need help creating a two dimensional vector that holds pointers of a user defined type

I am having trouble with two-dimensional vectors. In my old code I had
a vector in my class definition that looked like this:

std::vector<ABC*> vec;

....which means I am declaring a vector whose elements will contain
pointers to ABC objects.

....and in my constructor I have:

// make sure vector is empty before populating it
vec.clear();

for (int i = 0; i < 5; i++)
{
  ABC* abcobject1 = new ABC(i);
  vec.push_back(abcobject1);
}

....and in my destructor I have:

for (std::vector<ABC*>::iterator it = vec.begin();
     it != vec.end();
     it++)
{
  delete *it; *it = 0;
}

This is all well and good and appears to work OK. Now, let's suppose
that I want to make vec be a two-dimensional vector instead of a one-
dimensional vector. That is, the definition in my header file now
looks like this:

// vec is a vector of a vector of pointers to ABC objects
std::vector< std::vector<ABC*> > vec;

OK, now here's where I'm messing up. My constructor looks like this:

for (int i = 0; i < 7; i++)
{
  for (int j = 0; j < 5; j++)
  {
    ABC* abcobject1 = new ABC(j);
    vec[i].push_back(abcobject1);
  }
}

When the loop first iterates (i and j both equal zero) how can it
push_back an item onto vec[0] when vec[0] itself doesn't exist yet? I
think there needs to be a vec.push_back(...) line somewhere earlier,
but I don't know where and what should be pushed back. Can someone
help me?

Oh, and my destructor looks like this:

for (std::vector< std::vector<ABC*> >::iterator itOuter = vec.begin();
  itOuter != vec.end();
  itOuter++)
{
  for (std::vector<ABC*>::iterator itInner = (*itOuter).begin();
    itInner != (*itOuter).end();
    itInner++)
  {
    delete *itInner; *itInner = 0;
  }
}

I have no idea if that's correct, since I haven't gotten that far yet.