Mat Template Pointer is NULL

edit

Thanks Berak. The reason for not using std::vector is because I'm not able to set a pointer to walk through the data. I created a test app and found that although they specify a vector as being continuous memory, I'm unable to walk a pointer through the data successfully. The code below outputs junk when I use the pointer and correct data (all zeros) when I use the vector. I'd like to use the pointer approach because I'm writing a tight processing loop.

 std::vector<int>C(10);
int *pC = &C[0];  // this doesn't work either
pC = (int *)C.data();

cout << "using pointer C = ";
for( int ii = 0; ii < C.size(); ii++){
    cout << pC[ii] << " ";
}
cout << endl;

cout << "C = ";
for( int ii = 0; ii < C.size(); ii++){
    cout << C[ii] << endl; }

"The code below outputs junk when I use the pointer and correct data (all zeros) " -- you're wrong, the data is not set to 0 automatically

The following code works perfectly:

#include <iostream>
#include <vector>
using namespace std;

int main(void)
{
    vector<int> vec(10, 0); // Set values to 0 by default.
    vec[vec.size() - 1] = 123456; // Set last value.
    int *pvec = &vec[0]; // Get pointer to vector contents

    for (size_t i = 0; i < vec.size(); i++)
    {
        cout << *pvec << endl;
        pvec++;
    }

    return 0;
}

Thanks for you help on this.
I figured out what was screwing up my code. I had a C.push_back(1) after the pointer was assigned. Since vectors try to use continuous memory, the push_back must have triggered a mem alloc and copy to the new memory. My pointer was basically pointing at the old memory (just a theory).

 std::vector<int>C(10);
 int *pC;
pC = &C[0];  // Bad! Since vectors use continuous memory, the push_back triggered the allocation of new memory for C thus making the pointer invalid; 
C.push_back(99);    
pC = &C[0];  // Good, this gives us the correct memory location because it's done after the push.

cout << "using pointer C = ";
for( int ii = 0; ii < C.size(); ii++){
    cout << pC[ii] << " ";
}
cout << endl;