run-time created 2 dimensional vector

[E0p9LyTq]

Both Visual C++ 2017 and TDM-GCC 4.9.2 initialize the elements of a created 2D vector without having to push back any values. The vector elements seem to be initialized to zero upon creation. 32-bit and 64 bit compilation results in the same behavior.

Is this correct behavior? Am I missing some key idea and fooled that two compilers seem to work without undefined behavior?

#include <iostream>
#include <vector>

int main()
{
   std::cout << "Creating a 2-dimensional vector, enter row size: ";
   int row_size;
   std::cin >> row_size;

   std::cout << "Enter column size: ";
   int col_size;
   std::cin >> col_size;
   std::cout << '\n';

   // create a 2 dimensional vector
   std::vector<std::vector<int>> aVector(row_size, std::vector<int>(col_size));

   // let's display the newly created 2D vector before setting values
   for (int row_loop = 0; row_loop < row_size; row_loop++)
   {
      for (int col_loop = 0; col_loop < col_size; col_loop++)
      {

         // let's display the newly created 2D vector
         std::cout << aVector[row_loop][col_loop] << ' ';
      }
      std::cout << '\n';
   }
   std::cout << '\n';
   
   // initialize the vector with some values
   for (int row_loop = 0; row_loop < row_size; row_loop++)
   {
      for (int col_loop = 0; col_loop < col_size; col_loop++)
      {
         aVector[row_loop][col_loop] = ((row_loop + 1) * 100 + col_loop + 1);
      }
   }

   // let's display the filled 2D vector
   for (int row_loop = 0; row_loop < row_size; row_loop++)
   {
      for (int col_loop = 0; col_loop < col_size; col_loop++)
      {
         
         // let's display the filled 2D vector
         std::cout << aVector[row_loop][col_loop] << ' ';
      }
      std::cout << '\n';
   }
   std::cout << '\n';
}

A test run with either compiler:

Creating a 2-dimensional vector, enter row size: 5
Enter column size: 3

0 0 0
0 0 0
0 0 0
0 0 0
0 0 0

101 102 103
201 202 203
301 302 303
401 402 403
501 502 503

[jlb]

The vector elements seem to be initialized to zero upon creation.

Actually they are "default" initialized. For numeric POD types this will yield zero.

Is this correct behavior?

Yes.

[E0p9LyTq]

Ah, I see why now.

Because I supplied vector count size for each dimension.

Thank you, jlb, I was really confused.