it anyway. Other than the one value you pushed back, the extra space is not “elements of the vector”, it’s just unused space.

As far as the standard is concerned, the implementation could use it to store something meaningful, if it wanted. For example, an implementation could legally store some eye-catcher value in the unused memory, which conflicts with your desire for the unused memory to contain null pointers.

You could write code like this:

v.push_back(some_value);
if (v.capacity() > v.size()) {
    size_t oldsize = v.size();
    v.resize(v.capacity(), NULL);
    v.resize(oldsize);
}

There’s no guarantee this will actually leave the memory set to 0 once you resize back down again, but it probably will. So it might be good enough for debugging. If the purpose you have in mind is not debugging, please say what it is, because if not debugging then either your purpose is illegitimate or else one of us has misunderstood something.